Rocket thrust and ground level

[Bullgrit]

Is thrust upward (like a rocket) affected by a surface under it?

That is, take a rocket sitting on the launch pad. What if we removed the launch pad under the rocket -- the rocket sits above a bottomless hole. Will it's thrust and launch be affected with nothing under it?

Take that rocket as it is already in flight hundreds of feet in the air. Would putting a surface under/behind it affect it's thrust once it is already going up?

I'm thinking that having a surface under the thrust would increase the thrust's effect, but then I think about this:

I got to thinking about this by thinking about Iron Man's flight. I know IM uses a fictional thrust, but let's pretend his rocket boots are actually rocket boots. Say Iron Man is hovering several hundred feet in the air. He slowly drifts to the side until he is hovering a couple of feet over a flat skyscraper roof. Does he gain altitude because of the surface now under him, or does he stay at the same level?

Writing this up, I just thought of helicopters. Does a helicopter rise (on its own) when it passes over a rising surface (hill or building)?

Bullgrit

 

[Ed_Laprade]

I'd have to say no, but I'm no rocket scientist!

 

[Raven Crowking]

Is thrust upward (like a rocket) affected by a surface under it?

No.


RC

 

[Starman]

Nope. You can read here and here for more info than you probably want.

 

[Thornir Alekeg]

My understanding is that a helicopter works not by downward thrust, but by reducing the air pressure above the rotors in relation to the pressure below the rotors because of the shape and motion of the rotors. It is similar to the way an airplane wing or a sail on a boat work. As a result, passing over a building should have no effect on a helicopter.

My assumpmtion is that a rocket would not lose anything significant from being over a hole rather than a platform. It is the energy of the burning mass leaving the rocket that provides the thrust.

Trying to think in other terms, I think of the way a hose turned on suddenly will kick back in your hand. If you put it against a surface, it will kick harder, but I think that is because pressure builds up in the hose. The same might happen with a rocket, if it were flush with the surface but as soon as the rocket moves enough to allow that pressure to escape, it is back to the power of the burning fuel providing the energy.

 

[Umbran]

Is thrust upward (like a rocket) affected by a surface under it?

In general, no.

For every action there is an equal and opposite reaction. The rocket exhaust flies out the back with great energy, and so the rocket goes forwards with great energy.

Any interactions the exhaust has with ground after that are not relevant, unless the exhaust is reflecting off the ground, and striking the rocket again on the rebound (which is generally not a good thing - most rockets are launched above pits to minimize that kind of backblast).

Helicopters can sometimes take advantage of "ground effect" when flying very low, but normally the ground beneath isn't a concern for them either.

 

[Bullgrit]

...

 

[Man in the Funny Hat]

Is thrust upward (like a rocket) affected by a surface under it?

That is, take a rocket sitting on the launch pad. What if we removed the launch pad under the rocket -- the rocket sits above a bottomless hole. Will it's thrust and launch be affected with nothing under it?

For practical purposes, no. However, rocket propulsion comes from expanding gases. If it were resting on its engine bell on the ground and then the engines fired up the expanding gas has to go somewhere or else... The "or else", I assume, would be to increase the pressure of the gas, exerting greater force against the ground and the bell itself. I think it would then depend on the weight of the rocket itself as to how that would affect the initial acceleration. But, once above a few feet even a Saturn V's F-1 engine would, I assume - no longer have its acceleration affected.

Take that rocket as it is already in flight hundreds of feet in the air. Would putting a surface under/behind it affect it's thrust once it is already going up?

I believe that just as it would when sitting on the ground initially the effect would be insignificant. Newtons law would suggest that the rocket doesn't CARE what's behind it - only that the thrust of the engine itself remains constant.

Writing this up, I just thought of helicopters. Does a helicopter rise (on its own) when it passes over a rising surface (hill or building)?

My trivia serves me well. There is a little-known (to the general public) phenomenon called ground effect.
Ground effect in aircraft - Wikipedia, the free encyclopedia
When at 1 blade length or lower in altitude a helicopter is subject to ground-effect. But, ground effect has to do with the vortices of air created by wings and especially wingtips which is disrupted at very low heights - only VERY close to the ground. Due to the nature of Ironman's boot propulsion I don't think he would be affected, nor would a rocket.

 

[Man in the Funny Hat]

Any interactions the exhaust has with ground after that are not relevant, unless the exhaust is reflecting off the ground, and striking the rocket again on the rebound (which is generally not a good thing - most rockets are launched above pits to minimize that kind of backblast).

IIUC, it's not "backblast" but destructively intense vibration created by swirling exhaust right at the base of the rocket. To eliminate that vibration lauch pads are designed to vent the exhaust well away from the rocket while it rises above the pad. Also, I don't think large rockets are designed to rest their weight upon their engines - they rest the weight upon the frame around it, which would mean that again, you can suspend it up in the air a few feet to facilitate a better exhaust flow. If you watch the close up footage of rocket engines on the pad there is a general cloud of stuff as they start and then everything is sucked down and away by the high-speed flow of gases as it throttles up.

 

[Plane Sailing]

I remember back in the 1970's that there were plans for "Harrier-Carriers" for the UK, and one of the factors which influenced potential designs was the fact that vertical take-off and landing was more efficient when you didn't have solid ground beneath you, so the plan was to allow them to do the VTOL bit on grids, allowing the jet exhaust to vent below them rather than reflect off the ground.

Might not be directly relevant to your question, but included in case you find it interesting.