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Lunar Lithobraking

If your goal is simply to deliver a science package somewhere on the moon, might it be easier and cheaper to design the package to survive lunar impact? As an example, consider the Lunar X Prize, with the goal to land a device that can transmit HD video back to Earth.

The question has two parts.

  1. Is it possible to engineer a package to survive lunar impact intact?
  2. Would it be cheaper (in mass, Δv, and/or $) to use this method of landing over retrorockets?

Lunar Impact

Thoughts

Advantages:

  • Much lower mission Δv (-3km/s?).

Disadvantages:

  • High peak force on package during lithobraking.
  • Lower precision on final destination.
  • Dust thrown by impact may cover package.

Could use spent rocket and fuel tank from trans-lunar injection as a crumple zone to reduce peak deceleration. Springs, airbags, foam, etc could further lengthen impact time, reducing shock.

Research

Ranger Program – NASA's balsa wood moon crasher

Ranger Program – newspaper images

Airbag landers scale as v^2 while rockets scale as v.

SSE: Marginal Cost of landing on Moon

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    $\begingroup$ Depends on what do you mean with "science package"? There's e.g. planetary penetrators, research is ongoing and apparently they're testing for feasibility of loading them with epoxy resin protected science package and shooting them at high velocity into rubble piles (mimicking lunar regolith) to test their high gee impact survivability... I believe the tests are promising. But it's not really a door-to-door shipping system. Would you please clarify what exactly you have in mind? Cheers! $\endgroup$
    – TildalWave
    Jun 21, 2014 at 16:22
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    $\begingroup$ Funniest neologism I ever heard :-) $\endgroup$
    – peterh
    Sep 3, 2014 at 12:57
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    $\begingroup$ lithobraking "simulation"` youtube.com/watch?v=ZCBttJUu0Zk $\endgroup$
    – uhoh
    Jan 31, 2019 at 1:43
  • $\begingroup$ more lithobraking experiments i.stack.imgur.com/RmiSL.jpg from here $\endgroup$
    – uhoh
    May 19, 2019 at 8:34

2 Answers 2

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Fundamentally, all landers eventually use lithobraking - it's called touchdown, and the last few m/s are shed that way. Apollo lunar lander landing gear were design rated for a maximum of 5 feet per second vertical velocity at touchdown.

High Velocity Impact

Currently, artillery electronics can be hardened for short duration 30000 Gee accelerations according to public data; it's almost a certainty that the actual numbers are higher. Similar decelerations are acceptable provided the electronics are designed for that.

Typical science packages are not going to be able to survive that. Certain science packages might be able to do so, and plowing into a 100m long skid (or penetration) from 300m/s is shedding 45kj/kg in 0.6 sec or so, and about 6900 G's.

In order to survive this, special construction techniques are needed, and the types of experiments are limited severely.

Lithobreaking as Sole Method

The speeds needed for orbit are high enough that lithobraking is implausible for a a sole method even for the best hardened projectile electronics.

For translunar or more distant missions, the speeds and energies are higher still, and a shallow graze would result in a skip into orbit or past the target.

Lithbreaking as final process

The Pathfinder Rover is considered to have used Lithobraking via it's airbag bounce landing. This was used to shed a 14 meters per second at 18 G peak impact - too high for human safety, but well within human survivability. And, since it performed multiple science activities, easily within the realm of delivery for science payloads.

Similar systems could be used on the moon, albeit with much longer runs and higher bounces.

Price and Mass

The two competing issues are price and mass. For the Pathfinder rover, it was competetive; I've read (but cannot cite) that it was more expensive than rocket, but thought to be more likely to succeed. It was not more mass-efficient, but wasn't severely higher, and offered a number of other failure mode advantages.

The system was not practical when scaled up for larger rovers - both mass and price resulted in a return to thrust based.

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  • $\begingroup$ No, solid rockets and airbags were chosen because they were cheaper than the throttled rocket engines and legs used by the previous Viking missions. Before Mars Pathfinder landed, it was considered by those outside the project to be less reliable than the Viking approach. Many considered it to be nuts. Only after it worked was it considered a robust approach. $\endgroup$
    – Mark Adler
    Jun 24, 2014 at 5:17
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    $\begingroup$ By the way, the OP's question about "lithobraking" is not asking about taking out the last few m/s or even 100's of m/s. The question is explicitly about avoiding the use of retrorockets entirely, in order to eliminate the 3 km/s and associated cost and mass. $\endgroup$
    – Mark Adler
    Jun 24, 2014 at 5:21
  • $\begingroup$ The Mars landers shed a lot of velocity by aerobraking. Aerobraking isn't an option on the moon. Lunar impact velocity would be much higher. $\endgroup$
    – HopDavid
    Jun 25, 2014 at 7:21
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  1. No
  2. Yes

Though 1. depends on your "science package". If your "science package" is simply a large chunk of mass whose purpose is to vaporize and eject water from the lunar regolith (see LCROSS), then yes, it will "survive" in the sense of serving its purpose.

I am not aware of any penetrator designs with actual electronic science instruments and telecommunications equipment that have been shown to survive 2 to 3 km/s impacts, which would be required for an unassisted impact on the Moon. Penetrators are designed for hundreds of m/s, so you would need a rocket to remove about 99% of the energy before you could expect even a penetrator designed specially for that purpose to survive.

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  • $\begingroup$ @TildalWave I'm asking this as it relates to the Lunar X Prize, ie land a device that can transmit HD video. Military electronics can be survive 15500g's. That would mean decelerating from 3km/s in .02 seconds and would require 119 meters of stopping distance. Looks like we need about 40x more robust electronics... Mark Adler can you elaborate on #2? $\endgroup$
    – mLuby
    Jun 22, 2014 at 6:37
  • $\begingroup$ If you could somehow get something to still work after impacting, it would certainly be much less $\Delta V$ and mass. As for cost, that would depend on how much it cost you to develop and test the thing that still works after impacting at km's/s. $\endgroup$
    – Mark Adler
    Jun 22, 2014 at 7:25
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    $\begingroup$ @MichaelLuby Please include that information in your question. The point of my comment wasn't to answer it, but to help you identify issues with it so you can improve it and get better answers. That it's about Lunar X Prize does identify specific requirements well enough (although linking to and/or quoting question-specific requirements in it would be even better), for example, you wouldn't want that HD cam to remain buried deep in the lunar regolith after ballistic impact (or, tongue-in-cheek, "hard landing"). Thanks! $\endgroup$
    – TildalWave
    Jun 22, 2014 at 10:18
  • $\begingroup$ Good point TildalWave, I've updated the question to include that info. @Mark Adler, that's my feeling as well. Do you have any sources to back up our instincts? As for testing, I know NASA has a railgun they use for testing components at orbital velocities (not that that means it's cheap!). $\endgroup$
    – mLuby
    Jun 23, 2014 at 13:49
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    $\begingroup$ Cool, but that's a proposed, not existing rail gun, and it is a Navy gun, not a NASA gun. They are just proposing to put it on a NASA facility. Also it is intended to test the use of an EM gun to fire inert projectiles at 2-2.5 km/s for the purpose of destroying targets. Not for testing components. Though maybe someday someone will try that. $\endgroup$
    – Mark Adler
    Jun 24, 2014 at 5:31

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