British technology company to 'transform' air and space travel

Re:British technology company to 'transform' air and space t... by avmich on Tuesday December 01, 2015 @03:51PM (#10658648)

Thank you. Good points, but I think we'll have to do a full blown calculations to decide. In aerospace margins are usually thin and it's easy to have the sign error.

1) I fully agree that air-breathing engines are heavier per unit of thrust. Merlins - pretty good rocket engines by that parameter - lift, as you say, 150 times their weight; for air-breathing engines it's 5-10.

2) Aircraft engines usually don't need to be particularly thrusty; wings generate lift, so you can take off even with T/W < 1.

3) Liquid oxygen has density about 1.1 that of water. Kerosene, in comparison, is about 0.8. Difference less than 1.5 times, but the mass ratio is more like 2.5:1, so ditching oxygen tanks - especially at the start, when the system is relatively heavy - can save quite a bit. And why engines' T/W is relevant here?

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Re:British technology company to 'transform' air and space t... by Gravityloss on Wednesday December 02, 2015 @07:50AM (#10662610)

2) And now you're carrying wings to orbit too. More dead mass that could be payload...

3) My comment was to compare liquid oxygen with liquid hydrogen which is 0.07 of water density, so you get 16 kg of oxygen for 1 kg of hydrogen for the same tank volume. Skylon uses loads of hydrogen. It's those hydrogen tanks that make it so big. Such tanks are also a lot of dead mass to carry to orbit... Also costs probably scale with dry mass, not wet mass.

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