The Orbital Refilling Dilemma; Starship's Most Important Bottleneck Besides its Design
Neal Lachman
???? | ???? | CEO & Chief of Spacecraft Design | TitansSpace.com | Investor | Selene Mission | Crewed Mars Mission:2032 | Nuclear Propulsion | Normalizing Space Tourism | Spaceplanes | Space Stations
The Tyranny of the Rocket Equation, as explained in our critical analysis of Starship , renders rockets the most inefficient mode of transportation. Astronaut Don Pettit compares rocket mass fractions to those of other vehicles. He lists the?approximate numbers for propellant ?(or fuel when air is used as the oxidizer) to illustrate the general categories of mass fractions:
In other words, on average, a rocket, including Starship, will consist of up to 85% propellant, and only 15% structure and payload. It's also understood that about the same percentage of that propellant will be used to escape Earth's thick atmosphere. That is why, in order to start their journey to the Moon or Mars, Starships will need to have their tanks refilled in space.?
On the?Mars & Beyond section ?of the SpaceX website, they write about on-orbit refilling:
Starship leverages tanker vehicles (essentially the Starship spacecraft minus the windows) to refill the Starship spacecraft in low-Earth orbit prior to departing for Mars. Refilling on-orbit enables the transport of up to 100 tons all the way to Mars. And if the tanker ship has high reuse capability, the primary cost is that of the propellant, and the cost of oxygen and methane is extremely low.
This vision is not grounded in reality; orbital refilling is most likely never going to happen at the scale and frequency SpaceX envisions and requires for Starships.
Each future Starship taking off from Earth's surface (whether for a trip to the Moon or to Mars) will consist of two stages, the first stage is the so-called reusable Super Heavy booster at the bottom half, and the Starship itself, on top, is stage two. According to the latest iteration as per SpaceX , Starship stands at 120 meters (394 ft) tall, almost equally split between the two stages.??
SpaceX is developing two versions of Starship. One for humans and cargo transport, the second for propellant cargo.
As Pettit explains in the above-cited essay, most of a rocket's fuel will be spent in the effort to leave Earth. As such, since most of the on-board propellant of the Super Heavy booster will be expended in the effort to escape Earth's thick atmosphere, each Starship's tanks will need to be filled up with propellant (liquid methane [CH4] as fuel and liquid oxygen [LOX] as oxidizer) before starting its journey. This process is known as, orbital refilling/refueling or on-orbit refilling.
We believe small or even medium-scale orbital refueling for chemical rockets and spacecraft (such as satellites), as Orbit Fab intends or at the Titans?OrbitalPort ?Space Station or with the Titans Spaceplane , will be possible later this decade. In fact, it were the Russians who invented a refueling mechanism for their MIR station. The ISS gets refueled every few weeks. However, compared to the small amount of fuel required by the ISS, anything for a huge rocket like the Starship would be a Herculean task. Refilling hundreds of tons of fuel per Starship may be one of the rocket's most important bottlenecks at the start of its journey, and they add unknown (amounts of) points of failure for Elon Musk's deep space projects.?
Many arm-chair space experts would deem people like us Space Luddites as if we are opponents of development in space. On the contrary, we believe very much in the colonization, commercialization, and industrialization of Space, especially Earth's Moon, but we have to keep things realistic. It's of no use to try inventing Star Trek or Star Wars technology – we aren't living in movies. We can't travel at light-speed and we can't travel to Mars in a week, or visit the Sun. Heck, we can't even launch rockets without scrubbing launches, and even outright failing (exploding) now and then.??
Orbital refilling on a large scale will most likely remain another major fantasy. That doesn't stop people from fantasizing. The delusion with orbital refilling is so big nowadays that some?are advocating for smaller rockets, which would rendezvous with an orbital propellant depot, fill up the tanks, and then fly off/cruise to deep space. The premise of the proposal, at first, seems to make sense, were it not for the immense dangers of rockets exploding above our heads every now and then during a risky multi-hour refilling event. As explained in our Starship analysis , we have to move away from or at least drastically reduce utilizing rocketships.?
SpaceX got some money from NASA to figure out how to make on-orbit refueling happen. Nevertheless, in the real world, Starships are not shrinking in size on the design table. Starship is indeed the largest rocket ever designed, and as you may gather, it's a curse rather than a blessing. To make matters worse, Musk has recently declared that the Starship may end up being 150 meters tall - that means even more propellant.?
The Starship (or any future deep-space transport vehicle for that matter) needs fuel for:
Small-scale orbital refueling aside, nobody has proven refueling on a larger scale, and as of yet, we don't have the technology to store and transfer fuel from one rocket/vehicle/depot to another in LEO.
To reiterate, it's the huge scale, gigantic quantities, and frequency, not the concept of orbital refueling itself that is going to cause delays, disasters, or deaths. Remember, all this on-orbit activity will play out in Earth's Low-Earth Orbit, most likely between 100 and 200 kilometers altitude. Accidents that may result in explosions will accelerate the Kessler Syndrome , which is disastrous for LEO activity where every piece of debris poses a danger unless it burns up during reentry - if ever.?
The fastest cars in the world are?expected to achieve ?around 498 km/h (~310 mph). That's a fraction of the orbital speed something up there is traveling.?Orbital velocity in Low-Earth Orbit is between 6.9–7.8 km/s (24,840–28,080 km/h or 14,430–17,450 mph). This orbital velocity is not only incredibly fast, it's also dangerous for maneuvering.?
This would have been a significant dilemma for pairing Starships aft-to-aft (canceled now), and is still an orbital mechanics/astrodynamics hurdle for Starship's side-to-side refueling. Due to the incredible velocity, and due to this all being in the near-vacuum of space, it's nothing compared to aerial/in-flight refilling where aircraft can use the laws of aerodynamics to their benefit.??
In space, when two objects (e.g. Starships) are positioned side to side, it will look like there's no movement – they will appear as if they're stationary. Pairing/coupling these objects should be relatively straightforward with the help of thrusters. However, the process of transferring tons of propellant from a tanker to a Starship introduces many dangers with many additional potential points of failure. To reiterate, orbital refilling requires precision maneuvering using independent RCS systems that aren't part of Starship's current design.?
Since orbital refilling would occur at near-zero gravity, liquid fuel in the Starship tanker will float like everything operating in micro-gravity; it won't be simply draining into the receiving Starship. There will have to be some kind of (compressive) mechanism to move the fuel downward – a pump may not be fully efficient in this scenario.
Keeping two objects paired/coupled in orbit, pumping/transferring fuel, and all the necessary processes in between, require mechanisms that will add weight and numerous other complications (again, additional potential points of failure) to the Starships, and it may or may not work. Mind you, every single Starship will need to be paired 4, 5, 6, and probably many more times, to have enough propellant for its three-day journey to the Moon or eight to nine-month journey to Mars. Multiply this horror scenario unfolding above our heads 100, or even 1,000 times, as Musk envisions an armada ?of Starships taking off for Mars, Battlestar Galactica style, and you may realize why no sane government will allow this to happen over their land; no responsible regulator will issue permits for this dangerous spectacle.?
Last but not least, let's suppose, in the unlikely event that orbital refilling issues will be resolved, some irresponsible bunch of politicians arrange permits, and numerous Starships get on their way to Mars as part of a Battlestar Galactica-style armada. A whole new level of practical problems arrives: the fuel needs to be in a liquefied state during the entire (eight to) nine-month journey to Mars. It's a great mystery how this cryogenic cooling issue will be resolved in Starship as there have been no cooling mechanisms —including something as “relatively simple” as a sunshade— presented as a viable solution. Even then, who guarantees that their eventual cooling system will work??
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We were considering analyzing in-space (deep space) refilling options beyond orbital refilling, but that potential solution is so far out, that it would be a waste of time to even consider. Unless on-orbit refilling is proven as a safe, efficient, reliable option,?deep space refilling ?will be a pipe dream.
In our next article, we will explain why our solution for an end-to-end space transport system is much safer, cheaper, and many times more efficient than any other proposal.?
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