UK government invests in Sabre air-breathing rocket engine
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Re: UK government invests in Sabre air-breathing rocket engine
#2This is a dubious investment IMO.
Rockets that have shrouds have been thrown about for a long time, but they have one fundamental weakness. The shroud only works while in the atmosphere. After that, it's dead weight (since no significant aerodynamic forces come into play further up).
It's not quite the same as the SR-71 Blackbird engine which is a turbojet engine that morphs mid-flight into a ramjet. That's quite a spectacular piece of engineering especially since this was before the advent of computer-aided modeling. But the SR-71 was still very much an air breathing engine. The outer engine casing was still an essential part of its function.
Don't get me wrong; I wish these guys success and I hope my assessment is incorrect, but I just don't see how added weight to win over gravity lower in the atmosphere would help in the upper atmosphere and in space.
Re: UK government invests in Sabre air-breathing rocket engine
#3A couple of months ago there was an excellent BBC4 documentary about Alan Bond and Reaction Engines called The Three Rocketeers[1], which I believe has been re-posted on the internet.
Re: UK government invests in Sabre air-breathing rocket engine
#4This is a dubious investment IMO. Rockets that have shrouds have been thrown about for a long time, but they have one fundamental weakness. The shroud only works while in the atmosphere. After that, it's dead weight (since no significant aerodynamic forces come into play further up). It's not quite the same as the SR-71 Blackbird engine which is a turbojet engine that morphs mid-flight into a ramjet. That's quite a s…
I don't know the maths involved, but the idea presumably is that they save more weight not carrying (as much) oxygen than they lose by adding the rest of the engine. By being able to draw oxygen from the air whilst in the atmosphere, they only need to store a smaller amount of oxygen for when they hit space. This is the main saving I believe.
Re: UK government invests in Sabre air-breathing rocket engine
#5How and where does the nitrogen boiler(?) get its energy too cool helium to -170C at a constant rate? Is it effective to chill what must be ginormous amounts of air to -150C? It's all nice that the heat exchanger works, but where does that heat go?
There's a large discrepancy between the inlet air temperature (1000C vs 20C) in the article.
Somehow my sense for violation of the laws of thermodynamics is tingling.
Re: UK government invests in Sabre air-breathing rocket engine
#6How and where does the nitrogen boiler(?) get its energy too cool helium to -170C at a constant rate? Is it effective to chill what must be ginormous amounts of air to -150C? It's all nice that the heat exchanger works, but where does that heat go? There's a large discrepancy between the inlet air temperature (1000C vs 20C) in the article. Somehow my sense for violation of the laws of thermodynamics is tingling.
In the tests it's presumably just a continuous flow of pre-cooled liquid nitrogen from a tank & the heated N2 is probably dumped into the environment.
The design for a flight-capable engine is supposed to dump the waste heat into the fuel I believe.
Re: UK government invests in Sabre air-breathing rocket engine
#7How and where does the nitrogen boiler(?) get its energy too cool helium to -170C at a constant rate? Is it effective to chill what must be ginormous amounts of air to -150C? It's all nice that the heat exchanger works, but where does that heat go? There's a large discrepancy between the inlet air temperature (1000C vs 20C) in the article. Somehow my sense for violation of the laws of thermodynamics is tingling.
They have details on their web site:
http://www.reactionengines.co.uk/sabre_howworks.html
My reading of their explanation is that in a full system they would "pre-burn" some of the hydrogen and oxygen also used to power the main rocket motor to power the cooling of the helium that then cools the intake air.
[NB I have no idea whether that would actually work or not - just trying to interpret what their own explanation is!]
Re: UK government invests in Sabre air-breathing rocket engine
#8This is a dubious investment IMO. Rockets that have shrouds have been thrown about for a long time, but they have one fundamental weakness. The shroud only works while in the atmosphere. After that, it's dead weight (since no significant aerodynamic forces come into play further up). It's not quite the same as the SR-71 Blackbird engine which is a turbojet engine that morphs mid-flight into a ramjet. That's quite a s…
There's also the drag effects to worry about which pure rockets don't have to deal with.
This engine could end up being useful in the long run, but the clear path to cheap access to space involves reusability, and a more complex motor works against that.
Re: UK government invests in Sabre air-breathing rocket engine
#9This is a dubious investment IMO. Rockets that have shrouds have been thrown about for a long time, but they have one fundamental weakness. The shroud only works while in the atmosphere. After that, it's dead weight (since no significant aerodynamic forces come into play further up). It's not quite the same as the SR-71 Blackbird engine which is a turbojet engine that morphs mid-flight into a ramjet. That's quite a s…
I don't know the maths involved, but the idea presumably is that they save more weight not carrying (as much) oxygen than they lose by adding the rest of the engine. By being able to draw oxygen from the air whilst in the atmosphere, they only need to store a smaller amount of oxygen for when they hit space. This is the main saving I believe.
Indeed. O2 is much heavier than H2 so if you can scavenge the O2 out of the atmosphere on the way up then the amount you need to carry for the final burn in space is much smaller & the weight savings are huge.
Consider that the Space Shuttle external fuel tank contained 650,000kg of O2 and only 100,000 kg of H2. Save 10% of your O2 needs and suddenly you've got 60,000 kg to play with. The Shuttle payload was only about 25,000 kg.
Re: UK government invests in Sabre air-breathing rocket engine
#10How and where does the nitrogen boiler(?) get its energy too cool helium to -170C at a constant rate? Is it effective to chill what must be ginormous amounts of air to -150C? It's all nice that the heat exchanger works, but where does that heat go? There's a large discrepancy between the inlet air temperature (1000C vs 20C) in the article. Somehow my sense for violation of the laws of thermodynamics is tingling.
They have details on their web site: http://www.reactionengines.co.uk/sabre_howworks.html My reading of their explanation is that in a full system they would "pre-burn" some of the hydrogen and oxygen also used to power the main rocket motor to power the cooling of the helium that then cools the intake air. [NB I have no idea whether that would actually work or not - just trying to interpret what their own explanatio…
Thanks for the link, this clears up a lot:
http://www.reactionengines.co.uk/images/sabre/sabre_cycle_10...
There is no nitrogen in the system (not as consumable, anyway). Shoddy BBC reporting, as usual.
The heat exchangers use chilled hydrogen fuel they have to take up anyway, before it is burned in the rocket motor.
That makes a ton more sense.