# Braking spacecraft in a snow-filled tank?

#### trurle

Reading recently a Russian MOD aerospace journal, i stumbled upon (pretty unrealistic) scheme to hard-land payloads (metal ingots) on moon.
It involve catching 1.7 km/s slugs with the pipe filled by regolith or water, and even more hardcore option of digging up the payloads embedded in the Moon`'s natural regolith layers.
Well, the approach and materials have severe problems.
1) Braking pressures are too high (~0.3 GPa for water, and ~0.4 GPa for fine regolith) - therefore for most payloads soft-landing is more mass-effective option even assuming the payload is surviving.
2) Regolith is too good heat insulator, resulting in large cycle time and may be even regolith sintering/fusing to payload
3) Water is poorly compressible, producing a large pressure spike (water hammer effect)

After thinking a bit, seems "payload trap" filling with loose (40 kg/m3) snow may be much better.
a) The braking pressure is much lower at ~12 MPa
b) The over-pressure pulse is partially dissipated by breaking and melting snowflakes, transmitting less energy on "payload trap" walls.

Severe doubts are remaining. Could anybody help with the items below?
1) Penetration model is very far extrapolation (from the high density bullets striking sand bags to large low-density containers striking snow at double velocity). Is any relevant software/equation for estimating pressures in such impact scenario? I suspect 12 MPa braking pressure for 40 kg/m3 snow at 1.7km/s is not an accurate estimation.
2) Payload container veering off course and colliding with trap wall may be a problem. I can imagine fin-stabilized container self-guiding in core of denser snow supported by less dense snow envelope, but would it be sufficient? Any thoughts about path stabilization at such violent braking?
3) Is it plausible to make low-density snow similar to natural one? Typical snow-making machines which are designed for high throughput produce snow about 450 kg/m3, which is likely too dense for purpose. Ice block grinding method produce snow of ~250 kg/m3. Are any methods for lower-density snow?
4) Is the input shutter to prevent large loss of braking medium (snow?) necessary? Any design alternative to shutter? Need to prevent somehow braking medium from being ejected from input aperture each time a container is received.
5) Likely still need about 1-3% of container to be hydrazine for deorbit from storage orbit and fine guidance. Any chance to develop a fully inert container still hitting a small (few meters at most) aperture?

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#### mfb

Mentor
How did you calculate the stopping distance (or, closely linked, the acceleration)?

#### trurle

How did you calculate the stopping distance (or, closely linked, the acceleration)?
Yes. It was not in the summary sheet because it was an array depending on transfer orbit.
Roughly 235 calibers (for 3 g/cm3 spherical containers) for low orbits.

#### mfb

Mentor
Where does that number come from?

Having an unguided bullet approach a base at orbital speeds with an error margin of a few centimeters doesn't sound very safe, by the way.

#### trurle

Where does that number come from?

Having an unguided bullet approach a base at orbital speeds with an error margin of a few centimeters doesn't sound very safe, by the way.
I use for penetration estimation equation
PenetrationCalibers=2*(2800/DensityOfBrakeMedium)*(DensityOfContainer/9000)*(Speed/750)^2

Regarding guidance margin, i also think the container must be guided. Even in that case, with current technologies margins may be meters.

#### mfb

Mentor
That formula can't work well for high speeds. I would expect 1.7 km/s to be high in that context. It is also weird that it uses the width not the length of the projectile.

#### trurle

That formula can't work well for high speeds. I would expect 1.7 km/s to be high in that context. It is also weird that it uses the width not the length of the projectile.
The measurement of penetration in projectile widths is common in ballistics. Anyway, for this particular calculation the projectile (container) is spherical therefore width and length are the same.
Apart from generic doubts on accuracy (which i also have) do you have better penetration equations?

#### mfb

Mentor
I don't have better equations. 1.7 km/s is too fast to use formulas for typical guns but too slow for high velocity approximations (e.g. what matters for most impacts on the ISS).

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