30 replies to this topic

[MatsNorway]

I was sitting on the bus when a strange question came to me.

How is a pulse/scram/ram jet compared to afterburners?

How about efficiency?

How about power?

anything else worth mention?

anyone got figures for pulse jets/ram jets? i only got machine drawings.

The SR-71 had a ram jet configuration.

Edited by MatsNorway, 13 February 2010 - 19:19.

[Tony Matthews]

I was sitting on the bus when a strange question came to me.

How is a pulse/scram/ram jet compared to afterburners?

How about efficiency?

How about power?

anything else worth mention?

anyone got figures for pulse jets/ram jets? i only got machine drawings.

The SR-71 had a ram jet configuration.

What's a bus?

[MatsNorway]

What's a bus?

Buss

[primer]

Afterburners make sense for aircrafts that will spend only small time of their flight in supersonic regime. Ramjets are more suitable for missiles and such, which will spend most of their time in supersonic regime.

Efficiency is not the only criteria though, now is it. You also have to consider range.

[MatsNorway]

Afterburners make sense for aircrafts that will spend only small time of their flight in supersonic regime. Ramjets are more suitable for missiles and such, which will spend most of their time in supersonic regime.

Efficiency is not the only criteria though, now is it. You also have to consider range.

Could you have one or more ram jets popping out of airplane body like the wheels when needed.

Does the ram produce lots of power at all?

or is it used mainly because it goes well in high speeds?

Edited by MatsNorway, 13 February 2010 - 19:46.

[primer]

Could you have one or more ram jets popping out of airplane body like the wheels when needed.

Which aircraft do you have in mind? The typical passenger airliner will use high bypass turbofans (and perhaps GTF) for the next 40-50 years or more. Those military a/c that need to go supersonic already use low bypass (bypass less than one) turbofans or turbojets and just use the afterburner when they need to accelerate and punch through the transonic regime real quick.

Adding a totally different type of stowed engine will be design inefficient. All that weight in an airframe that cannot handle Mach 5, for what? Manned aircraft do not even need to travel at very high speeds. You have your AESA radar to 'outsee', and you have your missiles which travel far faster than any aircraft.

Does the ram produce lots of power at all?

Does an internal combustion engine produce a lot of power?
What is the application? You can build a ramjet capable of more thrust than a GE-90-115B, but it will not fly by itself. You would have to consider the packaging of the airframe, payload, range and other mission requirements. You can only make the whole thing so big since at higher Mach drag is more of an issue.

or is it used mainly because it goes well in high speeds?

Yes, going very fast is a ramjet speciality. Good for missiles. Not coming to a passenger jet near you, I'm afraid.

[MatsNorway]

Well.. case closed really.


What about a smaller jet engine and more ram jets/pulse jets?

jet engine for lift of and cruise and pulse jet for combat flight.

you will save weight and money, since pulse jets is just a pipe really.

Looking away from the MASSIVE heat signature ofc.

Edited by MatsNorway, 13 February 2010 - 20:33.

[Paolo]

Pulse jet and ram jet are two very different devices.
Pulse jets were used on V1 in WWII, are cheap but impossible to modulate, incredibly noisy and a source of tremendous vibrations.
No pulse jets were ever used on piloted aircrafts, bar some experiments with guided V1 in Germany.
They're popular on RC models.
Ramjets are a much more advanced technology , but they cannot work at low speeds and need a different engine to accelerate aircraft to minimum functioning speed.
SR 71 Blackbird engines were "partial ramjets" which worked as normal jet engines in the lower speed range.

Edited by Paolo, 14 February 2010 - 09:05.

[Wuzak]

Pulse jet and ram jet are two very different devices.
Pulse jets were used on V1 in WWII, are cheap but impossible to modulate, incredibly noise and a source of tremendous vibrations.
No pulse jets were ever used on piloted aircrafts, bar some experiments with guided V1 in Germany.

Piloted pulse jet aircraft did not make it to production. A few Messerschmit Me328 prototypes were built and test flown. These had two Argus pulse jets, one on each side of the rear fuselage.

Rgere were several projects using the pulse jet that never got off the drawing aord.

[SteveCanyon]

Slight thread hijack but well worth it.
Check out the German WW2 research work - http://www.luft46.com

My fave being the Focke-Wulf FW "Triebflugel".

[gruntguru]

Slight thread hijack but well worth it.
Check out the German WW2 research work - http://www.luft46.com

My fave being the Focke-Wulf FW "Triebflugel".

I guess the main advantage being the ramjets could be brought up to a "viable" speed without the aircraft moving.

[Tony Matthews]

I guess the main advantage being the ramjets could be brought up to a "viable" speed without the aircraft moving.

It is VTOL too, very clever. I've just thought - if you had a similar arrangment on wind turbines you could have a constant power supply even when the wind isn't blowing...

[Greg Locock]

That made me laugh, but it isn't as bad as a recent NASA proposal.

http://www.theregist...uffin_vtol_pod/

I've just been reminded of air augmentation for jet engines. You shoot the exhaust down a venturi. This entrains air through the venturi, so increasing the bypass ratio and hence low speed thrust without any moving parts. We use it to use shop air for spot cooling in the engine dynos.

[Tony Matthews]

That made me laugh, but it isn't as bad as a recent NASA proposal.

http://www.theregist...uffin_vtol_pod/

I've just been reminded of air augmentation for jet engines. You shoot the exhaust down a venturi. This entrains air through the venturi, so increasing the bypass ratio and hence low speed thrust without any moving parts. We use it to use shop air for spot cooling in the engine dynos.

'Jetex' engines, at least one of the larger ones, had an 'optional extra' Augmentor Tube. I think the venturi principle is used in F1 by some teams to blow cooling air onto hot, sweaty little drivers in the pit garages, and a similar principle is used to shift lightweight granules or powder - possibly even grain.

I supose the 'Puffin' has a use as long as your car is only three minutes flying-time away.

[Greg Locock]

'Jetex' engines, at least one of the larger ones, had an 'optional extra' Augmentor Tube. I think the venturi principle is used in F1 by some teams to blow cooling air onto hot, sweaty little drivers in the pit garages, and a similar principle is used to shift lightweight granules or powder - possibly even grain.

Wow Jetex. What a crap system. The American ones are MUCH funnier.

Incidentally the thing is called an eductor, they are commonly called jet pumps and are widely used, as you say.

http://science.blogd..._of_Eductor.htm

The blast pipe in a steam locomotive is an eductor, as are the injector lift pumps in them. Those get some extra advantage from condensation as well.

[cheapracer]

I was sitting on the bus when a strange question came to me.

Mine is usually "will that hot girl getting on at this stop sit next to me or not"?

....cooling air onto hot, sweaty little pitbabes in the pit garages,

Fixed.

Edited by cheapracer, 14 February 2010 - 08:04.

[Tony Matthews]

Mine is usually "will that hot girl getting on at this stop sit next to me or not"?

Sometimes, Cheapy, I think you have a one-track mind. Just like me...

[Charles E Taylor]

There is more interest here than you might imagine.


Some serious players with big money.

http://forums.autosp...a...t&p=3279810 Post 253
http://www.af.mil/ne...sp?id=123099095


http://www.airspacem...detonation.html


Watch this workspace very carefully.



Charlie

[MatsNorway]

I liked this part.

"PDE can be combined with other engine cycles, such as turbines, rockets, or hypersonic scramjets, to optimize flight envelopes, said Fred Schauer, assigned to the AFRL Propulsion Directorate. "

Just as my playaround idea was.

[J. Edlund]

Well.. case closed really.


What about a smaller jet engine and more ram jets/pulse jets?

jet engine for lift of and cruise and pulse jet for combat flight.

you will save weight and money, since pulse jets is just a pipe really.

Looking away from the MASSIVE heat signature ofc.

When you need a short time increase in thrust it doesn't get much simpler than adding an afterburner to a jet or turbofan engine. The afterburner is essentially a large pipe equipped with flameholders and fuel injectors. For the most of the time it's just dead weight, so it's made as light as possible. Flame holders and fuel injectors must be properly cooled to keep the heat signature low.

[MatsNorway]

When you need a short time increase in thrust it doesn't get much simpler than adding an afterburner to a jet or turbofan engine. The afterburner is essentially a large pipe equipped with flameholders and fuel injectors. For the most of the time it's just dead weight, so it's made as light as possible. Flame holders and fuel injectors must be properly cooled to keep the heat signature low.

yea but how efficient is it?

its actually close to be just a big ass flamethrower.

A pulse jet gives less trust i guess but it would give advantages in pursuits and intercepts that lasts longer.

its really all about how long the catch up time must be within. Long time -> pulse jet short time -> afterburner

given the same amount of fuel in planes and no borders

Edited by MatsNorway, 15 February 2010 - 16:35.

[Bloggsworth]

A pulse jet would begin diverting all the shipping in the locality, the last one I heard, about 18" long, could be heard several miles away! - That's always assuming you could get it started, which was not often; twice actually.

Edited by Bloggsworth, 15 February 2010 - 18:29.

[Tony Matthews]

A pulse jet would begin diverting all the shipping in the locality, the last one I heard, about 18" long, could be heard several miles away! - That's always assuming you could get it started, which was not often; twice actually.

I think the model ones may be more tempremental than the full size - although I have no personal experience of them, but I wonder what the failure rate of V1 pulse jet engines was. Very low, I would guess. There is a nice photograph of a small pulse jet being bench-tested by Mario Illien in Karl Ludvigsen's Ilmor book. Looks like fun!

[J. Edlund]

yea but how efficient is it?

its actually close to be just a big ass flamethrower.

A pulse jet gives less trust i guess but it would give advantages in pursuits and intercepts that lasts longer.

its really all about how long the catch up time must be within. Long time -> pulse jet short time -> afterburner

given the same amount of fuel in planes and no borders

The materials in the turbine limits the temperature at the turbine inlet, typically at 1300-1500 degC or so with air film cooling, so a turbine engine always have to run lean with plenty of cooling air. Over the turbines the temperature decrease and is perhaps around 1000 degC after the turbine. What the afterburner do is to reheat the air between the gas generator (turbine engine) and the outlet nozzle. Instead of 1000 degC the temperature is increased to beyond 2000 degC which increase exhaust jet velocity.

Typical values of a fighter jet engine
Military thrust: 80 kg fuel/kNh
Full afterburner: 180 kg fuel/kNh

The afterburner provides a thrust boost by around 50%.

Need better range at high speed? Fit a larger turbofan! The weight handicap is soon earned back in the form of lower fuel consumption. An afterburning turbofan can consume it's weight in fuel in just about 4-5 minutes.

[SteveCanyon]

Need better range at high speed? Fit a larger turbofan! The weight handicap is soon earned back in the form of lower fuel consumption. An afterburning turbofan can consume it's weight in fuel in just about 4-5 minutes.

Useless trivia - on some types of aeroplanes with afterburning jets, it can be more efficient to climb to cruise altitude on the burners rather than 'dry' military thrust, as the extra performance getting up to where the engine burns little fuel in the cruise results in a lesser overall fuel burn when going from A to B.

This is the argument I used when I tried to get the engineers to fit afterburners to the 747's I flew ..... didn't work ....

[Wuzak]

Useless trivia - on some types of aeroplanes with afterburning jets, it can be more efficient to climb to cruise altitude on the burners rather than 'dry' military thrust, as the extra performance getting up to where the engine burns little fuel in the cruise results in a lesser overall fuel burn when going from A to B.

This is the argument I used when I tried to get the engineers to fit afterburners to the 747's I flew ..... didn't work ....

Hasn't there been a similar argument for cars? ie that it is more fuel efficient to get up to the cruising speed quickly rather than build up speed gradually.

Not sure that it actually works.

[SteveCanyon]

Hasn't there been a similar argument for cars? ie that it is more fuel efficient to get up to the cruising speed quickly rather than build up speed gradually.

Not sure that it actually works.

I'm not sure either, but I'm willing to take one for the team and fit an afterburner to my car to find out.

[gruntguru]

Hasn't there been a similar argument for cars? ie that it is more fuel efficient to get up to the cruising speed quickly rather than build up speed gradually.

Not sure that it actually works.

Absolutely correct. Acceleration should be done in the engine's peak efficiency band ie medium throttle, low-medium revs. This is because the energy required for acceleration to a given cruising speed is the same regardless of whether it is done quickly or slowly. (Note I am talking about the energy used actually accelerating the car - not the parasitics along the way.) This "acceleration" energy is not wasted, it is still present as kinetic energy while the car is cruising. Overall it makes sense for this work to be produced by the engine at the highest possible efficiency. Another benefit of higher acceleration is the cruising speed can be reduced if total travel time is the constant. lower cruising speed will reduce parasitic losses for a given journey.

Greg can probably enlarge on this given his solar car experience.

[MatsNorway]

Thanks for info Edlund.

We need efficiency numbers on the jet and some numbers on the pulse jet now

Need better range at high speed? Fit a larger turbofan! The weight handicap is soon earned back in the form of lower fuel consumption.

This part is no god. if you read the entire article about the PDE. its a engineer that says that the jet engine has basically reached it potential, improvements like 0,2% is considered to be massive. PDE tech might improve it to 5% (a bit offtopic)

An afterburning turbofan can consume it's weight in fuel in just about 4-5 minutes.

that doesn't sound efficient.

could we not roughly calculate efficiency based upon the numbers given from Edlund?

what is the efficiency on a jet engine?

when it comes to ideal acceleration the fast approach at least don't need to fight the air resistance as long as the the slow approach before reaching cruising speed.

someone mentioned that turbine engines are most efficient at full throttle too. guess its the same for jets.

Edited by MatsNorway, 16 February 2010 - 16:08.

[J. Edlund]

Useless trivia - on some types of aeroplanes with afterburning jets, it can be more efficient to climb to cruise altitude on the burners rather than 'dry' military thrust, as the extra performance getting up to where the engine burns little fuel in the cruise results in a lesser overall fuel burn when going from A to B.

This is the argument I used when I tried to get the engineers to fit afterburners to the 747's I flew ..... didn't work ....

Not really useless, just an indication that the weight handicap caused by a more powerful engine can soon be earned back if the higher thrust level is required for a longer duration.

You're talking about using the additional thrust for a short period only, getting up to cruise altitude.

Hasn't there been a similar argument for cars? ie that it is more fuel efficient to get up to the cruising speed quickly rather than build up speed gradually.

Not sure that it actually works.

Yes, but for different reasons. When getting up to speed quickly in a car the engine is operating more efficiently.

Thanks for info Edlund.

We need efficiency numbers on the jet and some numbers on the pulse jet now





This part is no god. if you read the entire article about the PDE. its a engineer that says that the jet engine has basically reached it potential, improvements like 0,2% is considered to be massive. PDE tech might improve it to 5% (a bit offtopic)




that doesn't sound efficient.

could we not roughly calculate efficiency based upon the numbers given from Edlund?

what is the efficiency on a jet engine?

when it comes to ideal acceleration the fast approach at least don't need to fight the air resistance as long as the the slow approach before reaching cruising speed.

someone mentioned that turbine engines are most efficient at full throttle too. guess its the same for jets.

Practical pulse jets are usually not that efficient, not to mention that the noise and vibration would limit their use in most applications.

An afterburning jet engine is very light (thrust to weight ratio around 8), that's a reason why the fuel consumption is so high in relation to the weight of the engine. The above numbers are also with the engine stationary at sea level.

The specific fuel consumption and efficiency of the engine is also not of the greatest concern, if that was the case, we would be flying around in planes that look more like the Concorde than your average Boeing or Airbus.

[dosco]

The specific fuel consumption and efficiency of the engine is also not of the greatest concern, if that was the case, we would be flying around in planes that look more like the Concorde than your average Boeing or Airbus.

Could you eplain this?