Are air breathing orbital vehicles superior to rockets?
Not for all destinations.
If you want to develop a reusable vehicle that can go to space there are multiple solutions. It can be done with rockets -and we will explain why we continue on that path- or you could invest in a futuristic new generation of turbine engine rocket engine hybrids that is being developed in various parts of the world.
Time for some Sabre rattling between these two options!
A breakthrough hypersonic engine
For more than 20 years now, work on air-breathing engines that can fly at Mach 5 to 7 and do not melt in the process has been steadily progressing. The Russian, Chinese, Indian and NATO militaries are quite close to fielding single use military system variants. However, due to the high temperatures involved, turning these systems into vehicles that can be used again and again has been an even bigger and elusive challenge. The dream is that in terrestrial applications, winged vehicles equipped with such engines could eventually enable commercial flights from Brussels to e.g. Sydney in about 4 hours.
A small UK company, Reaction Engines Limited or REL, has now created a breakthrough technology that successfully tackles this challenge by cooling the superhot ingested air into something manageable that doesn’t damage the engine and would permit reliable sustained flight, just like is the case in familiar commercial airplanes.
Since a favourable 2010 review by ESA of this crucial air precooler technology that is part of the SABRE engine (SABRE stands for Supersonic Air Breathing Rocket Engine) being developed by REL, the technology no longer has trouble finding the investment it needs for their LAPCAT program aiming to develop just such a terrestrial application. After years of struggling (REL was incorporated in 1989), they now have a solid financial funding basis to produce an engine demonstrator. BAE has become the working partner and strategic investor, while the ESA, UK Space Agency (and a host of other partners) provide extra funding. This year, on May 4 2017, REL broke ground for the construction of a new engine test facility where it plans to undertake the first ground based demonstration of its revolutionary SABRE™ air-breathing rocket engine. The test facility at Westcott, Buckinghamshire, UK will enable Reaction Engines to test critical subsystems along with the testing of a SABRE engine core, which will commence in 2020.
Military partners of the UK and Europe eye the technology as a candidate to power next generation reconnaissance or deep penetration vehicles that can snoop around enemy terrain while, thanks to their high speeds and the long endurance, being impervious to countermeasures.
This endurance is made possible because the SABRE engine is able to create its own oxidizer, which means that -contrary to rockets- part of their propellant is distilled from the air that the engines ingest. As a result they still would have to carry the same amount of fuel but almost no oxidizer on most missions (a Propellant consists of a fuel and an oxidizer), this results in a far lighter and smaller vehicle than would otherwise be the case.
The same engine offers the potential to create a two-stage-to-orbit vehicle that basically consists of a first stage airplane equipped with the air-breathing SABRE engine, that upon reaching mach 7 at high altitude, switches over to rocket mode and, after reaching its highest altitude, detaches a second vehicle that can be either another SABRE equipped upper stage or a small rocket kick stage that lifts the satellite to its final orbit.
Large versions of the vehicle, dubbed Skylon and with a length of some 84 meters, can even make it to orbit as a true SSTO and attach itself to the ISS or any other orbiting structure to drop of crew and payload. In fact, it was this promising application that its inventor, Alan Bond, was trying to convince ESA and the world of since he proposed the concept in the 1980’ies.
Not the right technology for single vehicle deep space travel
Because of the myriad of advantages and potential applications, this project certainly deserves all the funding it can get to be developed into flying vehicles. But, for our ends -interplanetary point-to-point services- it is not the right technology. We are convinced an older technology is holds more promise to perform that job.
While the SABRE technology certainly is impressive, if REL gets it into production, it will only be functional in the immediate vicinity of Earth (LEO). That is because what is being touted as its biggest advantage – being able to land and take off on (very long) runways – is also its biggest disadvantage for in space applications: it has heavy wings and needs a runway.
This means that it has no use for in-space transportation of goods, which require a separately developed vehicle. While good for runway-to-runway transportation around the globe, and drop offs in LEO, it is not the universal vehicle that a Single Stage to Orbit could be.
The fact that Skylon produces its own cryogenic oxidizer by ingesting it in the engine and cooling it until it liquefies with its high-tech Precooler system (from bleeding hot at 1000°C to -150°C) also is of no use in space or on other planetary bodies, as only our Earth’s atmosphere has the right composition to pull off this trick.
Source: Reaction Engines Limited
And what with the supposed weight advantage? Doesn’t the fact that Skylon only needs to loft part of its fuel offer tremendous cost advantages? Well. No. Skylon all together is a much more complex vehicle and the engines are heavier than rocket engines. Skylon will not offer the reliability required of an interplanetary SSTO or for a variety of in space and deep space applications. Currently the projected weight advantage of using less fuel but a very complex engine do not outweigh the benefits in cost and reliability of the simpler and more universally usable design of a rocket SSTO.
While saving fuel is important in the sort of very competitive environment of international air travel, this efficiency argument becomes moot when trying to design an interplanetary SSTO vehicle that simply needs to be as simple and reliable as possible. Such an SSTO simply needs technology that can be operated virtually autonomously for many years in a dynamic space environment. SABRE at this point can neither demonstrate multi-year reliability nor is it the right technology for the task. The cost of the fuels used and produced therefore is less important than the demonstration of the reliability of the SSTO vehicle as the first one of its kind. Future generations of course would become more efficient.
An SSTO that relies on rocket technology can offer point-to-point access to any spot on Earth and any spot within the solar system. The fact that it unlocks access to every imaginable spot, provided we refuel it with a second vehicle for certain destinations, shows the true potential of developing this one SSTO vehicle. Furthermore, it would only require one R&D cycle as the versatility of the vehicle eliminates the need to develop a fleet of separate and specialized transportation vehicles that would have to work in tandem and do a complicated dance to get you to a certain destination.
While ambitious, developing an SSTO has become a very reasonable challenge that merely extends current capabilities. This is because with the likes of SpaceX, Blue Origin, and past developments like the Delta Clipper and even the NASA’s in space satellite refueling technology demonstrator programs that were flight proven and demonstrated on the ISS, we now have multiple clear examples of intermediate vehicles and technologies that help to build the technological case for a Single Stage To Orbit in space refuelable Demonstrator.
If asked which project to invest in, we at One Stage To Space only see one option: Invest in our rocket technology as it is the only one that will offer you truck like universal access to any spot in the solar system, including any spot of land on Earth, while being fully reusable over a span of many years. It outperforms all competition in its versatility.
The degree of flexibility offered by this transportation solution would make it a technology that has no problems in finding both commercial and government customers in sectors as diverse as satellite delivery, space tourism, rapid transportation of high value goods, servicing (ant-)arctic bases on Earth -and on the Moon- and deep space science and colonisation support services.