ecos
Hazard to Others
Posts: 464
Registered: 6-3-2014
Member Is Offline
Mood: Learning !
|
|
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?
|
|
|
Science is my middle name
Harmless
Posts: 10
Registered: 6-1-2016
Member Is Offline
Mood: No Mood
|
|
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.
|
|
|
j_sum1
Administrator
Posts: 6321
Registered: 4-10-2014
Location: At home
Member Is Offline
Mood: Most of the ducks are in a row
|
|
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.
|
|
|
Fulmen
International Hazard
Posts: 1716
Registered: 24-9-2005
Member Is Offline
Mood: Bored
|
|
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.
|
|
|
ecos
Hazard to Others
Posts: 464
Registered: 6-3-2014
Member Is Offline
Mood: Learning !
|
|
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
|
|
|
Dornier 335A
Hazard to Others
Posts: 231
Registered: 10-5-2013
Location: Northern Europe
Member Is Offline
Mood: No Mood
|
|
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.
|
|
|
ecos
Hazard to Others
Posts: 464
Registered: 6-3-2014
Member Is Offline
Mood: Learning !
|
|
@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]
|
|
|
DubaiAmateurRocketry
National Hazard
Posts: 841
Registered: 10-5-2013
Location: LA, CA, USA
Member Is Offline
Mood: In research
|
|
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]
|
|
|
Dornier 335A
Hazard to Others
Posts: 231
Registered: 10-5-2013
Location: Northern Europe
Member Is Offline
Mood: No Mood
|
|
| 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.
|
|
|
![[*]](images/xpblue/default_icon.gif){kind=icon}
