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tdoesstuff t1_j1rbw65 wrote

any chemical propellant engine can work in space and in our atmosphere


3d_blunder t1_j1rfhhc wrote

All rocket engines work in atmosphere and vacuum.


Science-Compliance t1_j1rhf68 wrote

Eh, not really. A lot of electric rockets (aka ion) produce so little thrust, that they effectively don't work in atmosphere.


HervG t1_j1ridkk wrote

Something not working effectively is still working. You know like most people


Science-Compliance t1_j1rk2l3 wrote

Maybe I didn't explain well enough. Ion engines produce thrust equivalent to the weight of a piece of paper. The lightest breeze would render your engine unusable, or the slightest amount of drag assuming still air. Even in perfectly still air, you would never get anywhere because drag is proportional to the square of velocity, so your top speed would make walking a much more attractive option.


Bubbagumpredditor t1_j1rdqis wrote

Any basic rocket should work regardless of atmosphere.

What you might be asking about is a reactionless drive or something that doesn't really exist.

There are hybrid rockets/engines that work like a rocket but may use air for oxidant, but those are a whole other thing.

If you want to see more stuff about some of the non standard rocket engines you can look up aerospike rockets, or ion drives, which are real.


Science-Compliance t1_j1ri0sb wrote

Ion engines, at least in their current state, aren't really suitable for atmospheric propulsion. They produce so little thrust that a light breeze would be a big problem.


robot_egg t1_j1rfc5m wrote

The optimal design of the nozzle changes depending on the ambient pressure, but liquid fueled chemical rockets work both in air and in space, and the pumping system can be designed to allow some throttling range.


Mandelvolt t1_j1rgg9u wrote

Look up the aerospike engine, or linear aerospike engine. They are designed to work optimally at different (edit: altitudes).


Science-Compliance t1_j1rgx7z wrote

Different altitudes / ambient pressures, not temperatures, and no, they're not "optimal". You pay a penalty for using an aerospike over a standard nozzle, but unlike standard bell nozzles, they're less bad at a wider range of altitudes.


ERROR_396 t1_j1rpnp0 wrote

The nozzle geometry is optimal at a wide range of pressures. He never said aerospikes are the best, just they work optimally at various pressures, rather than just one pressure like with a bell nozzle


Science-Compliance t1_j1rs56t wrote

Not sure if I didn't explain well enough or you just don't understand, but, anyway, the point I was trying to make is that the aerospike operates at lower efficiency at every altitude than a bell nozzle tailor-made for each altitude would. Over a range of widely varying altitudes, though, the aerospike nozzle, is, cumulatively much more efficient than a bell nozzle that only has one exact design altitude at which it is neither over- or under-expanded. Neither a bell nozzle nor an aerospike exhibit "optimal" expansion in any real-world scenario.


NikeLeon t1_j1rhg1f wrote

Not just different temperatures but also different atmospheric pressures. Nozzles on traditional rockets are shaped differently depending on which stage of the launch they're meant to propel the shuttle. Different atmospheric pressures require different nozzle shapes to properly burn the fuel [edit: properly direct the exhaust] for the most efficient thrust.


Science-Compliance t1_j1rks78 wrote

>Not just different temperatures

It has nothing to do with temperature.

>Different atmospheric pressures require different nozzle shapes to properly burn the fuel for the most efficient thrust.

It has nothing to do with properly burning the fuel. It has to do with the exhaust stream's static pressure at the nozzle's exit.


Edit: a very poorly sized nozzle will cause combustion instabilities, so in a sense, properly burning is kind of a thing, but before that happens, just having the incorrect exhaust pressure at the nozzle exit will make for a less efficient expansion of gases than optimal thrust independent of any combustion issues.


NikeLeon t1_j1rnd3o wrote

Much better put than I could articulate. I recalled it had to do with the yield of thrust per unit of fuel consumed.


Science-Compliance t1_j1rr8b9 wrote

Nozzle under-expansion: There is energy left in the exhaust that could have been converted into momentum with a longer/more expanded nozzle. This energy goes into expanding the plume outward into the air aft of the vehicle that could have been used to push the vehicle forward. You get a certain amount of "pressure thrust", which is the pressure of the exhaust relative to the ambient air pressure pushing against the vehicle, but this is small compared to "momentum thrust".

Nozzle over-expansion: You have expanded the exhaust gases past the point where they are at the same pressure as the ambient air, and the atmosphere is actively pushing back against your thrust stream. Extreme over-expansion will cause the exhaust plume to creep back into the nozzle, detaching from the nozzle wall and eventually leading to combustion instabilities.

A vacuum nozzle can never be over-expanded, since the static pressure of vacuum is essentially zero and the exhaust plume will always have positive total pressure. Vacuum nozzles' sizes are limited by other considerations such as mass and structural integrity.


wakebakey t1_j1rhnel wrote

What you are thinking of is a Space Plane. Ramjet and aerospike engines get associated with space planes often


Science-Compliance t1_j1rjd4g wrote

If you're talking about rockets that are currently out there, the only ones that would really be suitable are liquid or hybrid chemical rockets. Liquid chemical rockets use fuel and oxidizer that is in liquid form. A hybrid rocket has typically a fuel that is solid and an oxidizer that is liquid. Solid chemical rockets also work in and out of atmosphere but aren't really throttleable.

Ion engines, as you said, produce too little thrust to be useful in atmosphere and even require different mission planning in space than chemical rockets due to their incredibly low thrust. It's conceivable that with enough electric power, you could make ion engines produce thrust closer to chemical rockets, but we're talking about obscene levels of power required to achieve this, which would currently be in the realm of sci-fi.

Nuclear rockets are also a thing that have been tested and use the heat of a nuclear fission reaction to accelerate a fuel such as liquid hydrogen to produce thrust. Due to the low molecular mass of hydrogen, they produce less thrust than typical chemical rockets but are more efficient than chemical rockets and are also throttleable.


space-ModTeam t1_j1rlaol wrote

Hello u/MyShatsRRadioactive, your submission "Question about rocket thrust / engines in space and in atmosphere…" has been removed from r/space because:

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PerformerGreat t1_j1rj4vc wrote

Hydrogen is always my go-to for thrusters. works everywhere, powerful and only needs ice. even once i've progressed in my save most of the time i don't switch to ion. would rather use that power for JumpDrives than for running my thusters, such as ion or atmospheric.


EDIT: oops, thought this was the space engineers subreddit. my bad. but it's still good advice. so i'm gonna leave it.


iamethra t1_j1rnirk wrote

Hydrogen is a huge PITA to work with though which is part of the reason why a lot of modern rocket engines are moving to methane (SpaceX Raptor and Blue Origin BE4 among others).


MouseDestruction t1_j1rkgpa wrote

The type of rocket that pushes against itself as opposed to pushing against the atmosphere (rocket engine vs a jet engine basically).

I do think a throttle is going to be extremely hard though, you are riding an explosion directly, it's not like a car, there is nothing to put breaks on. Your only real option to go faster is make a bigger explosion, your best option to slow down is to turn around and fire engines opposite way, but gravity is free, fuel is not, and fuel adds mass to spaceship, so using a gravity well to slow or speed up saves a lot of money.

And only the main engine will be big enough to slow you down with any sort of speed (like 2 weeks or months or years), it's going to be equivalent to how long you spent speeding up.

Even if you use gravity and your engine you can only take so many g forces for so long. I think it's something like 1.3g you can handle for long periods? Outside of scifi 'inertial dampeners' you are stuck to a certain amount of acceleration or deceleration.