Compressed oxygen for jet engine (?)

as jet engines lose there productivity at altitude due to thining air/oxygen , why isnt injecting compressed air directly along side the fuel a good idea ? , is this already being done ?
would /does this not increase ceiling height and allow sub orbit levels or possible leave atmosphere altogether ?
 

Borek

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Where are you going to take the compressed air from?
 
liquid oxygen held in tanks , maybe the amount needed is far greater than could feasably be carried
 

SteamKing

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If you an find a jet engine capable of working at speeds of 5 miles / sec., sure, you could achieve orbit. You see, achieving orbit is based on how fast you can go, not how high.
 

Borek

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liquid oxygen held in tanks , maybe the amount needed is far greater than could feasably be carried
Design you are talking about is called a rocket, and the main problem with it is that you have to carry all fuel with you.
 
no this is a jet engine until it reaches altitude and thin oxygen supply externally , even then i dont think you can call this a rocket if it has turbines ?
 
ps, as you climb would friction and energy needed to move forward reduce increasing your speed
 

russ_watters

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no this is a jet engine until it reaches altitude and thin oxygen supply externally , even then i dont think you can call this a rocket if it has turbines ?
Why would it have a turbine?

In any case, the closest to this would be our air-launched vehicles like Spaceship 1/2.

The main reason it isn't done is there just isn't much benefit to having both in the same engine. You don't just combine their strengths, they limit each other.
 

AlephZero

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Maybe you are thinking about something like this? http://www.reactionengines.co.uk/sabre.html [Broken]
 
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etudiant

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as jet engines lose there productivity at altitude due to thining air/oxygen , why isnt injecting compressed air directly along side the fuel a good idea ? , is this already being done ?
would /does this not increase ceiling height and allow sub orbit levels or possible leave atmosphere altogether ?
There have been lots of designs to improve the productivity of jet engines, but a shortage of oxygen has never been an impediment.
However, there are Russian sourced designs that use liquid hydrogen fuel to cool the compressor.
This allows a higher pressure ratio and a more efficient engine.
It is worth noting that compressor exit temperature is the critical parameter in turbine engines.
One can use cool air to keep the turbine blades from melting, but where will cool air come from for the compressor? This is the long pole in the tent, imho. Better engines will come when we find materials that can perform while at 1500*C, rather than 1100*C as at present.
 
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as jet engines lose there productivity at altitude due to thining air/oxygen , why isnt injecting compressed air directly along side the fuel a good idea ? , is this already being done ?
would /does this not increase ceiling height and allow sub orbit levels or possible leave atmosphere altogether ?
I was thinking the same Idea, but to add to the O2 that comes though the air intake would be a unknown factor due to the speed of the craft, the you could use the Liquid O2 to enhance the performance of the aircraft, but you should look at other designs for said engine
 
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The short of it:

1) What you're describing isn't a jet anymore, it's an "airbreathing rocket" - a rocket that uses air while in the atmosphere, oxidizer when not. Whenever you're feeding oxidizer, it's running as a rocket, full stop - your compressor and intake are now out of use, and you need a very different turbine to drive your rocket hardware like the turbopump.

2) What you're describing is totally impractical for a jetliner. Liquid oxygen is hazardous, heavy, bulky, an added expense, cryogenic, and a big long laundry list of other things.

3) Airbreathing / dual-mode rockets have been looked into for quite a long time, but they're a tough issue. Regular jets only work at relatively low speeds. Getting to orbit is not about flying high, it's about flying incredibly fast - the vast majority of the needed energy is used for acceleration. Hence you're hauling a lot of extra mass (an airbreathing engine) for a proportionally little gain. If you want to operate in airbreathing modes at higher airspeeds, you either need to burn supersonic or vastly increase the air's density. The former is known as a scramjet - an active topic of research. Examples of the latter include LACE wherein the air is fully liquefied, and SABRE (proposed for Skylon) which combines cooling with high pressures. All of them present challenges. Scramjets, for example, face "frozen combustion" problems, wherein the air is flowing so fast that in the time it takes for the propellant to burn, it's already long since moved out of the engine and thus not useful for propulsion.
 
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After this described engine switches over to using onboard oxygen, the compressor at the front becomes redundant and the rear turbine become an unnecessary impediment to exhaust gases.
Also both of them become extra dead-weight which the engine now working in rocket mode has to propel in addition to the rest of the vehicle.
 
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Willis Hallis, although I could not find what you were specifically asking for (a jet that supplements oxygen at higher altitudes) I did find this, the SABRE is an experimental hybrid jet/rocket engine that you may be interested in. http://www.gizmag.com/sabre-engine-afrl-feasibility-study/37092/ Anyway, I would imagine that even carrying around dead weight compressors out of the atmosphere sure beats dropping stages, although I could be wrong. (And likely am.)
 

rbelli1

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The space shuttle at separation of the boosters was going fairly slowly as most of the booster burn was done straight up. The boosters weigh 182000kg once they are used up. In order to keep those rockets attached you would need to accelerate that 182000kg from essentially 0 velocity to orbital velocity. That would take a lot of fuel and get you no benefit.

I can't find any details of velocity for other rocket launches but I wold imagine that most of them use a similar get out of the atmosphere before accelerating plan. You want to minimize the amount of atmosphere you need to swim through.

BoB
 
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The space shuttle at separation of the boosters was going fairly slowly as most of the booster burn was done straight up. The boosters weigh 182000kg once they are used up. In order to keep those rockets attached you would need to accelerate that 182000kg from essentially 0 velocity to orbital velocity. That would take a lot of fuel and get you no benefit.

I can't find any details of velocity for other rocket launches but I wold imagine that most of them use a similar get out of the atmosphere before accelerating plan. You want to minimize the amount of atmosphere you need to swim through.

BoB
I am not sure if I understand what you meant here, correct me if I'm wrong but are you saying the shuttle goes straight up (more or less) and then thrusts horizontally at the apoapsis (once it's beyond the atmosphere that is) to achieve orbit? It was my understanding that the shuttle performed a gravity turn and finished circularization at the top of the ark. What you are saying (going shortest distance through the atmosphere to increase efficiency) makes sense but that makes me wonder why any rocket would perform a gravity turn if this was the case. Please let me know if I am completely wrong.
 

rbelli1

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The boosters end up about 150 miles off the coast so while there is a horizontal velocity is is a tiny fraction of orbital. You want to avoid high velocity in the densest part of the atmosphere.

BoB

EDIT: I did the calculation and it is very approximately 1/4th of the orbital velocity at separation (much more than I thought based on the distance to splashdown) so "small fraction" is a bit loose with the terminology. However accelerating 182000kg through 21000kmph delta V is still a lot of fuel.
 
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Thanks Rbelli1, that makes sense to me, also is drag in relation to speed linear or exponential?
 
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Willis Hallis, is this rocket/jet hybrid you proposed a ramjet? I think that injecting oxidizer into the combustion chamber would be easier to do on a ramjet than another type of engine with a compressor turbine. What I am asking is what type of jet engine do you think it would be feasible to inject packed oxidizer into as a supplement for high altitudes?
 

jbriggs444

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Thanks Rbelli1, that makes sense to me, also is drag in relation to speed linear or exponential?
Drag is often modeled as being quadratic. That is not the same thing as exponential.
 
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Drag is often modeled as being quadratic. That is not the same thing as exponential.
Does that mean that a fast aircraft is less efficient than a slow aircraft? (Assuming the only variable is speed)
 
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Does that mean that a fast aircraft is less efficient than a slow aircraft? (Assuming the only variable is speed)
Yes I think it does mean that, the skin of supersonic aircraft can get quite hot, which amounts to energy being wasted.
 

rbelli1

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Maximum efficiency is not always maximum efficacy. Getting from New York to Paris in the smallest possible time is desirable. Sometime you need to trade goods for other goods.

BoB
 
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Maximum efficiency is not always maximum efficacy. Getting from New York to Paris in the smallest possible time is desirable. Sometime you need to trade goods for other goods.

BoB
Interesting point, fuel efficiency is of little consequence if the flight system is a missile.
New-York to Paris looks safe for the time being though.
 

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