Rocket thrust and ground level

[MarkB]

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.

 

[Bullgrit]

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.

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

 

[Umbran]

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.

 

[Janx]

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.

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.

 

[Pbartender]

...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.)