Landing on regolith

[farmerben]

Engine thrust can blow regolith all over the place, possibly damaging the engine.  A solution to this could be a return to the so called "pendulum rocket fallacy".   Putting the engines higher up reduces the regolith dust.  

In KSP engines like the spider, twitch, and thud are ideally suited for this.   But, does this work in real life?  We also have to keep engine exhaust a safe distance from the hull.  

 

A mushroom shaped lander may be ideal with the crew and payoad in the mushroom stem, the fuel in the mushroom cap.  This not only eliminates the need for ladders and cranes, it offers a thick radiation shield for the crew compartment.  The moon blocks 1/2 of all the radiation, the fuel and oxidizer can absorb nearly all of the rest.

[Terwin]

Starship HLS plans to use  "high‑thrust RCS thrusters located mid‑body to avoid plume impingement problems with the lunar regolith" when within 100m of the lunar surface.

Not really mushroom shaped, and will still need elevators and such, but it sounds like even half way up Starship should be enough to avoid the primary issues.

One thing we do not want, would be plume impingement on the body of the rocket itself, which is something that looks highly likely if we were to return to rockets with the fuel tank directly under the engine.

[TheSaint]

Remember that the Moon has no atmosphere. So when the exhaust plume impinges on the surface, the dust that it kicks up doesn't form a cloud around the lander like it would on Earth. It gains velocity and then just keeps going until it reaches the end of its ballistic arc. You can see this most prominently in the videos of the lunar rovers. Watch the dust coming off of the tires.

The dust just comes up off of the tires and then falls down again. It doesn't linger in large clouds like you see around vehicles driving in dust on Earth.

Same goes with the engine exhaust. The dust is mostly just going to scatter directly away from the engine bell and then settle back down to the surface, not cloud around and settle on the engine, lander, etc.

If you mount your engines above the crew compartment, in a mushroom shape, with the engines around the perimeter of the cap, you make the problem worse not better. Now the engines are impinging the regolith in a circle around the crew compartment and blasting dust in all directions, including back towards the center of the circle where the crew compartment is. You probably won't kick up any debris big enough to do any damage. But if you put the engine underneath the crew compartment then all of the debris goes in the correct direction: away. 

[kerbiloid]

There was a thread about it, and I had written:

The engine exhaust speed ~= 3.5 km/s.

Lunar orbital velocity ~= 1.6 km/s
(To throw stones in orbit).

Lunar escape velocity ~= 2.3 km/s
(To throw stones outside of the Moon at all).

Moon orbital velocity ~= 1 km/s
To reach LEO, or the Earth surface, or LEO in opposite to the satellites direction = 2.3 + 1 = 3.3 km/s

Moon escape velocity ~= 1.4 km/s
To leave the Earth Hill sphere = 2.3 + 1.4 - 1 ~2.7 km/s

Mars transfer insertion velocity ~= 3 km/s
To reach Mars ~= 2.3 + 3 ~= 2.3 + 1.4 - 1 + 3 ~= 5.7 km/s

All of that (but the latter) is < 3.5 km/s of the exhaust speed.
As the jets in any case should be directed towards the lunar surface, they will anyway cause a shot of stone shrapnel which will:
pread around all the Moon;
partially reach LEO (including opposte orbits, where they can hit ISS at 15 km/s speed);
partially start orbiting Earth to fall back on the Moon a little later at random place;
partially even reach Mars and hit Mars, Phobos, and Deimos.

So, any landing on regolith is dangerous for everything around the lander itself, and you can just either decrease ISP below 1.5 km/s (and thus, spend more fuel), or land only on a solid plarform to be first prepared.

Also, even if the jet is tilted from the lander, there are rocks around, so the shrapnel will be ricochetting from the rocks into the lander.

So, you have to provide the landing zone with a landing platform without regolith.
Either concrete, or metal,

Spoiler

or just drop a nuke and land on glass.

But wait...
Isn't the spoiler text exactly that what should be done very first?

[lrd.Helmet]

9 minutes ago, kerbiloid said:

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or just drop a nuke and land on glass.

But wait...
Isn't the spoiler text exactly that what should be done very first?

That doesn't seem like such a great idea.
 

 

[kerbiloid]

12 minutes ago, lrd.Helmet said:

That doesn't seem like such a great idea.

It's not necessary to drop a big, 100 Mt nuke. Let start with a little and comfy one, like Davy Crockett.

It's a pity that 1950s+ sci-fi never mentioned a second navigator opening a window and dropping down a landing nuke.

[farmerben]

I the engines are angled 8 degrees out they still provide 99% downward thrust.  The hull would be protected against rocket exhaust and most of the debris.

 

There is also the problem of engine exhaust being reflected back up when close to the surface.  Landing legs must reach much lower than engines due to this.  

[kerbiloid]

Where the jet touches the ground, the stones will be colliding and playing billiard, spreading up as a wide sheaf of trajectories.

Only the sheaf axis will be at -8° angle. The sheaf itself will be wide, and some of its stones will be flying in retrograde direction, towards the ship side.

Edited May 3, 2022 by kerbiloid