Ramjet effect on drone

[oz93666]

TL;DR

Ramjet

I am following someone on Youtube who has built a rocket shaped (4 proprllers) drone for the world speed record ...
He was having problems with internal heat from electical control gear and battery inside the sealed rocket (about 1kw waste heat)
Speed is 500km/hr , I suggested a 4mm hole in the tip of the nose and 8mm hole at rear which should alow sufficient air cooling ..
I said this will increase thrust because the air exiting is hotter .. a bit similar to a ram jet ... was I correct about the aditional thrust?

 

[Lnewqban]

Please, see:

 

[Baluncore]

At 500 kph, the ram air pressure will be about 1.7 psi.
The rocket envelope will need to handle that internal pressure, along the sides and at the rear.

 

[sophiecentaur]

TL;DR Summary: Ramjet

I suggested a 4mm hole in the tip of the nose and 8mm hole at rear which should alow sufficient air cooling ..

I couldn't say whether the idea of a 'ramjet' would apply; it's more complicated than that. Locating two places on the craft which would have high and low pressure would cause internal circulation.

Maybe, if the front and rear orifices were shaped like the basic ramjet design, you could expect ramjet mode but afaik, the ramjet needs to do work, just to cause sufficient air supply. When added, the fuel produces enough for hit and a lot more (propulsion). 500km/h is far short of the supersonic speeds at which ramjets become efficient.

Aerodynamics is not one of those subjects where just intuition works. This may be an example of where it doesn't. All the aerodynamic ducks have to be in a row. (That produced an interesting mental image when I had written it.)

 

[oz93666]

At 500 kph, the ram air pressure will be about 1.7 psi.
The rocket envelope will need to handle that internal pressure, along the sides and at the rear.

Oh ... interesting , 1.7psi... but there is a outlet hole in the rear ...
Trying to figure this out... imagine the rocket shaped plane , 30cm long, with a small entry hole and a larger exit hole , air is heated inside rocket , exit velocity is greater due to expanded heated air ... wouldn't this produce thrust?

 

[jrmichler]

I said this will increase thrust because the air exiting is hotter .. a bit similar to a ram jet ... was I correct about the aditional thrust?

Yes, this is a well known effect. Search terms p51 mustang radiator thrust will bring up several good hits. The Wikipedia article is a good place to start, and suggests that Meredith effect is an even better search term.

 

[Baluncore]

I would expect the efficiency of both cooling and thrust development to be proportional to the temperature difference between the inlet air and the temperature of the electronics.

The length of the internal ducting needs to be minimised to reduce the internal boundary layer drag. That may be a challenge on a long body.

The system will need to be optimised for the altitude and air temperature at which the speed record test will be made. What will that be?

 

[oz93666]

The system will need to be optimised for the altitude and air temperature at which the speed record test will be made. What will that be?

Here's the video for those interested ....

 

[ShadowKraz]

It seems as though the thrust produced would be very minimal, probably not enough to overcome the drag the holes would produce. Or the drag the components needing cooling would produce internally.

 

[sophiecentaur]

At 500 kph, the ram air pressure will be about 1.7 psi.

And the exit hole will reduce that pressure by 'a lot'. It could average out to just a bit above zero excess pressure - to drive the air circulation.
I'm not sure about the energy budget in drones but the energy cost would affect range. Too hard.

 

[Baluncore]

And the exit hole will reduce that pressure by 'a lot'. It could average out to just a bit above zero excess pressure - to drive the air circulation.

The pressure will fall from the inlet to the outlet as the radiator will partly obstruct the airflow, while the heated air will increase in volume. If there is thrust, then I would expect a positive internal pressure to accelerate the air at the exit.

 

[sophiecentaur]

The pressure will fall from the inlet to the outlet as the radiator will partly obstruct the airflow, while the heated air will increase in volume. If there is thrust, then I would expect a positive internal pressure to accelerate the air at the exit.

My point was that you implied an internal pressure of 1.7 on the basis of the pressure at the inlet would compromise the envelope. Is that figure of 1.7 relevant and everywhere inside.?

The main question here is whether a Ramjet system would be worthwhile at the low speed of 500kmph. The cooling effect of the air flowing over the body could be increased with a free path through the body (a tubular shape), forgetting all about any attempt at ram jetting.

 

[Baluncore]

Is that figure of 1.7 relevant and everywhere inside.?

All stagnant space connected to the inlet would be subjected to that internal pressure.
A pinhole, on the front of a hypersonic vehicle, will allow the entry of high pressure hot air that will burst the envelope, or damage the internal structure. That is why a volume of shot is thrown out as a defence, so the incoming missile's velocity can be turned against the missile.

The cooling effect of the air flowing over the body could be increased with a free path through the body (a tubular shape), forgetting all about any attempt at ram jetting.

A tube the length of the body would have high internal surface drag. It would be better to use the outside surface of the body for conductive or radiative cooling, than to have an internal duct that must increase the diameter of the body.

Any radiator housing, external to the body, would need to be short, and provide more thrust than it causes drag.

 

[berkeman]

[Mentor Note: Thread prefix changed A-->I]

 

[oz93666]

It seems as though the thrust produced would be very minimal...

Agreed ... this idea was really for cooling of electronics and battery , the rocket was originally sealed which led to electronics fire , they are now trying water cooling! which seem ridiculouse when a reasonable air flow would do.

 

[ShadowKraz]

Agreed ... this idea was really for cooling of electronics and battery , the rocket was originally sealed which led to electronics fire , they are now trying water cooling! which seem ridiculouse when a reasonable air flow would do.

Or even heatsinks to aerodynamically designed radiators on the outside.