Criteria for Rough-Field Landings

The image in Figure 1 is some kind of artificially-generated image that I found on LinkedIn.  It shows what could happen with a SpaceX “Starship” variant trying to make a rough-field landing on the moon,  when not properly equipped to do so.  It digs in unevenly and topples over,  and a rocket falling over is a guaranteed fatal explosion! 

This sort of thing is a definite risk,  because the lunar regolith is a weak “soil”,  quite unable to support large loads concentrated upon small areas.  It actually rather-closely resembles Earthly sand-dune sand.  The rocks in it that do not touch each other,  cannot reinforce its strength.  (The same is true of most Martian regolith.)

Figure 1 – Concept Image of Vehicle Not Configured for Rough-Field Landings

The "how to design for rough field landing" image in Figure 2 shows what I estimated for a lunar "Starship" variant properly equipped for a rough-field landing on the moon.  A rough-field design for Mars would be similar,  but the numbers would be different,  and it would be very difficult to protect such externally-mounted legs during Mars atmospheric entry. 

Figure 2 – Options for Adding Rough-Field Capability to Lunar “Starship” Variants

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There are three critical design criteria a rough-field lander must meet: 

(1) The transient pressure underneath the pads (or other contact surfaces) during the touchdown event cannot be allowed to be any greater than the bearing strength of the lunar regolith,  which is rather similar to Earthly sand-dune sand.

(2) The minimum span across the polygon created by the landing leg outer contact points must exceed the height of the vehicle center of gravity above the surface. 

(3) The pads need to “tip toward the center”,  so that the lead pad cannot “dig in” if there is horizontal velocity at touchdown.

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These three critical design criteria were amply demonstrated to be appropriate,  by the Apollo LM lander,  and by the Surveyor probes before it.  There are too many today who ignore,  or never learned,  these well-established criteria.  It shows in the recent overturned commercial lunar landers.

There is an image of the "Blue Ghost lander" in Figure 3.  It is the Firefly Aerospace commercial lander design that was actually quite successful landing on the moon.  Note the squat low form relative to the landing leg pad span,  and the large size of the landing pads,  that do indeed “tip” toward the lander body in the center.  It meets the same criteria by which the Apollo LM and the Surveyor probes were designed.  So,  its success at a rough-field landing should not be much of a surprise.

 

Figure 3 – Image of the Firefly Aerospace “Blue Ghost” Lunar Lander

The take-home lesson here is simple:  if making rough-field landings upon the moon (or Mars),  the lander vehicle design must meet the three critical design criteria listed and highlighted above. 

Vehicle designs that do not meet these criteria should not be sent to the moon (or Mars) until a properly-constructed,  hard-surfaced,  and very strong landing pad has been built there to receive them!  How to build such landing pads in such hostile places is the topic for a future article. 

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