# Rocket thrust and ground level



## Bullgrit (Jun 15, 2010)

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


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## Ed_Laprade (Jun 15, 2010)

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


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## Raven Crowking (Jun 15, 2010)

Bullgrit said:


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




No.


RC


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## Starman (Jun 15, 2010)

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


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## Thornir Alekeg (Jun 15, 2010)

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.


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## Umbran (Jun 15, 2010)

Bullgrit said:


> 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.


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## Bullgrit (Jun 16, 2010)

...


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## Man in the Funny Hat (Jun 16, 2010)

Bullgrit said:


> 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.


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## Man in the Funny Hat (Jun 16, 2010)

Umbran said:


> 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.


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## Plane Sailing (Jun 16, 2010)

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.


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## MarkB (Jun 16, 2010)

Incidentally, "ground effect" is what makes hovercraft hover, but its effectiveness diminishes rapidly if the reflected updraft isn't channelled back into the base of the vehicle, which is why hovercraft have skirts (the skirt provides no actual structural support, it simply keeps the air contained).

There might be a minor ground effect as the rocket lifted off, but it would be almost imperceptible (even aside from the lack of skirt and the fact that the exhaust gases are being channelled away, a conventional rocket presents only a miniscule fraction of its surface area towards the ground), and would taper off to nothing as soon as the rocket achieved a little altitude.


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## Bullgrit (Jun 17, 2010)

Plane Sailing said:
			
		

> 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.



I think it's interesting and relevant.



			
				MarkB said:
			
		

> Incidentally, "ground effect" is what makes hovercraft hover



So a hovercraft running off the end of an aircraft carrier would plummet. But a helicopter doesn't. (I just remembered that some helicopters are based on CVs.)

Bullgrit


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## Umbran (Jun 17, 2010)

Man in the Funny Hat said:


> IIUC, it's not "backblast" but destructively intense vibration created by swirling exhaust right at the base of the rocket.




I was using the term more as colloquial description than to describe specific technical effects.

Yes, vibration is a problem.  Also that exhaust is bloody hot, and can damage the body of the vehicle.


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## Janx (Jun 17, 2010)

Umbran said:


> 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.
> 
> ...





This.

I Am Not a Rocket Scientist, so my next comments should only be taken as somebody who has watched a few Discovery channel shows... 

Rockets don't push against the ground with their exhaust for propulsion.  They rely on Newton's laws.  

this is proven by rockets in space travelling to the moon.  The exhaust at some point is most assuredly not point at anything and pushing, yet still has effect.

the more practical example is sitting in a small boat and throwing balls off it.  The boat will be propelled in the opposite direction as the ball.

As for helicopters and hovercraft lift mechanisms (and ignoring ground effect discussions a bit):

hovercraft get their lift by directing air in high downward force.  The skirts and such do probably rely on ground-effect and such.

A helicopter, though its blades may generate a downward wind (like a ceiling fan) is relying on a different effect.  Here's where I'm not fully certain of which mechanism it uses...

the rotors are either trying to "screw" into the air to provide lift, which i don't think is likely, or like an air plan wing, are trying to create a high and low pressure differential that causes lift.

Given that rotors also seem to be able to change their pitch, this would seem to support the cork-screw effect, but I'm pretty sure that was never a viable flight mechanism.

Somebody else can probably provide clarity on the nature of helicopter lift.


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## Pbartender (Jun 18, 2010)

Janx said:


> ...or like an air plan wing, are trying to create a high and low pressure differential that causes lift.




This.  Helicopter rotors are airplane wings that spin in a circle.  (For that matter, most airplane propellers work the same way.)


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