Rocket or Scramjet: Engine Powering Hypersonia

Hypersonia is a vision of a world powered by high speed flights. Thus, the propulsion system of such civilian hypersonic planes must be powerful enough. The race to propel such planes is between using supersonic ramjet (scramjet) or ‘throttleable’ rocket engines. Regardless, of which eventually emerges as the preferred choice, what matters is the success, steadiness and safety of the emergent. And how companies and countries act.

An engine is a vital component which drives and powers a vehicle. Therefore, it must be the one which works reliably and seamlessly. This requires making the right choice about airframe-engine configuration & integration and energy source. These choices determine usability, range, durability, and maintenance-ability of the aircraft.

The focus is not detailed technicalities of the binary types of engines. Rather, it is my perspectives of shaping the choice. To start with, Boeing’s recent decision to cancel its spaceplane programme with DARPA, is a pointer to the direction which this will go.

Several reasons have been offered in the media for this chop. To mention, none of these condemns either Boeing or DARPA for any misgiving. I however think the reason is more strategic. With a sizeable number of companies and start-ups developing innovative and reusable rocket engines for lofting satellites and passengers into space, it is not common-sensical to keep investing into a spaceplane that will not offer similar services as regarding cost and performance.

For Boeing, such strategic thinking might have made the choice of a scramjet a better option. I think Boeing is looking beyond the present, and focussing on the future. A forward-thinking about its space-civilian powertrain fusion. An engine platform capable of launching satellites and lobbing passengers into low orbits. And at the same time, suitable for ferrying people and cargo in endo-atmospheric flights.

The recent launch of the twin-engine, yet powerful and capacious 777x by Boeing, further strengthens this hunch. Such plane (the Airbus A350 as well) is an indication of the aviation and aerospace industries’ trust in the rising expansion and growth of the civilian flight market. The choice of an engine is however still not straightforward when it comes to hypersonic flight.

Rock or Scram?

The mechanics of both rockets and scramjets are somehow similar. They are both internal combustion engines. Rockets burn fuels at high temperature and eject hot air under high pressure at the back to provide thrust. Likewise, a scramjet is a simple device that at over Mach 3, injects hot air at high pressure and temperature and force it out at the back for high-speed propulsion.

The subtle difference between both is that a rocket burns fuels and often carries its own oxygen for combustion. An addition which increases the weight of the craft and potentially makes it ignitable. A scramjet, however, uses atmospheric oxygen at the performing Mach level; hence less fuel is required to be carried, aside from those required to power the integrated turbojet engines necessary for building up speed and perhaps for landing. Nonetheless, there are workarounds to these mechanical deficits.

The Synergetic Air Breathing Rocket Engine (SABRE) made possible by an innovative hyper-fast air-cooling device puts rockets in the running. Developed by British Reaction Engines, this unique innovation makes a rocket engine to reach low hypersonic speeds without the need of an internal oxidiser or oxygen. The device in nanoseconds, rapidly cools down the hot air that a scramjet would ingest so that a controlled explosion takes place inside the rocket engine. Such reduces the on-board weight of the craft as environmental oxygen is used for internal combustion.

There are other rocket engines on the market catering for space launches. Most of these are either using greener fossil fuel derivatives and chemicals as propellants, but require carrying on-board oxidiser for combustion. These are potential candidates for powering endo-atmospheric flight. The customers and public however need to be convinced.

I love pyrotechnics. As a curious child I used to make small explosives with raw carbide used in Dane guns and from match-stick heads. I carry around a scar on my torso as a prove of these experimental adventures. It happened that there was an uncontrolled explosion and a projectile hit my body. Thank God it wasn’t my face or head that was hit. That is by the way.

The anecdote is not scare mongering, but to share about my love for tinkering and building stuffs. By the way, rockets by today’s innovative companies are very controlled explosives. I would ride on top of one if given the chance.

The images of vertical rockets powering into space awe people. Am impressed as well. I think it is in so far, they are carrying equipment, robots and trained astronauts, but not the cheering crowd. Not impossible, but it would take a lot of convincing for people familiar with planes taking off vertically to ride on those rockets as they are now. The virtual prototype of Reaction Engines’ SABRE-powered LAPCAT A2 hypersonic plane, nonetheless, gives an image of a craft that can take-off and lands horizontally on a runway, though.

Moreover, companies and start-ups with mothership-launched plane-like rockets, which can land horizontally can serve a niche civilian hypersonic market. The small size of the crafts and adventurism posture of the offerings will only attract a certain customer base. Even though, the motherships themselves are like giant cargo aeroplanes, which can be used for subsonic civilian flights. A thoughtful consideration in the light of balloons and dirigibles as suitable but slower motherships for launching rockets.

These crafts have a seamless integrated engine-airframe integration and configuration. The virtual prototypes of LAPCAT A2 and Boeing hypersonic plane show this design. So also, are the Virgin Space and Space X rocket-powered crafts. This is a needed advancement, as a mere slap-on of a scramjet engine unto existing airframes will not do. Considering that the problem being faced by Boeing 737 Max stems from the ungainly integration of its new engines to the existing airframe. An avoidable situation had the plane been built ground-up with its engines.

The case for scramjet-powered hypersonic flight is stronger when seen as a radical technology, but an incremental leap on the existing vertical take-off aeroplanes. Jet engines have over 90 years of integration and configuration with airframes. Even though the aerodynamics of hypersonic planes are different from both subsonic and supersonic ones. A complexity that will even make the building of a safe and fully certified hypersonic plane more challenging, but not impossible.

A bit of physics here. A supersonic ramjet engine operates by using fast rushing and compressing air to burn little fuel in the combustion chamber to generate thrust. Unlike, its nearest cousin – turbojet, it has no moving parts (a good candidate for 3D printing) and only functions at a speed equal to or over Mach 3. Its advantage over its sibling – ramjet is that it maintains the flow of air at supersonic speed, hence maintaining its efficiency and performance to reach a potential Mach 15 speed as opposed to the maximum Mach 6 for a ramjet.

A ramjet with its speed performance can power a low speed hypersonic flight. A scramjet is however preferred for its superior performance under unpredictable stratospheric conditions – the near space ambience where hypersonic flight travels. Both however will attain supersonic speed (Mach 1-3) before going hypersonic. As mentioned earlier, the aerodynamics of both high-speed levels are different.

Sonic boom – which generates loud noise and disruptive sound waves, even though are being tackled by NASA and start-ups, occurs at supersonic speed. This doesn’t however occur at hypersonic speed because the sound waves generated concentrate at the tip of the craft as opposed to the advancing front of a supersonic craft. So, the curiosity is that can hypersonic speed be achieved from take-off? Is it possible to generate enough air turbulence on the ground to achieve a rapid but smooth minimum Mach 3 speed?

Such ‘add-on’ innovation if possible, will involve an unprecedented engine design and airframe integration and configuration. My thinking is that there is a possibility of incorporating a battery- or fuel-cell- powered electric motor into the engine as a vacuum to suck in air at high speed into the inlet of the scramjet engine. The image of a giant Dyson vacuum cleaner comes into mind.

The power density of such a system might be a mitigating factor. Hence, a compact fossil- or hydrogen-fuelled internal combustion engine acting as a pump can be considered as an option. Stretching my imagination further, a very compact green energy powered rocket that has its exhaust turned towards the inlet of the scramjet can be explored.

These are possibilities. All these additional devices will affect the mass of the engine and the aircraft. This is however not far-fetched. The extra weight of a turbojet add-on to the combined cycle scramjet engine being considered for powering the proposed Lockheed Martin built Mach 6, SR 72 reconnaissance-bomber-drone variants, is not considered a hindrance. The turbojet powers the plane to Mach 3 before the scramjet kicks in.

The increase in weight must, nonetheless, be factored into the design process of an add-on scramjet engine. The engineering of such designs and configurations are mind-boggling. Nonetheless, advances in 3D printing, computational fluid dynamics, 5G factory and communication, AI, data analytics and quantum computing can make these happen. Already companies and start-ups working in high speed flight are using these technologies to produce never seen before contours and shapes.

The decision to use a rocket or scramjet depends on physical and chemical factors such as mass (weight), fuel density, payload mass, thermodynamics and aerodynamics. And on economic, financial, market, organisational, political and regulatory choices and preferences.

 Conclusion

The aerospace and aviation industries need a change. Point-to-point long distance travels are dominating their attention, as exemplified by planes such as Boeing 787 and 777x and Airbus A350. To wit, hypersonic flight will further contribute to industrial expansion and enriching customer experience as more people travel more on longer routes and to farther airports in extant and emerging cities.

A cost-effective, practical and stable hypersonic engine platform is an architectural innovation. Any nation that masters it will excel in the emerging DigiMech economy. Britain has a deep expertise in both aerospace and aviation industries. And there is an emerging industry in hypersonic engine (SABRE) technology that will make the nation to excel. This will spur imitators and innovators alike as both public and private policies and investments are directed toward this novel sphere.

SABRE is a product of a long entrepreneurial and innovation process. Its DNA is linked to HOTOL concept, a horizontal take-off and landing spaceplane conceived by British industry and government three decades ago. Spurned by potential European partners but embraced by the Americans, it was discontinued for lack of broad support. It was due to Alan Bond, the co-founder of Reaction Engines, who worked on HOTOL that the cross-pollination with SABRE and LAPCAT A2 happened.

Britain revolutionised aero flight when Frank Whittle OM invented the first practical turbojet engine over 90 years ago. This is another moment for the nation to take a lead again with a radical innovation. Its entrepreneurs, innovators and policymakers fostering workable international partnerships can make this happen.