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Author: Subject: Jet Engine vs Rocket motor
ecos
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[*] posted on 25-3-2016 at 16:00
Jet Engine vs Rocket motor


Hi All,

I was watching a video about the concept behind jet engines : https://www.youtube.com/watch?v=KjiUUJdPGX0

Jet Engines
I noticed that jet engine depends on burning fuel using oxygen in the air.
Inside the combustion chamber we have fuel(petrol) and oxidizer (air) => Temp = 450 degree , Pressure = 35 par (info from the video).

this means we saved the weight of the oxidizer and the plane carry only fuel.
The air sucked inside combustion chamber would have other gases like Nitrogen which would be heated in high temperature and it would expand more.

Germans designed a flying bomb similar to jet engines : https://en.wikipedia.org/wiki/V-1_flying_bomb
just carrying fuel and sucking oxygen from air.
cars with jet engine car reach 714 mph : https://www.youtube.com/watch?v=AM8Lj6MppaI
I think this is a high speed like a rocket.

Rockets

rockets carry oxidizer(extra weight compared to jet engines) and fuel.
I know that the temperature and pressure inside the rocket motor can reach high values (Temp = 1000 degree )

of course, rockets would have higher speed but short travel distance compared to jet engines.


hints: I am not considering flying in space which is free from Oxygen, I just focus on rockets that are flying in air.

my thoughts here :
would rockets one day would have jet engines instead of propellant ? if not, would planes one day use rockets instead of jet engines?
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[*] posted on 25-3-2016 at 23:08


I think that it would be more efficient to strap a rocket to the back of a high-altitude jet with a launchpad so that 35,000-65,000 feet is cleared reducing the weight of fuel and emissions.
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[*] posted on 25-3-2016 at 23:21


Different engines for different purposes. It is that way and will always be that way.

For space, jet engines are not an option.
For extreme altitudes, jet engines are problematic.

For rockets, the necessity of carrying oxidant has pushed the design into regions where jets just never go -- higher energy density (by weight) solid fuels, higher temperatures. With these comes refuelling (or indeed re-use) issues.

Because jets have moving parts that rockets don't this limits operational temperature somewhat.

Applications of jet engines and design of jet engines have developed hand in hand. Exactly the same could be said for rockets. The two will never be interchangeable.
However, even though you excluded space from the applications you are interested in, you might be interested in Skylon.




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[*] posted on 26-3-2016 at 00:39


Google "combined cycle rocket", these are next-gen engines that run on air at lower altitudes and switch to onboard oxidizers at high altitude.



We're not banging rocks together here. We know how to put a man back together.
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[*] posted on 30-3-2016 at 06:13


Thanks all for replies.

I see the jet engine as hybrid rocket (fuel is liquid , oxidizer is gas).
I still wonder why rockets don't have mechanical parts in the combustion chamber , won't it be much efficient to add moving parts to (liquid propellants or hybrid propellant):
1- control the temperature (compressors , turbines ,..)
2- efficiently mix the fuel with oxidizer
3- adjustable nozzle to control the speed
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[*] posted on 30-3-2016 at 07:48


Rockets don't have any moving parts in the combustion chamber for a number of reasons. The temperature should not be controlled, it should be as high as possible to give highest efficiency. The pressure too should be maximised. This introduces a technical problem: the gases will eat anything inside the combustion chamber. For example the RS-25 has a chamber pressure higher than 200 bar and ~3500 K combustion temperature. In addition, the gases exit the nozzle at 4500 m/s, which is nearly theoretical performance.

The only moving parts are outside the combustion chamber, for example the turbo pumps. A very complex system of preburners and pumps is required to minimise the losses. Adding more parts inside the chamber will only slow down the gases and require excessive cooling!

An adjustable nozzle is difficult because it too needs cooling, something achieved by circulating fuel inside its walls. When rocket engines need to operate at very different pressures aerospike nozzles are used instead.
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[*] posted on 30-3-2016 at 16:44


@Dornier, as you said the higher temperature and pressure the higher the efficiency. this return us back why airplanes doesn't substitute jet engines with liquid/hybrid rocket motor ?

[Edited on 31-3-2016 by ecos]
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[*] posted on 30-3-2016 at 16:46


Quote: Originally posted by ecos  
@Dornier, as you said the higher temperature and pressure the higher the efficiency. this return us back why airplanes doesn't substitute jet engines with liquid/hybrid rocket motor ?

[Edited on 31-3-2016 by ecos]


Cost obviously.

would you rather want to design an engine that uses an atmosphere's oxygen which has significantly lower upkeep cost, or a rocket engine with liquid oxygen in a kept-cooled tank and environment, this costs much more!

Liquid oxygen also burns at much higher temperature because you need sufficient air to build up pressure, and therefore efficiency. You'd need exotic materials to sustain a constant re-use-able engine nozzle for such purposes. Modern turbojets can suck in enough air to be 10x more efficient.

[Edited on 31-3-2016 by DubaiAmateurRocketry]
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[*] posted on 31-3-2016 at 04:07


Quote: Originally posted by ecos  
@Dornier, as you said the higher temperature and pressure the higher the efficiency. this return us back why airplanes doesn't substitute jet engines with liquid/hybrid rocket motor ?

[Edited on 31-3-2016 by ecos]


You said it yourself earlier. Carrying oxygen onboard is heavy, so the effective specific impulse of jet engines is around ten times higher than that of rockets!

Rockets are used where oxygen isn't available and jet engines are used where it is.
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