Commercial Rockets

SKYLON — Britain, no estimate

A company called Reaction Engines Ltd is hoping to build a true orbital spaceplane — a vehicle which can go from runway to orbit to runway without any stage separations. As yet there is not even any funding to start building the plane, but they are making good progress on the propulsion system which would make this possible, which they call SABRE, for Synergistic Air-Breathing Rocket Engine. (Yeah, the word “synergistic” in there is kind of gratuitous.) This motor is based on a trick whereby they scoop up air like a ramjet, but then they subject it to very rapid cooling, which greatly increases its density. They then turbopump it into a rocket combustion chamber, along with liquid hydrogen which has been warmed by the incoming air (but not directly — there’s a complicated intermediary loop filled with helium which also gets involved with the preburner and turbopumps). Some excess hydrogen which was used for the cooling gets burnt separately in a set of ramjet-like nozzles arranged in a ring around the four nozzles of the rocket engine; this happens at medium speeds and altitudes where the amount of fuel needed for cooling exceeds the amount burned by the rocket. The result is a rocket which, for its initial boost phase, needs no onboard oxygen and gets much of its lift from wings, thereby saving tons of propellant. Then, once they get up near 2 km/s speed (to be more precise, mach 5) and 26 km altitude, they close the intakes and start using lox, making it a conventional rocket from then on.

The middle of this single-stage craft would have a cargo bay, in which they could put a passenger capsule for space station taxi service. They say they might pack up to thirty people in there, if there’s anywhere for that many to go. It would not have any windows or any pilot’s seat.

The SABRE engine is apparently fairly far along in development, at least in incomplete small-scale form. They’re testing the high speed phase in the blast of jet engine exhaust, and those tests reached mach 5 successfully in 2019. Unlike a ramjet, the SABRE can produce quite strong thrust at high speeds, and it can also operate from a standing start. It might be the one engine which works in all phases of airplane and rocket flight. If Skylon succeeds, it could make giant boosters obsolete, except for the biggest payloads.

But we’re not going to reach orbit with something all that much smaller than a conventional rocket; it’ll still need fairly enormous tanks of liquid hydrogen, and they’re thinking that despite the savings, the finished plane would be many times the size of a Space Shuttle, with a length exceeding eighty meters — nearly as big as a New Glenn, though much lighter. But at reentry time, this large size will be an advantage: because it weighs little in comparison to its size, especially with empty tanks, it can lose a lot of speed in the upper atmosphere without subjecting its ceramic skin to much heat. Their proposed design has the fuselage tapered to a point at both ends, with two stubby wings each bearing a SABRE nacelle, and some control fins at the tips. They’ve also got a conventional rocket nozzle of a much smaller size at the back end, for low-thrust orbital maneuvering. Most of the weight would be in the middle: the wings, engines, and cargo bay are right at the center, with the lox tanks abutting the bay fore and aft. The majority of the plane’s length consists of nothing but hydrogen tanks.

We don’t know yet if the Skylon plane will ever be built, but the SABRE engine has an interested customer in the US Air Force, so it’ll probably get used in some way. Meanwhile, a group in China is working on an engine that sounds similar.

The company claims they have solved the most obvious problem with the design, which is the accumulation of ice on the air cooler, but are not yet revealing the solution to the public. Personally I have grave doubts as to whether this issue is really solvable.

Though the SABRE engine might make a single-stage spaceplane possible, that doesn’t mean that the single-stage approach is the correct one to use. A more sensible approach would be to make a suborbital spaceplane powered by SABREs, and then drop a conventional rocket stage from it. The plane might be able to release a second stage with both an altitude and a speed that could exceed that provided by a conventional first stage booster, and by doing so, put up a substantially larger payload than the single-stage version of the plane would be able to lift. That upper stage might be made recoverable by giving it a heat shield, and if not, it might be quite small, like not much heavier than its payload. Lately, Reaction Engines Ltd has been mumbling that yeah, they’ll approach two stage designs first, before trying to build anything like the Skylon. However, the Skylon is the approach for which we have a clear design and specs, so that’s what we’re documenting here.

One question which interests me is what the SABRE’s specific impulse would be in the atmosphere in different parts of the flight. On the one hand, like an airplane engine it ought to have an impulse many times the exhaust velocity, since it only takes a small mass of hydrogen to propel a large mass of air out of the back, but on the other hand, as it speeds up, the refrigerated air scoop may rob it of more than half of its thrust. I would bet that the cutoff for when to close the intakes is determined not by how thin the air is, but by how close they are to the point where the exhaust velocity can no longer exceed the intake speed. It’s in these conditions that a scramjet has the advantage, on paper, because it doesn’t need to slow down the incoming air before burning it. Some of Reaction Engines’ figures suggest that the specific impulse in air-breathing mode might range from 40 to 90 km/s; I would guess that the higher number (if achievable) applies at minimum speed and altitude. That big number means that not only does it benefit from saving the weight of lox, it also benefits from consuming hydrogen at a lower rate for the same thrust, because the reaction mass is multiplied due to all the nitrogen and other gases which get pushed through the rocket along with the hydrogen and oxygen. Hopefully the engines run rich, like most hydrogen rockets, or they might end up producing nitrogen oxide smog. (And hopefully by then we will be seeing a decrease in higly toxic rocket fuels, especially solid fuel.)

Skylon: mass 325 t?, diam 6.3 m?, thrust 5800 kN?, imp 4.5 km/s?, type Gh (sort of), payload 17 t? (5.2%?), cost unknown.