There are some technologies that I have been reading about essentially my entire life. It’s interesting to now read breathless articles about a new exciting technology, and realize that I read similar articles in the 1980s.

Of course, many technologies have materialized in the last 30 years, but some seem frustratingly difficult, and it’s hard to even know if we are really any closer now than when I was 20.

One of the frustrating technologies is the single stage to orbit spaceplane. A recent article from the BBC announces a new initiative to develop a space engine, and everyone seems very enthusiastic about what will result (as I was in the 1980s when I first read about such engines).

For a little background, the term “spaceplane” can refer to several types of vehicles. Essentially the term is used for any vehicle that flies like a plane in the atmosphere but can also go into space. Atmospheric flight, however, might only include the return to Earth phase. So the Space Shuttle is a spaceplane. It takes off vertically using rockets, but when it comes back to the ground it uses its wings for lift and lands like a plane.

So far the Space Shuttle was the only manned spaceplane. There was also the Soviet Buran, and SpaceshipOne, both of which were unmanned. Two other spaceplanes were intended to make it to orbit and took off horizontally, ascending part of the way like a jet – the X-15 and the X-37. Neither of these went fully into orbit, however.

Therefore – we have yet to develop a spaceship that has all the features of a real spaceplane: it can take off like a plane, fly to high altitude then propel itself into orbit, return from orbit and also land like a plane, and carry people. This would be a single-stage-to-orbit craft, without any disposable parts. Theoretically it would just need to be refueled, and then it can take off again.

This kind of craft could potentially significantly reduce the cost of getting to orbit. SpaceX has approached this problem by developing rockets that can land vertically, making that major component reusable, and reducing the cost to get stuff into orbit. But for the Falcon rockets only the first stage is reusable, not the second stage rocket.

Sabre Engine

The news item is this:

Reaction Engines Limited (REL), the UK company developing a revolutionary aerospace engine, has announced investments from both Boeing and Rolls-Royce.

The hope is that with these major companies, including a leading aerospace company, investing in the development of this engine, the company will be able to see it across the finish line. But, the technological hurdles remain significant.

There are several concepts for spaceplane engines, and the Sabre engine is only one. The overall idea is to have an engine that works similar to a jet engine in the atmosphere, but can convert over to a rocket once above the atmosphere. The biggest potential advantage of this design is that oxidizer (such as liquid oxygen) is the single heaviest component of a conventional rocket. A spaceplane engine, however, could burn oxygen from the atmosphere, and so would not have to carry that oxygen with it.

This could have huge weight savings. Remember the rocket equation – you have to carry fuel to lift the fuel you need to lift the fuel, etc. Reducing the weight of fuel could therefore significantly reduce the overall cost to orbit.

A single-stage-to-orbit design would also save money because there are no disposable stages. Further, this could potentially reduce the accumulation of space junk in orbit.

Now for the big hurdles, the things that have kept this technology from being realized for the last 30 years, and continues to be a challenge. Current spaceplane engine designs are significantly heavier than conventional jet engines or rocket engines, even if you add the weight of the empty oxygen tanks. The weight of fuel is shed as the fuel is burned and expelled as propellant. The extra weight of a heavier engine, however, stays with the craft all the way into orbit. So far the heavy weight of such engines more than offsets the savings in fuel weight. So any future design would have to put a premium on reducing that weight, otherwise most of the potential advantage is gone.

Perhaps even more of a technical challenge is that the aerodynamics of horizontal take off to orbit are brutal. Rockets fly vertically, and get out of the atmosphere by a relatively short line. Horizontal take-off craft have to fly very fast through the atmosphere for a much longer time. This causes temperatures and stresses that are significant. The craft also has to remain stable, through supersonic speeds while it’s weight might be shifting due to burning fuel.

Even more challenging is how to handle the air at that speed. At supersonic speeds the air being taken in to provide the oxygen for the engines will be heated to over 1000C. The biggest challenge has been to design an engine that can cool that air down to sub zero temperatures in a fraction of a second. Some designs have a precooling component, but again that adds weight.

So while the idea of simply getting into your spaceship, taking off from the ground and then flying into orbit, or even beyond, is extremely appealing, the technical challenges are massive. So far this technology has been like the jetpack or flying car, a romantic idea but not practical or economically viable.

I am excited to read about this new investment in the development of one type of spaceplane engine, and I hope that they can make it over the finish line, but my excitement is tempered by decades of disappointment. It is also reduced by the knowledge of how difficult the technological challenges are.

The key thing to remember is – not only do they have to develop an engine that can get the job done, it has to be cost effective or otherwise the primary point is lost. It does seem, however, that this research is worth the investment. Like fusion reactor research, the potential payoff is huge, even if the challenges are equally huge.

It is interesting to think back to the 1980s, to what technologies I was anticipating. I think I would be surprised that 30 years later we still don’t have a fully functional spaceplane. I think I would be more disappointed than surprised if 30 years from now we still don’t.