Is engine thrust noticeably increased near the ground?

Question

I know next to nothing about rocketry and space flight. I assume that the majority of the force generated by a rocket's engine is simply the result of throwing gasses out of the bottom very fast, and relying on Newton's third law of dynamics.

But when the rocket is near enough to the ground for the exhaust to be meaningfully interacting with the surface (e.g. during take off, or during SpaceX's booster landings) does the engine thrust increase to any significant degree? Perhaps due some of the gaseous particles hitting the ground and then bouncing back upwards?

If yes, can you give any indication of HOW much? Is it enough that launch/landing mechanics need to take it into account, or is it no more significant than, say, air densities/pressures varying due to temperature or breezes?

Answer 1

There is very little, if any, ground effect at the launch of a large rocket. Having those hot gases bounce off the ground and back onto the rocket might well spell the start of a very bad day for the rocket. The exhaust from a launching rocket is directed away from the rocket by a flame trench to militate against these potentially very bad effects.

There might be some ground effect thrust enhancement (or possibly dis-enhancement) during the controlled landing of a rocket under thrust, e.g., on the Moon or on a barge. These effects will generally be extremely transient in nature, and hence can be pretty much ignored.

Answer 2

In the context of your question, whether there is a thrust-enhancing ground effect either during take-off or terminal landing, the answer is apparently "no".

Citing "Mars Exploration Entry, Descent and Landing Challenges" (2006) by Braun and Manning:

"Terminal descent thrusters can not spend any more than a few hundreds of milliseconds within a meter or so of the surface without digging trenches, launching small rocks into the landing gear and producing destabilizing ground effect backpressure on the bottom of lander."

Also, from "Tradespace Model for Planetary Surface Exploration Hopping Vehicles" (2012) by Philip M. Cunio:

"After this, development proceed with the production of a quick-release launch stand, which minimized stand dynamics and ground effect during launch. This enabled launch and hover testing, as, shown in Figure E-9."

(These are verbatim quotes from the papers, grammatical and punctuation errors intact.)

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I found two other papers which may contain relevant information ("Development of a terminal landing rocket system for Apollo-type vehicles" by Jack F. Lands Jr. 2012, and "Investigation of Effect of Ground Downstream of Overexpanded Dual-Bell Nozzle During Vertical Takeoff and Landing" by Yonezawa et al 2010), but haven't read either of those, and the previous two citations seem to give us the answer.

You obviously know about aircraft ground effect. Helicopters also experience an "in ground effect" increase in performance when hovering somewhere within one main rotor diameter of the ground. This is because the rotor wash compresses the air beneath the rotor, increasing the air density and allowing for an increase in lift characteristics for the "rotary wing" in a similar way that a fixed-wing aircraft experiences an increase in lift when close to the ground.

Rockets don't experience this effect in the same way because they do not work on the principle of aerodynamic lift. The turbulence generated by the thrust serves to destabilize the vehicle rather than enhance its performance.

Cool question.