Yes that was part of what I was asking.
Thank you.
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There have been some experimental engines that breathe air while it’s available, and then switch to an internal oxidizer once they’re out of the atmosphere. They never turn out to be practical, though.
Air augmented rockets are used for some missiles. They do not seem practical for achieving space flight though, one reason being obviously that all their advantages disappear once out of the dense part of the atmosphere.
We’ve got a decommissioned Soviet Kholod air-breathing missile inside an office building near us. Probably going cheap because the technology didn’t work very well.
I get all this, what I don’t get is how an aerospike engine uses the exhaust gas to optimize the ideal nozzle shape for a given velocity. I’ve tried a couple of Youtube vids but I’m not seeing how this works in reality. Can someone explain this like I’m 8?
According to the qualitative description in that FAQ, it is not changing the the velocity of the exhaust gas to optimize the expansion, rather the pressure (if any) of the outside atmosphere pushes on the exhaust and constrains the expansion. I do not see anything there about changing the nozzle geometry during flight.
Everyday Astronaut has a pretty good page/video on the subject.
You can get a pretty good hint as to what’s going on with just this image, though. See how the ambient pressure presses the exhaust flow to the surface of the spike? That’s how you achieve no flow separation at arbitrary pressure. At lower pressure, the exhaust stream starts to widen as it reaches the end of the spike, but it remains in contact. It widens just as much as it needs to in order to expand to ambient pressure.
Oh no.
It has significantly less usable propulsive force ON EARTH than on the moon or Mars.
Reason:
0) surrounding air is not used by a rocket motor. It is not fuel, nor something to push against, nor something to get lift from like an airplane. It is merely ‘stuff’ that happens to be around the rocket.
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Rocket engine thrust is reduced by ambient pressure. In the case of the Apollo Lunar ascent module’s rocket motor, that loss of thrust would be about 16%
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Once your rocket is moving, the air(if any) starts dragging back on it. Obviously Earth’s high-pressure atmosphere causes a LOT more drag than that of Mars (1/170th as dense) or that of the Moon (3 quadrillionths as dense)
In addition, while it does not at all affect the propulsive force, the local gravity does reduce the actual propulsion one gets from the propulsive force. With again Earth gravity impeding launch much more than weaker Mars or feeble Moon gravity.
Side note: To get a couple of humans and their life support system into orbit around the Earth, you need something roughly like the Gemini rockets. To get a couple of humans and their life support systems off of the Moon, you need only something about the size of a car. Though granted, that’s mostly because of the difference in gravity, not atmosphere.
Wouldn’t the low gravity also be the cause of the lack of atmosphere?
Not really. The relationship between surface gravity and atmospheric density is not direct. Titan’s surface gravity is lower than the moon’s but it’s atmosphere pressure at the surface is higher than the Earth’s. Venus’ surface gravity is slightly lower than the Earth’s but it’s atmospheric pressure is much higher than the Earth’s.
Thanks, interesting. This gravity stuff is tricky.
Sigh. “Its” not “It’s”
Or to be more precise, gravity is one factor that goes into determining atmosphere, but it’s not the only one, and other factors are far more significant.
As I understand the aerospike engine, it’s just the inversion of a cone. The spike is internal, in the center of combustion, but shaped with a similar angle (starting thick then tapering to a spike, at the rate that a cone would flare out), while the cone is external to the combustion. It’s a different solution to the same problem. It has it’s set of advantages and disadvantages along with every other rocket exhaust nozzle. This one IIRC trades lower overall efficiency for a wider range ‘not that bad’ efficiency, and has it’s issues like the spike overheats.
I read through the article, thank you for posting it. I get it now!