Re:Ask HN: Why can't jets use a rocket engine like Raptor?
by Gibbon1 on Tuesday October 03, 2023 @12:24AM
(#37747965)
One way to look at it is to use the particular fuel and engines ISP. Specific impulse which really is about pounds of thrust per pound of fuel per second.
Rocket each pound of fuel + oxidizer you burn a second gives produces a few hundred pounds of thrust. Call it 300.
Air breathing engine doesn't carry oxidizer. A rocket needs to carry ~3 lbs of oxidizer per lb of fuel. Using the 300 number above an air breathing engine would have an ISP of 900.
It gets worse of the rocket because prop and turbo-fan engines also use air for reaction mass[1]. Which boosts the calculated ISP even higher. Call it 5X higher. Now your ISP is 4500.
So yes you can use rockets for an aircraft. But it won't travel as far as one with a piston or turbofan.
1 Thrust is basically mass flow X deltaV. Where power is mass flow X deltaV squared. So it's more efficient to create thrust by accelerating a large mass flow a small amount (turbofans) than a a small amount of mass a large amount (rockets).
Rocket each pound of fuel + oxidizer you burn a second gives produces a few hundred pounds of thrust. Call it 300.
Air breathing engine doesn't carry oxidizer. A rocket needs to carry ~3 lbs of oxidizer per lb of fuel. Using the 300 number above an air breathing engine would have an ISP of 900.
It gets worse of the rocket because prop and turbo-fan engines also use air for reaction mass[1]. Which boosts the calculated ISP even higher. Call it 5X higher. Now your ISP is 4500.
So yes you can use rockets for an aircraft. But it won't travel as far as one with a piston or turbofan.
1 Thrust is basically mass flow X deltaV. Where power is mass flow X deltaV squared. So it's more efficient to create thrust by accelerating a large mass flow a small amount (turbofans) than a a small amount of mass a large amount (rockets).