Advanced Propulsion Resources

And he concluded that what mankind must do to save itself is to launch an enterprise aimed at leaving the earth... The only thing I could say was this: that if I came to the conclusion that this was what mankind needed, if I wanted to contribute something to save mankind, then I would probably go into nuclear physics, because only through the liberation of atomic energy could we obtain the means which would enable man not only to leave the earth but to leave the solar system. Leo Szilard, 1932.

[ This article is currently under construction. Last updated: 23/02/2025. Update notes: added sections on the Pulsed Plasma and Nuclear Salt Water Rockets. Working on Nuclear Fusion Propulsion section next. ]

For those interested in learning about Advanced Propulsion methods related to spaceflight or wish to delve into Breakthrough Propulsion Physics or wish to become a Propulsion Physicist, here's a list of books, papers and other resources that the author of this article has found well worth reading or watching with some suggestions. This author has categorized the resources in various topics that are of particular interest to his areas of interest and research. This area of Applied and Theoretical Physics is a vast topic covering multiple disciplines intertwined with Materials Science and Engineering. There are literally hundreds of potential propulsion system configurations conceived depending on mission type within the solar system and beyond. The author of this article has only listed publications he has read and recommends so is by no means a complete list. Any suggested additions the reader can recommend, feel free to drop the author a line.

One will need a good grasp of Physics and Mathematics (differential and integral calculus as a minimum) to understand the various physical principles involved. It is strongly recommended doing a degree in Physics at a local university or if that's not an option, there are plenty of resources online to get one started. OpenStax (Rice University) is an excellent peer reviewed resource with free online textbooks covering Physics, Astronomy and Mathematics. For another good free Physics textbook, read Motion Mountain which uses a different approach to teaching Physics with less emphasis on the maths and more on the understanding of the physical concepts.

A well rounded Propulsion Physicist should read from a variety of sources and keep up with the latest developments because in this field new insights and developments come up often. The author recommends joining LinkedIn, there are a lot of highly skilled and very smart people on this platform who post relevant updates or articles which can be very informative (and some not so informative or wrong), so cross check their claims, facts, sources, figures and assess accordingly. There are free repositories that one should closely monitor. Included are some sites that the author takes time to read on a regular basis, these also provide useful commentary on the latest developments, useful organisations to join and other items:

• arXiv
• viXra (not peer reviewed)
• NTRS
• Not Even Wrong, by Peter Woit.
• Backreaction, by Sabine Hossenfelder.
• Centauri Dreams, by Paul Gilster.
• Space Settlement Progress, by John Jossy.
• Quanta Magazine
• Physics World
• Scott Manley runs a Youtube channel with interesting space related news.
• Marcus House also runs a channel with the latest news.
• LyX, when one is ready to publish physics papers, use this excellent and free WYSIWYM LaTeX editor. It will handle any mathematical equation one throws at it including Feynman diagrams, graphics etc, does the lot.
• NASA TOPS, do this course which explains the methods and tools of the trade to practice Open Science and how this will benefit one's research and career. Practicing Open Science has many advantages for the advancement of all areas of Science, this is the way for 21st century research, read this article by Lisa L. Lowe.
• Kerbal Space Program, all work no play? This game is actually very useful to visualise some plausible spaceflight propulsion systems based on real physics, go get it. Paul Gilster also features in this interview. Note unfortunately the development studio for KSP2 has recently closed. Workaround: download KSP 1.12.5 from Steam. Download CKAN, the Comprehensive Kerbal Archive Network to manage and automate the installation of mods. There's a large modding community with many mods that can enhance the gameplay of the original KSP. The particular mods the reader might be interested in are called: Near Future Propulsion, Kerbal Atomics and Far Future Technologies by Nertea (Chris Adderley).
• Australian Institute of Physics, suggest joining up as a member your local Institute of Physics, the AIP publishes a magazine and other interesting Physics related news. It is also good to network with fellow Physicists. Physics is an experimental science and one won't solve all the mysteries of the Universe alone by pen and paper. One will need at some stage advice from experts in their field and eventually experiments need to be carried out to ask Nature whether she agrees with a hypothesis, and if not, throw the model in the bin and try again, welcome to Science.
• American Institute of Aeronautics and Astronautics, definitely suggest joining up as a member, they have a vast array of pertinent resources, a regular informative magazine, discount on books and courses one can do online. They also have a Sydney section here in Australia.
• APEC, (Alternative Propulsion Engineering Conference), by Tim Ventura. There's an enthusiastic alternative propulsion physics community who report on their findings on a regular basis, they cover advanced topics such as gravity control, warp drives, FTL propulsion, inertial propulsion and other topics. Recommend keeping an eye on developments however beware of the fringe physics. It should be noted that claims of new propellantless propulsion drives come up on a regular basis and one common theme often noticed is that the new physics doesn't appear to be understood or explained well or highly speculative however the device is claimed to work nevertheless. A thrust is apparently observed in the lab vacuum chamber and all spurious interferences have been apparently taken into account and the thrust by the device is claimed to be real. It is important to keep an open mind as even if one of them worked would be ground braking however extraordinary claims require extraordinary evidence, the author looks at this as a good exercise to assess their claims and which alarm bells are ringing and if the propulsion physics is viable.
• Nuclear and Future Flight Propulsion Technical Committee, more good advice for budding Propulsion Physicists. As one can read, the career prospects are not that good especially if one isn't that interested in low energy chemical rocket propulsion systems like this author.
• Spoiler alert and reality check for near term human spaceflight: The Specific Impulse, Isp, measured in seconds, is a measure of the propellant utilisation efficiency and is defined as the engine thrust (F in Newton) divided by the propellant mass flow rate (? in Kilogram/second) x (g0 acceleration due to gravity on Earth which is approximately 9.81 m/s2). An analogy for Specific Impulse is how many litres of petrol a car uses per 100 Km, the less petrol used the more efficient the car is. Where the relativistic mass is the same as rest mass (where velocity is much less than the speed of light c), it is also related to Specific Energy (also known as the Gravimetric Energy Density), Jsp (Joule / Kilogram) in the simplified formula as follows:

• For chemical combustion rockets with low Jsp, the Isp is too low to get a crew to Mars and back in reasonable time frames (15 months at best with chemical rockets) which means astronauts may get sick due to space radiation exposure. It would be illegal to send a crew to Mars today if using chemical rockets because it would mean exposing the crew to deep space radiation for well over 3 months. This would exceed the radiation dose levels permissible in a workplace environment. According to this author's RadiaCode 103 (sensitive to gamma radiation), the natural background radiation where this author lives in Sydney today is 0.07 μSv/h (micro Sieverts per hour). For comparison, a typical dental x-ray would give an adult a radiation dose of 5 μSv, a full chest x-ray gives 100 μSv, flying on an airplane at high altitude for a 6 hour flight would typically give an adult a dose of around 35 μSv due to cosmic radiation. Our human bodies have evolved to handle low doses of ionizing radiation from the ground, cosmic radiation and other sources. The Potassium-40 isotope in our bones and bananas for eg is radioactive. The human body can repair itself when exposed to these low doses of radiation however too much radiation in a short period of time can cause radiation sickness if one receives a dose of 1 Sv or more. NASA doesn't allow their astronauts to accrue more than 1,000 mSv (1 Sv) over their whole career (there is a proposal to reduce this to 600 mSv). Some studies suggest 1 in 20 people will develop cancer once exposed to 1,000 mSv. Astronauts on the International Space Station accrue on average 23 μSv/hour. On the Moon's surface, background radiation from Galactic Cosmic Radiation (GCR) charged particles has been found to be 57 μSv/hour with the benefit of one half geographical shielding by the Moon's surface. In deep space on transit to Mars the radiation was found to be 75 μSv/hour, and on the surface of Mars it was found to be 28 μSv/hour.

• Near LEO we are somewhat protected by Earth's geomagnetic field which deflects low to medium energetic particles, Earth's atmosphere also stops some of the remaining radiation getting through except for highly energetic cosmic radiation and provides shielding equivalent to about 10 metres of water (liquid water being very good at blocking radiation hence used at nuclear power plant pools to absorb radiation from the reactor core). The Moon and Mars don't have a geomagnetic field. The Moon has a negligible atmosphere (known as an exosphere) consisting of trace amount of gases. Mars has a very thin atmosphere equivalent to only 10 cm of water. This is bad news for crew working on the surface of the Moon or Mars for any significant extended periods of time. It gets worse: staying long term in a microgravity environment is not good for our human bodies and radiation from highly energetic cosmic particles bombarding the metal alloy hull of the spacecraft can create secondary radiation showers inside the spacecraft module making the radiation worse inside for the crew. Astronauts that went to the Moon weren't outside Earth's geomagnetic field long enough to get long term adverse health effects due to space radiation. There are psychological factors to consider as well: how would one feel being couped up in a small spacecraft environment for months on end away from our loved ones and no fresh baguette and camembert onboard? All these issues need to be resolved to make space as safe as possible as a permanent living and workplace environment for humans.

• The Isp for Nuclear Thermal Propulsion (NTP) is generally twice that of the best chemical rockets available today, Nuclear Electric Propulsion (NEP) even better and Nuclear Fusion Propulsion (NFP), now we're talking, so any Propulsion Physicist should make it their business to learn everything they can about Nuclear Physics and Plasma Physics, this is the way for reasonable time transits within our solar system. See the Discovery II concept as an example of a possible NFP design.

• So someone designed the next NFP engine for the next iteration Discovery III or decided to move on to using antimatter for greater Isp? Great, next issue which one will need to deal with is how one will get all that hardware outside Earth's gravity well and build it in orbit. NASA's Discovery II study showed it would need 7 heavy lift launches to get all the hardware for in situ orbital construction which may or may not be a problem in the near term future but something to think about when designing a propulsion system.

• There are many other options to the above propulsion systems which we'll cover later however note sunlight intensity at Jupiter is 1/27 that at Earth, solar panels become less effective the further one ventures out. A good Propulsion Physicist should also think about the energy requirements for the propulsion system and how one will go about powering up the engine. If one needs to convert the entire mass of the Universe into energy to power up a fancy "trans-warp hyperdrive" or a propulsion system is for an epic "multi-generational mega-structure" for interstellar travel then no one will likely build it or even afford it even if it works on paper.
• Back to Earth, the job prospects for a Propulsion Physicist aren't that good if one isn't a US citizen. This author lives in Australia where there is a legislated nuclear ban in place, the job prospects in this field are next to none if one is mainly interested in propulsion systems with Isps of the order of NTP and onwards. This author has other passions which happen to pay the bills, do this as a second job if needed or try win the G?DE-Award. One could try get employment with the university however working in academia isn't for everyone and has its issues especially if one's research interests don't line up with the physics department, more on these issues here. Be prepared for a lot of travelling and ad-hoc employment during postdoc work, getting a tenured physics related position can be difficult and a long road but is doable if one tries hard enough however note many physics graduates don't actually end up landing a physics related job and this again depends which country one lives in however here in Australia the Space plasma power and propulsion group of the Australian National University do interesting work in the field of ion and plasma thrusters.
• One could also work for other research organisations such as the CSIRO or ANSTO that hire physicists however the research one will be conducting is unlikely to be in propulsion physics related to spaceflight. Another option is to serve in the Royal Australian Air Force - Defence Space Command helping to protect the country potentially from nuclear ballistic missiles launched by a foreign actor gone crazy or protecting our satellites from threats for eg.
• In the private sector there are a few Australian companies one could work for such as Neumann Space who also do interesting work in plasma thrusters, VXB Aerospace here in Sydney who have developed their own Hall Effect Thruster and Black Sky Aerospace who make their own solid rocket motors and fuel. Gilmour Space Technologies have also developed their chemical rocket and launch facilities for sending payloads into LEO and HyImpulse / Southern Launch use a LOX/Paraffin hybrid propulsion rocket for LEO launches as well. The Australian Space Agency also have good resources on how to start a space related career.
• Thought of a propulsion system or space tech that's so brilliant that no company in the space sector will want to launch their next satellite without one? One could think about setting up shop and work for yourself. The space sector is very fluid and has its risks (many startup companies rise and fail) and will most likely be very stressful however if the business turns out to be profitable the reader could be their own boss and free to decide what research one wishes to pursue down the track whilst still getting a wage. The National Space Industry Hub in Sydney can help one get started and they have a free Space Foundations course worth undertaking. Regardless what happens, if the reader has a passion for physics, follow it.

Below is a list of books and other resources in general Physics and Astronomy worth reading:

• The Feynman Lectures on Physics, by Richard Feynman.
• Modern Physics, by Kenneth S. Krane.
• Gravitation, by Charles W. Misner, Kip S. Thorne & John Archibald Wheeler.
• 100 Years of Gravity and Accelerated Frames, by Jong-Ping Hsu & Dana Fine.
• A stubbornly persistent illusion, by Stephen Hawking.
• Relativity, The Special and General Theory, by Albert Einstein.
• Quantum Physics of Atoms, Molecules, Solids, Nuclei and Particles, by Robert Eisberg & Robert Resnick.
• The Quantum Theory of Fields Volume 1, by Steven Weinberg.
• The Quantum Theory of Fields Volume 2, by Steven Weinberg.
• Quantum Field Theory in a nutshell, by Anthony Zee.
• Electrodynamics: An introduction including Quantum Effects, by Harald J. W. Müller-Kirsten.
• The Quantum Vacuum, by Peter W. Milonni.
• Advances in the Casimir Effect, by M. Bordag, G. L. Klimchitskaya, U. Mohideen & V. M. Mostepanenko.
• The Philosophy of Vacuum, by Simon Saunders & Harvey R. Brown.
• And yet it moves, by Mark P. Silverman.
• Nothingness, by Henning Genz.
• The Road to Reality, Roger Penrose.
• The lightness of being, by Frank Wilczek.
• Lost in Math, by Sabine Hossenfelder.
• Mathematics for Physics & Physicists, by Walter Appel.
• Mathematical Handbook for Scientists and Engineers, by Granino A. Korn & Theresa M. Korn.
• Introductory Astronomy & Astrophysics, Stephen A. Gregory & Michael Zeilik.
• Spherical Astronomy, by Robin M. Green.
• Lectures on Theoretical Physics, by David Long.
• Introductory Nuclear Physics, by Samuel S. M. Wong.
• Nuclear and Particle Physics: an Introduction, by Brian R. Martin & Graham Shaw.
• Introduction to QCD, by Peter Skands.
• How to become a GOOD Theoretical Physicist, by Gerard 't Hooft.

He who loves practice without theory is like the sailor who boards a ship without a rudder and compass and never knows where he may cast. Leonardo da Vinci, c.1509.

Inner Solar System Voyages (up to Mars)

Was it not for the pleasure which naturally results to a man from being the first discoverer... this service would be insupportable. James Cook, 17th August 1770.

There are a vast number of rocket launch platforms available today using either solid, liquid or a combination of propellants to get hardware into Low Earth Orbit (LEO), Medium Earth Orbit (MEO), Geosynchronous Orbit (GEO) and cislunar orbits. The main hurdle is to get out of Earth's gravity well outside 100 Km, once in orbit things are relatively more straightforward propulsion wise. So why are we stuck with chemical rockets when we have other propulsion concepts that have far greater Isps? The issue with many propulsion systems with high Isps is that they provide low thrust which is inadequate to get off the ground and reach a 100+ Km stable orbit however they are good at providing this low thrust over a long duration of time (good efficiency) whilst chemical rockets have a high enough thrust (power density or acceleration) to get tonnes of hardware into orbit however they run out of fuel within several minutes hence heavy lift rockets that send satellites to GEO or cislunar are large with typically a few rocket stages to get around this as dictated by the rocket equation.

A Hall Effect Thruster (HET) for eg doesn't have enough thrust to get us from ground to orbit even though its Isp of the order of 5,000 s is far greater than a typical chemical rocket engine like the RS-25 with an Isp of 366 s at sea level. However the RS-25 can provide a thrust at sea level of 1.86 Mega Newton (MN) compared to the thrust of a high performance HET of the order of 5 N at 100 kW input power. To build a HET with its high Isp and a high enough thrust would make it so large and the power systems would be so big it would not be practical to build. Even a NTP thruster would be inadequate for this application.

Are there any other options apart from chemical rockets to get us into orbit? There might be, let's have a look:

• Kinetic energy space launch systems using a centrifuge: SpinLaunch, a startup is attempting this however this is limited to relatively small payloads and has many other issues such as payloads exposed to extremely high gees if they can get it working.
• The Space Elevator: although the concept is feasible, the technology to make the carbon nanofibres long enough to make the tether isn't there yet (steel cable tensile strength isn't adequate for the job).
• Nuclear Pulse Propulsion: early versions of the Project Orion proposed detonating atomic bombs behind a spacecraft with a pusher plate / shock absorption system yielding impressive Isps however it was realised detonating nuclear bombs in Earth's atmosphere isn't a good idea and would have to be used in deep space only. However this propulsion concept requires a large stockpile of hundreds of atomic bombs to be carried onboard the spacecraft which makes this concept not so feasible especially with the Nuclear Test Ban Treaty in force.
• Beam powered propulsion: The US Air Force tested small lightcrafts propelled by high powered lasers which are focused to heat the air to very high temperatures which expands and then pushes the lightcraft upwards. This appears to have limited applications for any decent size payloads.
• Gravity Control Propulsion (GCP): this is a hypothetical concept and there is currently no evidence that this is feasible with our current understanding of physics and gravity. Lots of people have tried, all have failed. However research is ongoing for a Relativistic Theory of Quantum Gravity, there are many models proposed, all are conjectures until experimental data becomes available to ask Nature which model she agrees with (if any). And even if a model is found that agrees with Nature, there is no guarantee GCP would be viable. However see the section on Breakthrough Propulsion Physics further below in this article for more information.

As one can see the options other than chemical rockets are very limited. It appears the SpaceX model of full reusability of the lower and upper stages using liquid cryo propellants is one way to reduce the launch costs if they can get a reliable system working. Their latest Raptor 3 engine has an impressive thrust to weight ratio and uses liquid methane (LCH4) / liquid Oxygen (LOX) compared to liquid hydrogen (LH2) / LOX used for the RS-25. It is much easier to cool methane to a liquid state compared to hydrogen however the price for the tradeoff is a slightly lower Isp. There is also active research being conducted to improve chemical rockets by using High Energy Density Matter (HEDM) and Rotating Detonation Engines (RDE). At this stage you should read this excellent book that covers advanced cryogenic engines and their associated systems and have the rocket propulsion reference sheets handy:

• Advanced Propulsion Systems and Technologies, Today to 2020, by Claudio Bruno & Antonio G. Accettura.
• Rocket Propulsion reference sheets, by Werner J.A. Dahm.

Once in a stable orbit, the propulsion options available to us increases dramatically and this will depend on the payload, Δv (change in velocity) mission requirements and where one wants to go. In this section we are looking at propulsion options for LEO, MEO, GEO, cislunar and up to Mars.

Let's start with small micro-propulsion thrusters in the milliNewton (mN) thrust range typically used for microsatellites such as CubeSats in LEO. The thruster options can be classified into chemical (liquid, monopropellant, bipropellant, solid and hybrids), kinetic (cold gas and resistojets), electric (electrothermal discharge, electrostatic and electromagnetic for eg), propellantless (solar sail and electrodynamic tethers for eg) and many variants to the above. Some are more popular and cost effective than others with different power requirements and they vary in Isp and thrust capabilities as follows:

A common theme is many thrusters offer higher Isps with lower thrust capabilities whilst others offer higher thrust with lower Isps with various efficiencies. Gridded Ion Thrusters for eg can be throttled for higher thrust outputs at the expense of Isp and lower thrust output with higher Isp. What we need are thrusters that offer:

• Isp of the order 5,000 s +.
• Thrust output of the order 100 kiloNewton +.
• Efficiency in power conversion where applicable of the order 70% +.

Go through the papers below and study the propulsion physics that is applicable to each thruster design then go through the Satsearch article to see how these various thrusters look like in practice and think about the engineering that would be required to build them, they have handy datasheets which one will find interesting:

• Propulsion Technologies for CubeSats: Review, by Swood Alnaqbi, Djamal Darfilal and Sean Shan Min Swei.
• A comprehensive report on Electric Propulsion technology for space exploration, by Prithvi Giridhar.
• CubeSat thrusters and small satellite propulsion systems, Satsearch.

Some thruster technologies are more mature than others and are suited to the space available onboard the satellite and budget. Chemical propulsion isn't that practical for CubeSats for eg and often smaller electric thrusters are more suitable for station keeping for eg. For the larger satellites, SpaceX's Starlink constellation for eg use HET thrusters with Argon as the propellant. The same technology and hybrids can be ramped up for deep space exploration as is used for NASA's Psyche spacecraft.

Spacecraft need propellant to change their Δv at some stage in their mission however once the fuel runs out usually that's it for the spacecraft. Even if using gravity assist manoeuvers around the solar system or aerobraking/aerocapture using a planet's atmospheric drag to slow a spacecraft down or for orbital manoeuvring, small course corrections and attitude control is still required. We could think about building refuelling stations in orbit, use In Situ Resource Utilization (ISRU) methods or bypass the rocket equation altogether and use propulsion systems that don't require them to carry propellant onboard such as solar sails, electrodynamic tethers, laser propulsion etc. NASA launched the ACS3 small spacecraft below which is testing solar sail technology, these transform the momentum of solar photons to generate a propelling force. For an ideal solar sail, the pressure of sunlight photons at 1 AU is 9 N/km2 so the larger the sail the better however there are engineering practicalities to consider with larger structures such as the deployment mechanism and thickness of the sail. Potentially an interstellar precursor mission could be employed using solar sail technology by getting much closer to the Sun to obtain the required Δv to exit the solar system, the main disadvantage here is these would be suited for flyby missions only because deceleration at the destination might not be feasible.

Looking at larger satellites in MEO, GEO and spacecraft operating beyond up to Mars, let's recap some of the propulsion options that require a decent supply of electrical power, some of these are:

• Resistojets.
• Arcjets.
• Colloid Thrusters.
• Microwave Electrothermal Thrusters (MET).
• Gridded Ion Thrusters (GIT).
• Hall Effect Thrusters (HET).
• Field Emission Electric Propulsion (FEEP).
• Pulsed Plasma Thrusters (PPT).
• Magneto Plasma Dynamic thrusters (MPD).
• Pulse Inductive Thrusters (PIT).
• Variable Specific Impulse Magnetoplasma Rocket (VASIMR).

How are we going to power up these thrusters? The use of solar panels will depend on the spacecraft and operating area however as pointed out earlier, the sunlight intensity at Jupiter is 1/27 that at Earth, keep that in mind when we venture out beyond the asteroid field. CubeSats typically generate up to of the order of 100 W of electrical power due to their small panel sizes, large MEO and GEO satellites generate of the order of a few kW of electrical power via their panels and the International Space Station with its large solar array generates typically 120 kW. When it arrived at Jupiter in 2016, the Juno spacecraft generated 486 W of electrical power (note it uses hydrazine / nitrogen tetroxide chemical propulsion thrusters) however due to the high radiation environment near Jupiter, with radiation degredation of the solar cells, this has now reduced to around 400 W. Mission planners considered using a Radioisotope Thermoelectric Generator (RTG) however due to a shortage of Plutonium-238 nuclear fuel, the solar panel option was chosen. Generally Solar Electric Propulsion is useful for voyages up to the asteroid belt. For a 10 kW high power HET for eg, one would require around 20 kW onboard electrical power. To generate electrical power of the order of kWs and MegaWatts (MW) beyond the asteroid belt for larger spacecraft (and especially for human spaceflight), we'll need to use nuclear based power generating systems.

All the concepts just described above are covered in the early stages of this AIAA course which the author highly recommends undetaking before continuing on:

• Advanced Space Propulsion – On-Demand Short Course, AIAA.

Let's now revisit Jsp discussed earlier, the Specific Energy. From here on we'll call it the energy density for short. It is useful to look at the energy densities related to the various forces and then compare the Isps for the various propulsion methods.

It is clear from the above table that nuclear energy densities for fission and fusion based reactions are 10x6 to 10x7 larger than chemical reactions. Hence Isps for fission and fusion propulsion systems are greater than chemical combustion propulsion systems as shown below:

Nuclear Thermal Propulsion (NTP) with LH2 as the propellant (liquid hydrogen chilled to 20 Kelvin) offers twice the Isp of the best chemical rockets available today. It isn't new technology, nuclear rockets have been tested since the 1960s however due to various reasons haven't been spaceflight tested so far (although both the US and Russia have launched nuclear microreactors into orbit for electrical power generation such as the SNAP-10A). Below is a good introduction to nuclear rockets and describes the NERVA development program:

As one might have guessed by now, Isp is closely related to the velocity of the exhaust which is also related to the exhaust temperature. The higher the exhaust temperature, the higher the exhaust velocity and the higher the Isp. It is beneficial to understand how to calculate the Isp based on exhaust temperature to help us understand more advanced propulsion systems later on so as an exercise let's calculate the Isp for a NTP propulsion system using a solid core tube nest heat exchanger utilising LH2 as the propellant. According to the LANL diagram, the exhaust temperature for this propulsion system is given as 2,700 K. Note a big advantage of NTP is that unlike chemical combustion rockets, there is no oxidiser required to be carried onboard the spacecraft so we save space and mass not having to carry liquid oxygen as propellant as the heat is generated from the nuclear reactor core instead of chemical reactions. We'll need the formulas as follows:

Let's start from the bottom formula and work our way up, definitions:

• Gamma: is the ratio of the specific heat capacity at constant pressure (Cp) to the specific heat capacity at constant volume (Cv) of the propellant gas (hydrogen in this case), it provides a link between the gas properties and the energy conversion process in the rocket engine, for this calculation a typical ratio is 1.4.
• R: is the gas constant = 8.314 J/mol?K.
• Mw: is the molecular weight of the propellant, in this case diatomic hydrogen gas which has two atoms of hydrogen so we have 2 x 0.001 Kg/mol = 0.002 Kg/mol.
• Te: Exhaust temperature in Kelvin, in this case is 2,700 K.
• g0: acceleration due to gravity on Earth which is approximately 9.81 m/s2.

Time to get the calculator out: work out Cp based on the given Gamma, then Ve to finally get Isp which should come to approximately 903 s. According to the NERVA video above, the best Isp that we could get with NTP is around 900 s. The above formulas also show that using a propellant with a lower molecular weight yields higher Isps hence why the best Isp for a chemical combustion rocket engine at the moment, the RL10B-2 with an Isp of 465.5 s, uses LH2/LOX, hydrogen being the lightest element in the periodic table. Note the Space Shuttle Main Engine chamber temperature is around 3,500 K which is higher compared to NTP however the penalty comes from Mw which is around 0.018 Kg/mol for the exhaust water vapour molecular weight. Also note the square root dependency indicates that modest increases in Jsp using HEDM for chemical rockets for eg will yield a minor increase in Isp.

So should we get rid of chemical rockets and use NTP instead to go to Mars? NASA did a recent comprehensive study on this for a possible human crewed mission in the late 2030s using Venus Gravity Assist manoeuvres and the answer is not straightforward because there are many other factors other than Isp to think about which are laid out in their report:

• Mars Transportation Assessment Study (MTAS), NASA.

Among the other factors mentioned in the MTAS included thrust to weight ratio considerations of the propulsion system, which affects spacecraft acceleration and Δv. The study found a realistic thrust to weight ratio for the NTP engines would be 3.6 and full thrust lifetime of the reactor only 4 hours, anything above these figures still needs to be demonstrated. One major other factor is the nuclear fuel to be used for a NTP mission to Mars. The NERVA program used Highly Enriched Uranium (HEU) reactor cores in the fast neutron spectrum which contains Uranium-235 concentrations greater than 20%, this is to avoid using moderator materials for simplicity, reliability and maximise lifetime performance of the reactor core. HEU is used in naval propulsion reactors, nuclear weapons and in some research reactors. Due to security and weapons grade nuclear material proliferation concerns, together with the risk of this nuclear material ending up in the environment during a Rapid Unscheduled Disassembly (RUD) event at launch for eg, the current NTP development efforts are looking to use High Assay Low Enriched Uranium (HALEU) instead which contains U-235 concentrations between 5% - 20% (non-weapons grade), HALEU TRISO nuclear fuel can withstand temperatures up to 3,800 °C and has relatively low environmental risks even if there is a RUD. However this is a departure of the NERVA highly researched reactor designs studied in the 1960s and 70s and the research budget today to use HALEU instead of HEU is small compared to what NERVA's budget used to be, NASA views this as another development risk. Remember they want boots on Mars by the late 2030s to use the favourable planetary alignment window for a Venus Gravity Assist flyby.

Another factor which is also problematic is the long term (2 years+ for a human Mars mission) Cryogenic Fluid Management (CFM) to store and transfer the hydrogen propellant at 20K. It needs to be kept that cold to keep hydrogen in liquid form to minimise leakages over that time period and to minimise volume at launch which can be challenging to manage especially from small valve gaps. It can even diffuse through the metal lattice of cryogenic storage tanks over time.

To achieve the highest Isp possible and maximise the temperature of the propellant and hence exhaust velocity, NTP reactors operate at the extremes of materials capabilities and engineering design, 3,000 K being state of the art for solid core reactor designs using ceramics. Apart from being the propellant, hydrogen is also used as the reactor coolant and any imbalance of the hydrogen flow could cause hotspots and melted fuel elements in the reactor core. At these higher temperatures hydrogen also becomes more corrosive as well which limits engine lifetime to a few hours of thrust operation even with hydrogen resistant material coatings. We also need radiation shielding for the crew which adds a fair amount of mass to the spacecraft. One can get the idea now why the NERVA research and development went for a long time to work these problems which culminated to a flight test ready NTP engine the XE Prime however this didn't get to be flight tested due to the eventual cancellation of the program.

Back to MTAS, the NTP option proposes four Nuclear Thermal Rocket engines with an Isp around 900 s @111,206 N each (25,000 pound-force lbf) with a return trip time of 690 days including 30 Mars days stay on the surface however concluded from the study not to use this option in preference to the bi-modal [liquid methane / liquid oxygen (LCH4 / LOX) - Nuclear Electric Propulsion (NEP)] option.

Now is a good time to get a more detailed understanding how NTP works, read these two excellent books and the Master Thesis below:

• The Role of Nuclear Power and Nuclear Propulsion in the Peaceful Exploration of Space, IAEA.
• Nuclear Space Power and Propulsion Systems, by Claudio Bruno.
• Nuclear propulsion and space microreactors, by Shady Elshater.

So where are we up to today with NTP? NASA recently joined the DRACO program to develop NTP further using HALEU with a proposed flight test in 2027. Even if NASA don't end up using NTP to go to Mars it will open up more solar system destination options down the track with this new capability. Hydrogen is readily available in the solar system wherever there is H20 water.

Although NTP generally offers twice the Isp of chemical combustion rockets, it does pose challenges with hydrogen and its Isp is still much lower than electric propulsion we looked at earlier. The MTAS recommended using the chemical rocket LCH4/LOX option (also known as Methalox) instead for orbital manoeuvring which is a much more mature propulsion system and the CFM is more manageable providing the same amount of thrust as the NTP option at 111 kN. SpaceX's latest Raptor 3 engine also uses Methalox. In cruise mode it recommended using two pods consisting of 10 Xenon 100 kW Hall thrusters each with an Isp of 2,600 s as shown below. Xenon is easily stored in tanks under pressure and at room temperature hence no fancy cryogenic storage systems required for this however we need lots of electrical power to operate these Hall thrusters, 1.9 MW of it.

To generate that much electrical power we need a nuclear microreactor onboard because solar panels would be too big (remember the International Space Station large solar array discussed earlier generates around 120 kW) and the further we venture away from the Sun, the less power they generate as well. To generate 1.9 MW(e) of electrical power we need a microreactor that can generate at least 10 MW(th) of thermal power (heat) due to the inefficiencies of Thermo Electric Generators (TEGs), Thermionic converters and heat engines such as Brayton converters which ranges from only 8% to 20% efficiencies. Stirling converters do a bit better at 30% thermal to electric conversion efficiency as demonstrated in the KRUSTY 1 kW(e) experiment design:

If anyone can come up with an efficient method to convert high power nuclear energy directly to electricity without resorting to intermediary heat exchange materials and working fluids, DARPA would like to hear from them and so would NASA. If one can come up with the chemical formula for a material that is a room temperature superconductor to dramatically increase the efficiency for electric thrusters that would also be very helpful. Spacecraft radiator size is determined by the size and efficiency of the thermal to electric power conversion unit. The 2,500 m2 radiators are quite large to reject the waste heat generated by this conversion process and any gain in efficiency will mean a reduced size in the radiator requirements (reducing mass and launch requirements) to build the spacecraft in orbit.

It is a good exercise to be able to calculate the Isp of a Hall thruster however there are many factors involved, we need more background theory to be able to do this, go through the excellent resources below:

• Why Are There Two Different Types Of Electric Space Engines, And How Do They Work?, by Scott Manley.
• Fundamentals of Electric Propulsion: Ion and Hall Thrusters, by Dan M. Goebel and Ira Katz.
• High-Power Performance of a 100-kW Class Nested Hall Thruster, by Scott J. Hall, Benjamin A. Jorns, Alec D. Gallimore, Hani Kamhawi, Thomas W. Haag,, Jonathan A. Mackey, James H. Gilland, Peter Y. Peterson and Matthew J. Baird.
• The H10 High Power Density Hall Thruster, by Richard R. Hofer, Jacob B. Simmonds, Dan H. Goebel, Joel M. Steinkraus and Adele R. Payman.

To keep things brief here we'll use the simplified formula below for a Xenon Hall thruster (see Ch 2.2-2.4 Goebel and Katz for more detail):

Definitions starting with the bottom formula:

• Gamma: total thrust correction factor calculated by multiplying alpha (thrust correction factor for charged species terms) by F(t) (beam divergence correction factor calculated by taking the cosine of the average half angle divergence of the beam, typically 10°), use 0.984.
• Eta(m): thruster mass utilization efficiency and accounts for the ionized versus unionized propellant, beam current and mass flow rate and other factors, use 0.84.
• V(b): average beam voltage, going from MTAS, use 650 V.

Time to get the calculator, Isp comes to approximately 2,606 s which is around the Isp given for the MTAS Xenon Hall thruster specification.

Note the theoretical specific impulse calculations don't always quite agree with the measured Isp for the Hall thruster for the given parameters because the physics modelling sometimes isn't quite accurate and factors such as the beam ion optics configuration, anode and cathode efficiencies, multi-channel hall thruster field interactions and other factors come into play however research is showing that the average efficiency of these thrusters increases by increasing the beam voltage as is expected by the formula.

So where are we up to today with NEP? Recently Lockheed Martin received funding from the US Air Force Research Laboratory (AFRL) to develop NEP further and the associated power systems required for a spacecraft via the JETSON program. BWXT and Space Nuclear Power Corp will assist in the development of the microreactor to power the NEP system. Note NEP microreactors have several expected advantages compared to NTP reactors. They will operate at lower temperatures and in a steady state mode which will make them last longer because it is used to generate electrical power for the Hall (or other electric) thrusters (and spacecraft power requirements) and isn't interacting with very hot corrosive hydrogen propellant. NEP microreactors will also need to sit behind a dense and massive conic shield to abosrb gamma and neutron emissions to provide a radiation shadow to protect spacecraft systems and crew, this could potentially be more massive than the microreactor itself.

A bi-modal Methalox/NEP or NTP/NEP look to be good propulsion candidates for future crewed missions to Mars however from a radiation dose point of view neither propulsion options are still adequate if we want to maintain a maximum 1,000 mSv maximum career radiation dose limit for a Mars return trip unless we utilize specific passive and/or active shielding to protect the crew or we go faster. The concept NEP Hermes spacecraft shown below is an example and features a rotating habitat to mitigate long term microgravity effects on the human body and heavy passive shielding to provide some protection for the crew from long term exposure to space radiation. The NEP propulsion system is at the back behind the radiators.

In the next section we'll look at more advanced propulsion systems with higher exhaust velocities, greater Isps and thrust to allow crewed missions beyond the asteroid belt.

Outer Solar System Voyages (asteroid belt onwards)

I think that pretty much sums it up: space is a vast void, and you're really going to have to travel fast if you're going to have any chance of surviving. I also would not want to send people to Mars on a fragile and power-limited ship. If you send people that far, you have to give them a fighting chance to survive, and the only way you can do that is if you have ample supplies of power. Power is life in space. Franklin Ramón Chang-Díaz, 2007.

For crewed missions past the asteroid field to the outer planets, Jupiter, Saturn and onwards, we will need high performance propulsion systems to attempt to reduce mission timeframes to as practicable as possible. From here on and to keep this article brief we will be looking only at propulsion options that offer as a minimum both an Isp of 5,000 s and Thrust of at least 100 kN. We will attempt reasonable vehicle mass fractions and make educated guesses on the spacecraft architectures required. The vehicle mass fraction is defined as follows:

where Mi is the spacecraft initial total mass at launch fully fueled with the payload (also known as wet mass) and Mf is the final spacecraft mass after some of the propellant has been used up (also known as the dry mass). We also know the spacecraft will require a nuclear microreactor for electrical power generation as solar panels in these regions are of limited use and earth based or otherwise beamed power options to the spacecraft present many additional problems and infrastructure requirements. We also know that electric propulsion options discussed earlier are inadequate for these human spaceflight missions, although some have impressive Isps well above 5,000 s, they only provide a few Newtons of thrust. We know solid core heat exchanger NTP provides generally Isp double that of the best chemical propulsion options available however is still well below our Isp requirement as we have reached the current upper temperature limit of approximately 3,000 K of the best ceramic/metals (CERMET) materials available for solid core designs and we know that Isp is closely related to propellant temperature and exhaust velocity.

Looking back at the previous LANL figure what other fission reactor can we use to heat up liquid hydrogen propellant? Instead of using a solid core heat exchanger, we could let the Uranium fuel melt and use a molten core instead however the expected Isp is still too low for our requirements. Let's keep going and heat the Uranium fuel even further to at least 20,000 K until it becomes a gas. Enter the gas core NTP rocket. There are two types: open cycle and closed cycle however to meet our requirements we are only considering the Open Cycle Gas Core NTP option. We'll need to learn more on nuclear physics and space nuclear propulsion systems to continue so read the two excellent textbooks and then go through the resources and articles below:

• Roadmap to Nuclear Gas Core Rockets, by Colin Warn.
• Space Nuclear Propulsion and Power: Principles, Systems and Applications, by Bahram Nassersharif.
• Principles of Nuclear Rocket Propulsion, by William J. Emrich.
• Gas Core Nuclear Rocket Performance Characteristics, online book companion, by William J. Emrich.
• Chart of Nuclides NuDat 3.0
• Gas Core Nuclear Rocket Feasibility Project, by S. D. Howe, B. DeVolder, L. Thode and D. Zerkle.
• Magnetohydrodynamic Vortex Containment, Part 1 | Part 2 | Part 3 | Part 4, by Raymond J. Sedwick and Daniel A. Zayas.
• Overview and future of Open Cycle Gas-Core Nuclear Rocket, by Rittu S. Raju and John E. Foster.
• Beyond the Solid Core Nuclear Thermal Rocket: A Computational Investigation into Criticality and Neutronics Performance of Advanced Liquid and Gas Core Reactor Approaches for Next Generation Performance, Ph.D. Thesis, by Rittu S. Raju.
• Advanced nuclear power engine: A brief overview of gas core reactor for space exploration, by Yebing Zhang.

What is the Open Cycle Gas Core NTP good for? Let's have a look at the trip times using this propulsion system for a trip to Mars and Jupiter how the numbers look like. With an Isp of 5,000 s / vehicle mass fraction of 0.2, the one way minimum trip time Earth to Mars is around 23 days and to Jupiter is around 136 days or just over 4.5 months (see Emrich Chapter 4 Interplanetary mission analysis for more details on the orbital and flight time calculations).

Not fast enough? Later in this section we'll also look at other propulsion options as per the LANL figure such as fusion based propulsion yielding an Isp of 432,000 s @100 kN which could reduce travel times to anywhere in the solar system to less than a year regardless of favourable launch windows and antimatter based propulsion yielding an Isp of 18,110,000 s which could make interstellar travel more feasible by attaining relativistic velocities.

Let us look at the simplified formula for Isp based on internal chamber temperature and molecular weight of the propellant:

This formula tells us that the hotter we can get the propellant, the greater the Isp. If we use propellant with a low molecular weight this increases Isp. In the Open Cycle Gas Core Nuclear Thermal Propulsion rocket the propellant of choice with the lowest molecular weight is diatomic hydrogen gas (Mw = 0.002 Kg/mol). Using this simplified formula, it tells us we need the hydrogen to attain a temperature of at least 25,000 K to be able to get an Isp of 5,000 s. At these high temperatures there's a bonus. It turns out above 5,000 K hydrogen dissociates from being diatomic H2 to H so Mw becomes 0.001 Kg/mol. There's a useful online book companion calculator which allows one to see how the temperature gradient develops from the uranium core to the reflector based on the inputted parameters and calculates Isp for us, see Emrich p339 for more details on the other parameters.

There's also another bonus: Rittu S. Raju in his thesis has shown that the absorptivity of hydrogen is very small for lower temperatures up to 15,000 K and previous authors have suggested using small tungsten particles mixed with the hydrogen propellant to allow it to absorb heat more effectively from the plasma uranium core however this increases Mw and reduces the Isp for the system. In his thesis Raju shows on p76 that tungsten seeding of hydrogen is not required for temperatures above 20,000 K @1,000 atm pressure as the opacity is very high. The gas core option has the potential to offer high Isps / high thrust with reasonable thrust to weight ratios.

That was the good news. Although the propulsion physics is viable, in many respects getting a gas core open cycle NTP rocket engine working presents many engineering and material science challenges that may be more difficult compared to getting a fusion reactor working. However many of the techniques applicable to gas core open cycle NTP would also be applicable to fusion reactors. No one has tested an uranium gas core gone critical to date (according to the open literature) and all past research is based on modeling and non-nuclear experimental work. Several reactor core geometries have also been looked at in the past by several authors. The author of this article is looking into the viability of the Toroidal Gas Core Vortex Stabilised option as proposed by S. D. Howe et al using modern multiphysics High Performance Computing (HPC) validation simulations for plasma dynamics, uranium plasma core confinement and fuel retention, thermal hydraulics, cooling of engine components, neutronics, nozzle protection from high temperature plasma exhaust, radiation diffusion/shielding and startup/shutdown sequence with the addition of realtime stabilisation via magnetic field confinement and the viability of AI assisted Computational Plasma Dynamics (CPD) monitoring.

It is this author's opinion that the open cycle gas core option would only be viable if uranium fuel entrainment from the uranium plasma core to the hydrogen propellant stream can be kept to below 0.01% and the startup sequence to criticality would need to work with solid uranium rods (not gaseous Uranium HexaFluoride) and without using antiprotons as suggested by some authors. The shutdown sequence and recovery of the uranium fuel is also being looked by this author. The multiphysics simulations would need to validate all the above before this is tested via physical prototypes (which will be very expensive when using nuclear activated hardware and the rocket exhaust would need to be scrubbed if tested here on Earth).

How would a spacecraft with a gas core open cycle NTP rocket engine look like? No one has built such as a spacecraft in real life however lucky for us someone has already thought of this in Kerbal Space Program with the Emancipator rocket motor. This author is still learning how to build spacecraft in KSP and is still in the VAB (Vehicle Assembly Building) at this stage. The below images will be updated when complete.

Another propulsion concept currently being looked at by Howe Industries is the Pulsed Plasma Rocket (PPR), which also claims to offer an Isp of 5,000s @100kN. They recently received Phase II of NASA's Innovative Advanced Concepts (NIAC) funding to continue research & development.

This propulsion concept is essentially a more advanced version of the Orion Project mentioned earlier on in this article. Recall this proposed to generate nuclear explosions to impart a force on the pusher plate connected behind the spacecraft to generate thrust with an impressive Isp. Here the PPR proposes to use energised HALEU barrel bullets which are driven to criticality via a more refined method of the Pulsed Fission Fusion (PuFF) concept which results in a plasmoid which then expands and is ejected by a magnetic nozzle resulting in thrust. Note to date it remains to be seen if the magnetic nozzle concept is viable in practice to semi-contain these mini nuclear explosions in terms of degredation over time. It appears also the microreactor power requirements for this whole propulsion system will be significant. As pointed out earlier, Brayton power conversion efficiencies aren't that good. Another disadvantage of note is that for every nuclear pulse generated, one HALEU barrel bullet is consumed in the process which could be a problem if sourcing a fresh supply of nuclear fuel barrel bullets is an issue which makes this propulsion system not so conducive to ISRU in the near term. However if they can get this propulsion concept to work and it makes it to NIAC Phase III, it would be a significant breakthrough.

Instead of relying on many individual nuclear explosions to generate thrust behind a pusher plate or magnetic nozzle (ie nuclear pulse propulsion) why not use continuous nuclear fission reactions to generate continuous thrust? Pushing the envelope as far as fission based propulsion systems go, enter the Nuclear Salt Water Rocket (NSWR). In theory this could provide an Isp of 6,730s @12,900 kN using the fission products of an aqueous fissile propellant of water/Uranium235 enriched @20% (reactor grade fuel). The nuclear fission reactions are allowed to go supercritical continuously and the fission exhaust products are directed via a water cooled magnetic nozzle. The author also suggests in this paper that if the NSWR used instead a solution of 2% Uranium bromide with 90% enriched Uranium (weapons grade), this could yield a 90% fission yield increasing the Isp to 482,140s potentially enabling precursor interstellar missions. Like many of the above propulsion systems discussed so far, it does present significant engineering challenges which are potentially solvable however it should be noted that this author found no further trade studies on this concept after the above 1991 paper. This propulsion concept deserves a full multiphysics / neutronics / thermal hydraulics / CPD / magnetic nozzle protection and exhaust modelling study via modern HPC techniques to do a more in depth viability assessment of this propulsion system.

[To be continued... Nuclear Fusion Propulsion]

There will certainly be no lack of human pioneers when we have mastered the art of flight. Who would have thought that navigation across the vast ocean is less dangerous and quieter than in the narrow, threatening gulfs of the Adriatic, or the Baltic, or the British straits? Let us create vessels and sails adjusted to the heavenly ether, and there will be plenty of people unafraid of the empty wastes. In the meantime, we shall prepare, for the brave skytravellers, maps of the celestial bodies. Johannes Kepler, 1610.

Interstellar Travel

To drop a pea at the end of every mile of a voyage on a limitless ocean to the nearest fixed star, would require a fleet of 10,000 ships of 600 tons burthen, each starting with a full cargo of peas. John Herschel, c.1850.

If the aim is to leave our Solar System and build a propulsion system to get a spacecraft to our nearest neighbour star Proxima Centauri 4.25 ly away, then one should first familiarise ourselves with the challenges of interstellar travel and the space in between our Sun and other star systems in our Milky Way Galaxy.

Due to the vast distances involved, even at relativistic velocities, the transit time frames to go to most nearby star systems, especially with confirmed planets in the habitable zone, are daunting compared to our relatively short human body lifespans. Building faster starships may solve some of these timeframe issues (well over a century transit times) however the faster one goes, the propulsion, power and engineering requirement challenges increase accordingly. One could build a propulsion system to propel a multi-generation starship as one human lifetime may not be enough to reach a designated star system however there are many issues with this approach if anyone decides to build these epic mega-structures or world ships. One could resort to going into hypothetical "hibernation or sleep pods" or use cryopreservation methods for long duration voyages to star systems however these technologies haven't been invented or verified yet by humans (although there are people in cryosuspension today hoping to be revived in the future when medical science and nanorobotics are sufficiently advanced to make revival a slim possibility, these people are clinically dead however not necessarily information-theoretic dead).

Spoiler alert on FTL drives: If one is thinking now of Faster Than Light (FTL) propulsion, warp drives or taking a shortcut through spacetime via wormholes to the outer rims of our galaxy, don't get too excited. There has been a flurry of research papers on these topics that have explored various spacetime metric solutions to the above concepts within the General Relativity (GR) framework. Certain mathematical solutions within GR permit these concepts however unfortunately many have also shown these concepts are not solutions that can be utilised in the physical world. One deal breaker issue for example is the concept of negative energy density needed to create a warp bubble. Within a Casimir vacuum it is postulated that negative vacuum energies exist however this hasn't been observed in nature and if negative energy densities did exist it would make the vacuum unstable and we wouldn't be reading this today. Also note GR despite being a very successful model, isn't a complete model of spacetime and gravity. This doesn't rule out the concept of FTL, warp drives, wormholes or other spacetime engineering methods for propulsion purposes altogether however current Physics doesn't appear to support the above concepts in the physical world and further enquiries are required through Breakthrough Propulsion Physics.

Read these excellent books on interstellar travel below followed by the other resources. Interstellar propulsion related research is mainly carried by a few universities and organisations who occasionally get grants to do so however the funding appears to be sporadic and low priority for most organisations as at this stage going to Mars for eg seems to be one of the main priorities and it should be noted even if we launched a new interstellar probe tomorrow, unless there is a significant breakthrough in propulsion physics, the interstellar science return would be a 50 years+ wait. Because of the very long wait, it can be a challenge to keep the public (tax payer who ultimately fund these missions) engaged during the transit when less science data is transmitted back to Earth. Another issue facing deep space missions as pointed out earlier with Juno is sourcing the Plutonium-238 nuclear fuel for the Radioisotope Thermoelectric Generator (RTG) which is becoming problematic. Most organisations are also concerned about their next year's budget and this area requires long term thinking. NASA for eg commissioned the Breakthrough Propulsion Study below and Stage I was completed however the funding dried up for Stages II and III. This doesn't stop individuals and organisations carrying out their research in their own time or relying on donations/sponsors.

• Prospects for Interstellar Travel, by John H. Mauldin. Hard to get these days, here is a detailed book review.
• The Starflight Handbook - A Pioneer's Guide to Interstellar Travel, by Eugene Mallove & Gregory Matloff.
• Centauri Dreams, by Paul Gilster who also has an excellent Booklist.
• Deep Space Propulsion: A Roadmap to Interstellar Flight, by Kelvin F. Long.
• Breakthrough Propulsion Study, Marc G. Millis, Jeff Greason & Rhonda Stevenson.
• Marc G. Millis update and his last career presentation.
• Tau Zero Foundation, has a good summary of propulsion concepts.
• Interstellar Research Group, who provide regular updates on relevant research published.
• Initiative for Interstellar Studies, who publish a regular magazine called Principium.

Below is a list of current interstellar missions and related studies that have been carried out which also describe their Interstellar propulsion option proposed for the given mission scenario. Note Voyagers 1 & 2 are the only probes that have just entered interstellar space so far. Pioneers 10, 11 and New Horizons are on their way. All others listed are theoretical studies which rely on certain key technologies to be available to be viable missions. Except for Interstellar Probe and a few others most are currently at low TRLs.

• Local Interstellar Medium, 0.002 ly: Voyager 1 & 2 l Interstellar Probe
• Solar Gravitational Lens, 0.01 ly: SunVane | SunVoyager

• Deep Interstellar Medium, 0.43 ly: TinTin

• Proxima Centauri, 4.25 ly: Worldship | Icarus | Photon Sails | Starshot
• Ross 128, 11.01 ly:
• GJ 1061, 12.00 ly | Luyten's Star, 12.35 ly | Teegarden's Star, 12.50 ly:
• TOI-700, 101.40 ly (possible Earth 2.0 candidate):

What a wonderful and amazing Scheme have we here of the magnificent Vastness of the Universe! So many Suns, so many Earths, and every one of them stock'd with so many Herbs, Trees and Animals, and adorn'd with so many Seas and Mountains! And how must our wonder and admiration be increased when we consider the prodigious distance and multitude of the Stars? Christiaan Huygens, 1695.

Breakthrough Propulsion Physics

Spaceflight as we know it, is based on the century old rocket equation that is the embodiment of the conservation of linear momentum. Current space transportation systems are based on this principle of momentum generation, regardless whether they are chemical, electric, plasma-dynamic, nuclear (fission) or fusion, antimatter, photonic propulsion (relativistic) and photon driven (solar) sails, or exotic Bussard fusion ramjets. Moreover, special relativity puts an upper limit on the speed of any space-vehicle in the form of the velocity of light in vacuum. The only possibility to overcome these severe limitations lies in the finding of novel physical laws that allow constructing propulsion systems based on principles different from classical mechanics (momentum principle). Therefore, there has been a great deal of interest during the last decade in so called breakthrough propulsion physics. Walter Dr?scher & Jochem Hauser, 2007.

After reading about the challenges involved from above just to get to our nearest neighbour star, one might decide that this is all too hard and sit tight on planet Earth and hope our species eventually doesn't succomb to a natural or otherwise human related disaster like a major asteroid impact, climate change, worldwide epidemic, World War III and nuclear holocaust, overpopulation and lack of resources or decide to setup colonies on Mars and our local Solar System neighbourhood or investigate further and enquire about possible new Physics that we may yet discover that potentially could lead to new propulsion methods. After all we currently don't have a working Relativistic Quantum Gravity theory and there are many loose ends to resolve in the Standard Model. Why do the electron, muon and tau particles differ in mass when they have the same electric charge and the Koide formula is still a mystery. We don't quite fully grasp Spacetime, the Quantum Vacuum and many other current issues in Physics are still unresolved such as what is the root cause of inertia and the physical background process of matter/energy bending Spacetime. So in other words even with General Relativity, no one can really still explain why an apple falls to the ground without resorting to the hypothetical graviton particle. In this area assume nothing, question everything. This was one of the aims of NASA's Breakthrough Propulsion Physics (BPP) program when it was headed by Marc G. Millis who later wrote a book:

• Frontiers of Propulsion Science, by Marc G. Mills & Eric W. Davis.

[To be continued...]

BPP related items:

• Greenglow & The Search for Gravity Control, by Ron Evans.
• Ron Evans interview about Project Greenglow.
• The Good Kind of Crazy: The Quest for Exotic Propulsion, Scientific American.
• Assessing Hypothetical Gravity Control Propulsion, by Marc G. Millis.
• Is Gravity Control Propulsion Viable?, by Paul Titze.
• Progress in revolutionary propulsion physics, by Marc G. Millis.
• Inertial frames and breakthrough propulsion physics, by Marc G. Millis.

The pursuit of propulsion physics is not just for propulsion, but also another approach to make further progress in physics, even if the desired propulsion breakthroughs cannot be achieved. Marc G. Millis, 2017.

Corollary Research Topics

Those who explore an unknown world are travelers without a map; the map is the result of the exploration. The position of their destination is not known to them, and the direct path that leads to it is not yet made. Hideki Yukawa, 1982.

Artificial Gravity Systems for Spacecraft

By Rotating Structures

• One revolution per minute, by Erik Wernquist.

Artificial Gravity Deck Plating, in microgravity / non-rotating structures (hypothetical concept)

Passive and Active Shield Systems for Spacecraft

• Shielding Space Travelers.
• Space Radiation Environment and Its Effects, webinar.
• The Space Environment and Its Effects on Space Systems, by Vincent L. Pisacane.
• NASA Technical Brief: Design for Ionizing Radiation Protection.
• Transport methods and interactions for space radiations, NASA.
• Shielding factors for a fission-powered mars exploration rover, by Alexander C. Bendoyro, L. Dale Thomas, Jason Cassibry and William Emrich.
• Multi-layered shielding materials for high energy space radiation.
• ESA Radiation Test Database.
• Space Radiation Webinar.
• Simple Space Radiation Models.
• XCOM.
• Space radiation measurements during the Artemis I lunar mission.

AI Propulsion Physics

• AI courses, Institute of Applied Technology.
• NVIDIA Academy.
• A Physicist’s Guide to Machine Learning and Its Opportunities.
• Machine learning and theory.
• Tomorrow’s physics test: machine learning.
• A physicists’ guide to the ethics of artificial intelligence.
• AI used in fusion research | Quantum computers | Cosmology.
• AI Research Assistants.
• [ Python | NC Python | NC Learning AI | NC Private AI | BC Python Full Course | MIT OpenCourseWare ]
• [ GitHub | Jupyter | Kaggle | Raspberry Pi ]
• [ NASA TOPS | Zenodo | ORCID | DOI | ARDC | PID | DMP | NumPy | scikit-image | dealii | Astropy | PyPi | Sphinx | pyOpenSci | JOSS | Torax | PyTorch | PlasmaPy ]
• Data61.
• Google Gemini AI.
• [ NVIDIA Grace Hopper | CUDA | Modulus ]
• Awesome Nuclear.

Project EThandshake

As a side research project whilst learning Python and AI, this author is also interested in the mathematics and protocol theory of how a potential human made AI enabled interstellar probe would do a handshake communication learning protocol to a potential alien probe or ETI via radio, light or other signals that it may encounter in its voyages through interstellar space and how this would be implemented via Python code into the onboard AI computer system. The chances of one of our interstellar probes making contact with an alien probe or ETI is extremely low in our interstellar neighbourhood hence this is an interest from an academic point of view.

A bit of background to where this interest comes from, back in this author's university days, a month was spent during the summer break at the Sydney University Stellar Interferometer next to the Australia Telescope Compact Array at Narrabri, Australia as part of a scholarship through the Astronomy department. This author also holds an advanced amateur radio licence and has an interest in Radio Astronomy and SETI as well. Back in the days this author used his amateur radio gear to make contact with the MIR Space Station when it was still in orbit, see AMSAT Australia for further information. With a recent interest in Python coding and AI, the above seems a natural progression blending the various disciplines.

• Radio Astronomy, by John D. Kraus.
• The Alien Communication Handbook, by Brian S. McConnell.
• CosmicOS.
• Artificial Intelligence for Interstellar Travel.
• Artificial Intelligence Town Hall.
• OnAIR.
• REALM.
• Artificial Intelligence Will Let Humanity Talk to Alien Civilizations.

Cost effective Antimatter Factory and Engineering Systems (current antimatter production cost: $62.5 trillion / gram) and fuel supply base for the antimatter rocket.

• Antimatter tanks: Penning trap | Penning–Malmberg trap |
• Antimatter valves:
• Antimatter pipes:
• Antimatter pumps:

Alien: Prometheus - Cinematic Study. Look past the Sci-Fi, some of the scenes are simply outstanding. Some of the concepts that have been covered in this page ranging from advanced propulsion physics, interstellar travel, artificial gravity (inside Prometheus), Gravity Control Propulsion (Engineer's ship) and habitable exoplanets are applicable in this cinematic study and some more in the movie such as Artificial General Intelligence, the ability of a machine to interpret human thoughts, biotechnology, the origins of life, the Fermi paradox and how humans deal with long interstellar transit times, a few of these concepts are no longer Science Fiction. This author likes the scene where both ships are in the same field of view using two different propulsion systems. One could argue that the scientific and technological maturity of a civilization is directly correlated to the type of propulsion systems the civilization is able to construct for their spacecraft with the associated power systems (energy densities) required. Let's hope the aliens on the next planet are a bit more friendly and happy to share their ideas and thoughts on the Universe over a cup of tea.