British Skylon Engine Passes Its Tests 172
An anonymous reader writes "The BBC reports that the SABRE hybrid (part air-breathing jet, part rocket) that is intended to power the Skylon single-stage-to-orbit space plane has passed its final technical demonstration test, and is now looking for money (only £250m!) to prepare for manufacturing. If this goes ahead, travel into orbit from local airports (ideally, those close to the equator) will be possible. And quite cheaply. But might it have the same legal difficulties flying from U.S. airports as the Concorde did?"
Re:That name (Score:5, Informative)
Good show, old chaps, but change the name. Sooner or later, a Skylon will turn on you.
No worries. The UK Ministry of Defense communication satellites are already called "SkyNet".
Misleading Title (Score:5, Informative)
The engine doesn't exist yet. This was a test of the pre-cooler. It is a critical component and it was important.
Re:Dumb Question (Score:5, Informative)
Why does the input air need to be chilled? Does this have something to do with using hydrogen in a turbine engine?
Covered here [wikipedia.org]. It's actually an interesting read. Put succinctly, as speed increases, the temperature of the air increases, reducing efficiency.
Re:Screw US Airports (Score:4, Informative)
The US "legal troubles" were a stalling tactic* back when Boeing was trying to build their own SST. The original grass roots "ban the bang" campaign was British.
*Do you really think Congress wouldn't have lifted the landing ban had the US version made it off the drafting board?
Re:One problem (Score:4, Informative)
Re:The odd thing about the Skylon (Score:3, Informative)
Re:One problem (Score:4, Informative)
Because of the low ballistic coefficient, Skylon would be slowed at higher altitudes where the air is thinner. As a result, the skin of the vehicle would only reach 1100 Kelvin (K). In contrast, the smaller Space Shuttle is heated to 2000 K on its leading edge, and so employs an extremely heat-resistant but extremely fragile silica thermal protection system. The Skylon design need not use such a system, instead opting for using a far thinner yet durable reinforced ceramic skin
Re:Screw US Airports (Score:2, Informative)
New Mexico thank you, in the unfortunately named Jornada del Muerto basin.
http://en.wikipedia.org/wiki/Spaceport_America (wikipedia)
Re:Misleading Title (Score:4, Informative)
Let's see. Air at atmospheric pressure has roughly 1kJ/(kg*K) heat capacity. It doesn't matter that they ram-compress thinner air, what matters is that after the ram the air will have roughly atmospheric pressure. We can assume that just to get a ballpark figure. There's about 23% by weight of oxygen in the air. When you burn hydrogen in oxygen, you join 2 mass units of hydrogen to 16 mass units of oxygen. You end up using only 2.9% of hydrogen by weight compared to weight of air, if you want a stoichiometric burn. They supposedly [wikipedia.org] cool the air down by 1140K.
So for each kg of air, you have to remove 1.1MJ of heat, and you've only got 29 grams of hydrogen to boil off. Vaporization heat of hydrogen is 0.45kJ/mol, or 0.45kJ/1g. So the boiling hydrogen can sink about 13kJ of heat, about 1% of what you need to sink. That's a no-go. It will be a no go even if all they get after the ram is 2% of atmospheric pressure, so we can be pretty sure it's no go period.
We get 286kJ/mol for combustion of hydrogen with oxygen, so we have available about 8.3MJ of heat from burning enough hydrogen to use up oxygen from a kilogram of incoming air. That may work out. Feel free to look at the Sabre cycle [wikipedia.org] and fill in the blanks as to required flow rates and temperatures, even in an idealized fashion. It should give an idea of the project's feasibility. I'm sure real engineers have already done the legwork on all that. Just that it's not as simple as "lots of very cold liquid hydrogen".
The real thing is their proprietary and at the moment confidential frost control. They've got those long tubes, they could put acoustic waves into them, hmm.
Re:One problem (Score:4, Informative)
Actually, the precise military design requirement that borked it was that the shuttle craft had to be capable of re-entry and landing entirely over the US continental area. They didn't want to overfly Russia, China or Europe.
That meant that it needed to have a steep descent profile, which in turn meant a hot flight, which meant expensive and sophisticated heat protection which ended up not working very well.
The Skylon is entirely commercial, and will have a much more sensible re-entry profile. It will be able to slow gradually in the upper atmosphere and even, with it's unique engines, start up again and fly back into orbit should it wish....
Press release (Score:4, Informative)
Hypersonic engines are up against hard physics. The ram air heats so much in the inlet that it's hard for combustion to add much energy to make it go faster out the back.
The idea behind the SABRE engines is to cool the ram air before it is compressed. The heat exchanger to do this is what the press release is all about. With not much more than a ton of mass, it sucks 400 MW of heat out of the incoming air, dropping the temperature from 1500 C to -150 C in a few inches of heat exchanger that looks much like fabric because the tubes are so tiny.
The engine cycle also uses the temperature difference between the ram air and the LH2 to run the compressor. It takes close to 2/5th of the energy from burning hydrogen to liquefy it. The engines recover much of this by running a helium turbine on the temperature difference between the ram air and the liquid hydrogen flow to the engines. The turbine powers the compressor stage that raises the pressure of the -150 C air to rocket chamber pressure.
The design is extremely clever thermodynamics which also avoids most of the metallurgical problems of high temperature. Fabricating the air to helium heat exchanger was a very hard task. They have miles of tiny tubing, tens of thousands of brazed joints and they don't leak!
Using these engines and breathing air, the vehicle reaches 26 km and about a quarter of the velocity to orbit giving an equivalent exhaust velocity (back calculate from hydrogen consumption) of 9 km/s. That's twice as good as the space shuttle main engines. It is expected to go into orbit with 15 tons of payload out of 300 or 5% even though the rest of the acceleration is on internal oxygen that only gives 4.5 km/s exhaust velocity.
Leaving out the oxygen and using big propulsion lasers to heat hydrogen reaction mass, such a vehicle would get 25% of takeoff mass to LEO, reducing the already low cost by a factor of 5. That's enough to change the economics of power satellites from being too expensive to consider to a cost substantially less expensive than any fossil fuel.
But try explaining any of this in a press release.