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Author Topic: Hard landing for a lunar rover  (Read 3003 times)

Offline clb22

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Hard landing for a lunar rover
« on: 02/19/2010 12:55 PM »
Does anybody have good material on the concept of a hard lunar landing for a lunar rover? I am not talking about an impactor (2.5km/s) which would mean it's really hard to have any hardware survive, but more of a single "dump" propulsion stage that slows the lander, but doesn't do a controlled soft descend, but shuts down before and hits the ground in the 10-100m/s+ range.

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Offline Rhyshaelkan

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Re: Hard landing for a lunar rover
« Reply #1 on: 02/19/2010 01:27 PM »
 Spirit and Opportunity are good models. Their airbag landing would work well for the Lunar surface. You would need a longer [deceleration]burn on decent to compensate for no aero-breaking, but other then that I think it would be quite similar.
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Offline Bernie Roehl

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Re: Hard landing for a lunar rover
« Reply #2 on: 02/19/2010 01:56 PM »
Does anybody have good material on the concept of a hard lunar landing for a lunar rover? I am not talking about an impactor (2.5km/s) which would mean it's really hard to have any hardware survive, but more of a single "dump" propulsion stage that slows the lander, but doesn't do a controlled soft descend, but shuts down before and hits the ground in the 10-100m/s+ range.

What you're describing is sometimes called a "crasher" stage.

Yes, it can work, but the idea of taking something like a rover (which will have a lot of small, lightweight moving parts) and smashing it into the lunar regolith at 100 m/s seems unlikely to succeed.  Even if you do survive the impact, you'd kick up a lot of surface dust (even more than an engine would, I suspect).  That dust would cling to everything, and would interfere with the rover's moving parts.

I think you'd still need to have a small throttleable engine to cushion the final descent (or use airbags, like the other poster suggested).


Offline Downix

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Re: Hard landing for a lunar rover
« Reply #3 on: 02/19/2010 02:13 PM »
Does anybody have good material on the concept of a hard lunar landing for a lunar rover? I am not talking about an impactor (2.5km/s) which would mean it's really hard to have any hardware survive, but more of a single "dump" propulsion stage that slows the lander, but doesn't do a controlled soft descend, but shuts down before and hits the ground in the 10-100m/s+ range.

What you're describing is sometimes called a "crasher" stage.

Yes, it can work, but the idea of taking something like a rover (which will have a lot of small, lightweight moving parts) and smashing it into the lunar regolith at 100 m/s seems unlikely to succeed.  Even if you do survive the impact, you'd kick up a lot of surface dust (even more than an engine would, I suspect).  That dust would cling to everything, and would interfere with the rover's moving parts.

I think you'd still need to have a small throttleable engine to cushion the final descent (or use airbags, like the other poster suggested).


or a combination.  I still like my design which uses solid rocket motors which fire 15 seconds before impact + airbags.  The combination of them + the shock absorption plates should be enough to keep a rover intact.
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Offline savuporo

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Re: Hard landing for a lunar rover
« Reply #4 on: 02/19/2010 09:43 PM »
Question is, why ? Whats wrong with the landing rockets ? I mean, we could do rocket-powered landing in 1966, we can certainly do it easily now ?
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Offline clb22

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Re: Hard landing for a lunar rover
« Reply #5 on: 02/19/2010 09:49 PM »
Question is, why ? Whats wrong with the landing rockets ? I mean, we could do rocket-powered landing in 1966, we can certainly do it easily now ?

Because softlanding is hard, really hard, from a software, hardware, sensor, control etc. etc. perspective. Just having a "dumb" descend rocket that gives you a predefined amount of delta-v and then just let's the payload drop after it burned out is a LOT easier.

Regarding the point about a "crasher stage" and problem with lunar dust and impact above, we have done hard landings with airbags on Mars (also as previously mentioned). Rovers can be designed to take high g-loads on impact. So, I am less worried about the impact itself and more about the cushion technology we would apply in this case. Can a Mars airbag landing be emulated for the Moon?
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Offline savuporo

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Re: Hard landing for a lunar rover
« Reply #6 on: 02/20/2010 07:27 PM »
Because softlanding is hard, really hard, from a software, hardware, sensor, control etc. etc. perspective.
But thats my point. Apparently software, hardware, sensors and control were enough in 60ies to pull it off, and each of these has come a long way since.
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Offline Hungry4info3

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Re: Hard landing for a lunar rover
« Reply #7 on: 02/20/2010 07:50 PM »
Because softlanding is hard, really hard, from a software, hardware, sensor, control etc. etc. perspective.
But thats my point. Apparently software, hardware, sensors and control were enough in 60ies to pull it off, and each of these has come a long way since.

Yeah but look how many times it failed on both the Soviet and American sides. The lunar landers were thankfully controlled by humans when necessary.

Offline Proponent

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Re: Hard landing for a lunar rover
« Reply #8 on: 02/21/2010 08:50 AM »
Yeah but look how many times it failed on both the Soviet and American sides. The lunar landers were thankfully controlled by humans when necessary.

Just two of seven American unmanned lunar soft-landing attempts failed--Surveyors II and IV--and the likely failure modes were not intrinsic to unmanned vehicles.  In one case, one of three vernier engines failed to ignite, and in the other it is suspected that the solid-propellant retrorocket exploded at ignition.  Similar failures would have been just as fatal for manned landers.

Coping with rough terrain might be a bigger issue for unmanned landers, but I'd have thought that with advances in avionics and the availability of both maps and positional information at very high precision these days, unmanned landing really shouldn't be that tough.  We've had ground-hugging, terrain-reading cruise missiles for two decades now.
« Last Edit: 02/21/2010 09:14 AM by Proponent »

Offline mlorrey

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Re: Hard landing for a lunar rover
« Reply #9 on: 02/21/2010 08:57 AM »
Yeah but look how many times it failed on both the Soviet and American sides. The lunar landers were thankfully controlled by humans when necessary.

Just two of seven American unmanned lunar soft-landing attempts failed--Surveyors II and IV--and the likely failure modes were not intrinsic to unmanned vehicles.  In one case, one of three vernier engines failed to ignite, and in the other it is suspected that the solid-propellant retrorocket exploded at ignition.  Similar failures would have been just as fatal for manned landers.

What are the mass advantages of an active soft landing system vs using an airbag system?

Offline Proponent

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Re: Hard landing for a lunar rover
« Reply #10 on: 02/21/2010 09:33 AM »
Spirit and Opportunity are good models. Their airbag landing would work well for the Lunar surface. You would need a longer [deceleration]burn on decent to compensate for no aero-breaking

I'll bet, though, that the major reason airbags ever looked attractive for Mars is that given the atmosphere there it's possible to make a landing without any major rocket systems (though I think they've all had some rocket component).  And I don't think there are any more airbag Mars missions in the works, are there?  It may be that airbags, even on Mars, have turned out not to be as easy as was expected.

On the moon you're going to need a big rocket of some sort anyway, so there's less to be gained by not using rocket braking all the way down to a soft landing.  Now, I've never tried to design a small lunar lander with rover, so I'm not going to argue that airbags aren't the way to go, but I suspect that their past use on Mars may not be very relevant.
« Last Edit: 02/21/2010 09:36 AM by Proponent »

Offline khallow

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Re: Hard landing for a lunar rover
« Reply #11 on: 02/21/2010 05:54 PM »

And I don't think there are any more airbag Mars missions in the works, are there?

They work well for missions of a particular size. Given that NASA isn't officially developing more missions to the surface of Mars, it really doesn't say much.
« Last Edit: 02/21/2010 06:03 PM by khallow »
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Offline Proponent

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Re: Hard landing for a lunar rover
« Reply #12 on: 02/22/2010 12:31 AM »
Does anybody have good material on the concept of a hard lunar landing for a lunar rover?

I realize that I'm probably telling you something you already know, but have you looked at the Ranger Block II hard-landing capsules?  Of course, they had crushable balsa wood shells rather than airbags, but they might still be a starting point.  The NASA history website has an in-depth book on the history of Ranger.
« Last Edit: 02/22/2010 12:31 AM by Proponent »

Offline Proponent

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Re: Hard landing for a lunar rover
« Reply #13 on: 02/22/2010 03:48 AM »
Despite my previously-expressed skepticism about the value of airbags in general and on the moon in particular, the more I think about them, the more interesting they seem.  For a landing on a relatively smooth, airless body, the impact trajectory could be highly oblique.  This, of course, reduces the vertical component of deceleration on impact at the cost of some combination of horizontal deceleration and spinning the craft up.  For a vertical impact at speed v, the deceleration time is on the order of r/v, where r is the size of the airbag or other impact absorber.  Therefore the peak acceleration is on the order of v/(r/v) = v2/r.

For a horizontal impact on the other hand, the deceleration could be much lower if, as seems likely, the distance over which the craft slows down is much larger than its own size.  On the other hand, the lander will tend to spin up.  In the worst case, it would spin up to the point that the rotation velocity at its surface matches the impact speed.  In this case, the centripetal acceleration at the surface of the airbag is v2/r.  Since that's the same as the peak acceleration in the vertical-impact case, at first glance it seems we haven't gained anything.  But if the bag is larger than the payload which it protects, then the centripetal acceleration experienced by the payload is lower by the ratio of its size to the bag's, r.

The payload might even be free to rotate independently the airbag, say by floating it in a low-viscosity liquid (to take it to the extreme, use superfluid helium-3!).  This would partially decouple it from the bag's angular momentum, reducing the centripetal acceleration.  I believe the Ranger payloads were floated in a liquid, although the purpose of this was to allow the payload to right itself after landing, regardless of the orientation in which the lander finally came to rest.

Of course, a nearly horizontal impact might just introduce a few teensy-weensy problems of its own :).  For one thing, the lunar surface is not perfectly flat and glassy smooth, so our horizontal lander might need to withstand bumping into a few rocks or hillsides.  There will be a lot of frictional heating on the skin of the bag.  An oblique impact imposes different trajectory constraints than does a vertical impact.

I wonder if there might be something to be gained by spinning up the bag before impact.  Spinning it forward would reduce the initial friction at the cost of increasing the stopping distance, and spinning it backward would do the opposite.

EDIT: Added third paragraph, concerning freeing payload to rotate.
« Last Edit: 02/22/2010 06:52 AM by Proponent »

Offline mlorrey

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Re: Hard landing for a lunar rover
« Reply #14 on: 02/22/2010 04:41 AM »
Spin up is rather easy, you simply have the parachute pack have a lanyard that wraps around the uninflated package that lets the package roll out. On the last rotation, the bags inflate before the parachute system separates (to verify inflation before separation), and at sep the probe now has some significant spin on it.

The only downside is you dont have much choice over where the inflated package comes to a rest unless you are aiming for a specific crater that is large enough to treat as the golf hole, as one of the mars rovers achieved.

If you are shooting for one of those craters that seems to have a bunch of ice in the floor, they seem to be a few miles wide so aiming for that sort of a hole isn't a really big deal.

If the bag material was really tear resistant (which will be tough if you are landing on the moon) you could use some small rockets to spin it up before landing (if in vacuum) in counterrotation or "english" so the spin kills downrange velocity like a chipped golf ball landing on a green.

This all said, spin is not really necessary for a landing, neither mars rover had any spin.
« Last Edit: 02/22/2010 04:42 AM by mlorrey »

Offline clb22

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Re: Hard landing for a lunar rover
« Reply #15 on: 02/22/2010 08:18 AM »
Does anybody have good material on the concept of a hard lunar landing for a lunar rover?

I realize that I'm probably telling you something you already know, but have you looked at the Ranger Block II hard-landing capsules?  Of course, they had crushable balsa wood shells rather than airbags, but they might still be a starting point.  The NASA history website has an in-depth book on the history of Ranger.

Thank you for the tip. I knew that Rangers 3-5 included an instrument that should have survived the impact, but considering that all 3 missions (including Ranger 5 that gathered some data) failed I didn't look further into that.

Seems like the mission profile for the seismometer + transmitter part of the Ranger Block II series were floating on a layer of freon within a sphere (made of balsa wood as you said). Impact speed was estimated between 130km/h and 160km/h. The flight profile was pretty simple, a radar altimeter would initiate the seismometer capsule separation and retrorocket ignition at some point only 19 to 25km away from the lunar surface. The Ranger spacecraft would meanwhile hardimpact the surface. The retrorocket for the capsule was a solid rocket motor. The structure can be clearly seen mounted on top of Ranger 4 in the picture I attach (total mass of sphere + retrorocket ~100kg; mass of scientific payload within protective sphere: 3kg).

I think for a extremely basic, low-cost mission (Google lunar challenge etc.) such a mission profile could make sense. Landing a 4-5kg rover within a protective shell (doesn't even have to be an airbag, just cushioned within a lightweigh shell by either a liquid or some other kind of protection) and a simple as possible retro-rocket motor (be it based on monopropellant or even a solid) could mean the focus of the mission could be shifted to the rover development and away from a descend stage development and testing. After all, the whole descend mission plan involves only an off the shelf radar altimeter triggering the retrorocket at a predefined moment and a protective shell. No complicated throttling, no required Earth teleoperations, no precise landing manoeuvres, no extensive testing of softlanding on Earth, not even software is required.
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Offline Proponent

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Re: Hard landing for a lunar rover
« Reply #16 on: 02/22/2010 08:53 AM »
What are the mass advantages of an active soft landing system vs using an airbag system?

Good question.  Just doing a BotE estimate from basic physics, here's what I come up with.  Let the kinetic energy of the payload of mass mpay just before impact at velocity v be U ~ mpayv2.  To bring the lander to a halt, the bag must do this much work on the lander.  The work done by a bag of size L is on the order of the force exerted multiplied by distance traveled, i.e., PL2L ~ PL3, where P is the pressure in the bag.  Assume that on impact the bag compresses to something like half its original volume and not to a volume that's an order of magnitude less than initial.  Then even with adiabatic compression, we don't need to worry about whether P represents initial or maximum pressure, for an order-of-magnitude estimate.

I'll spare you the gory details, but the mass of the gas bag turns out to be mbag ~ mpayv2/cbag2, where mpay is the mass of the payload to be landed, cbag2, which has the dimension of velocity squared, is the strength of the bag material divided by its density.  For a strength of 100 MPa (typical for things like nylon, according to Wikipedia) and a density of 1000 kg/m3, cbag ~ 300 m/s.  This assumes that the mass of the landing system is not big compared to that of the payload.  I'm sure that's not a great assumption, but for an order-of-magnitude estimate to be used for comparative purposes it probably won't lead us far astray.

The expression for the mass of the gas in the bag takes a similar form.  The ideal gas equation can be written such that the gas density equals the pressure divided by the square of the isothermal sound speed, cgas2 = RT/mmol, where R is the ideal gas constant, T is the absolute temperature and mmol is the mean molecular mass of the gas.  Grinding through the details produces mgas ~ mpay(v/cgas)2.  If the gas is air at 300 K, then cgas ~ 300 m/s--coincidentally about the same as cbag.  If the gas is helium or hydrogen, then cgas rises by a factor of two or three.  In any case the mass of the landing system is, to an order of magnitude, that of the bag alone.

To summarize, the mass of the gas bag system per unit mass of payload to be landed is on the order of mbag/mpay  ~ v2/cbag2, where cbag2 ~ 105 m2/s2.

Now consider a rocket-based soft-landing system.  Again assume that it is relatively light compared to the payload (the rocket need handle only the relatively low-velocity terminal landing maneuver).  The impulse that the rocket must deliver is equal to the lander's pre-ignition momentum: mpayv.  If the rocket engine has exhaust velocity cexh, typically a few thousand meters per second, then the mass of propellant needed is mpayv/cexh.  In other words, the mass of propellant per unit payload mass is v/cexh.

The mass of the gas bag system scales as the square of velocity, whereas that of the rocket system scales linearly.  Hence below some velocity vcrit, the bag weighs less, and above vcrit it weighs more:  (vcrit/cbag)2 ~ vcrit/cexh, hence vcrit ~ cbag2/cexh ~ (105 m/s)/(3000 m/s) ~ 30 m/s ~ 100 km/h.

Now there are lots of finger-in-the-air numbers and approximations in this guesstimate, but the order of magnitude seems reasonable.  It's interesting that it corresponds to the Ranger capsule's impact speed, even if that capsule was to be cushioned by balsa wood rather than a gas bag.

Does anybody know the impact speeds of Mars Pathfinder or the MERs?

EDIT: Corrected the density of nylon-like materials, fixed a couple of typos.
« Last Edit: 02/23/2010 01:42 PM by Proponent »

Offline clb22

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Re: Hard landing for a lunar rover
« Reply #17 on: 02/22/2010 09:35 AM »
Does anybody know the impact speeds of Mars Pathfinder or the MERs?

Great post above. Very good explanation and analysis.

In 2002, JPL assumed the following impact survival conditions for the MERs. Those conditions are of course maximum thresholds, I believe the actual impact velocity data is not known (wasn't measured):
- Total impact velocity < 24 m / s @ no rocks, 16 m / s up to 0.7 m rocks
- Normal impact velocity < 14 m / s (loads and stroke out)
- Tangential impact velocity 21 m / s @ no rocks, 14 m / s up to 0.7 m rocks
- Grazing angle of impact > 30 or total impact 10 m / s
« Last Edit: 02/22/2010 09:38 AM by clb22 »
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Offline savuporo

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Re: Hard landing for a lunar rover
« Reply #18 on: 02/23/2010 03:59 PM »
I remember the discussions about supposed advantages of airbags for landing over at sci.space.tech a long time ago.
See this thread for example, and comments by Henry Spencer in particular.

Bottom line seems to be, that airbags dont win anything over rockets in terms of mass savings, while bumping up complexity immensely, and they do not allow precision landings.
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