Soft landing on the moon discussion

[Pyrotex]

The conclusion that a low, fast Lunar orbit makes landing more difficult than a high, slow orbit --- does that include the Potential Energy of the two orbits? You need delta-V to reduce your velocity, of course, but you ALSO need delta-V to reduce the "virtual velocity" that you would have built up from "falling" from your orbital altitude --- your Potential Energy.

Airbag impacts can protect a human in a car traveling at 60 mph or 88 ft/sec. That would be about 30 m/sec. It is likely we could build an airbag/lander system that could manage that. I suggest 40 m/sec as the maximum perpendicular impact velocity we should even consider.

On Earth, our gravity is 32 ft/sec/sec or about 10 m/sec/sec. At 1/6 of that for Lunar gravity, velocity would increase at about 1.7 m/sec/sec. So, from an initial velocity of zero relative to ground, that would take a fall of 23.5 seconds. This would correspond to a fall from a height of 23.5 sec * 1/2 * 40 m/sec = 470 meters.

So, our decelleration stage should cut in to burn off all actual velocity and bring the lander to a dead stop within 470 meters of the surface. The lander disconnects from the dec.stage, which burns another few seconds to insure that it does not land on TOP of the lander. During the 23.5 seconds (max) it takes to fall, the lander deploys it airbag system.

We should keep horizontal residual velocity to a minimum, just at the Mars Landers did. Rolling really fast can build up rotational accellerations and rates way too high to maintain structural integrity. Ideally: a straight vertical drop to the surface.

[TheBigDog]

Originally Posted by Pyrotex

The conclusion that a low, fast Lunar orbit makes landing more difficult than a high, slow orbit --- does that include the Potential Energy of the two orbits? You need delta-V to reduce your velocity, of course, but you ALSO need delta-V to reduce the "virtual velocity" that you would have built up from "falling" from your orbital altitude --- your Potential Energy.

Airbag impacts can protect a human in a car traveling at 60 mph or 88 ft/sec. That would be about 30 m/sec. It is likely we could build an airbag/lander system that could manage that. I suggest 40 m/sec as the maximum perpendicular impact velocity we should even consider.

On Earth, our gravity is 32 ft/sec/sec or about 10 m/sec/sec. At 1/6 of that for Lunar gravity, velocity would increase at about 1.7 m/sec/sec. So, from an initial velocity of zero relative to ground, that would take a fall of 23.5 seconds. This would correspond to a fall from a height of 23.5 sec * 1/2 * 40 m/sec = 470 meters.

So, our decelleration stage should cut in to burn off all actual velocity and bring the lander to a dead stop within 470 meters of the surface. The lander disconnects from the dec.stage, which burns another few seconds to insure that it does not land on TOP of the lander. During the 23.5 seconds (max) it takes to fall, the lander deploys it airbag system.

We should keep horizontal residual velocity to a minimum, just at the Mars Landers did. Rolling really fast can build up rotational accellerations and rates way too high to maintain structural integrity. Ideally: a straight vertical drop to the surface.

What about the Ralph Kramden method of straight to the moon. Never enter lunar orbit. Just aim for the center of the moon with just enough energy for its gravity to pull us out of earth orbit. Then brake as needed for soft landing. Is that an option?

Bill

[Janus]

Originally Posted by TheBigDog

What about the Ralph Kramden method of straight to the moon. Never enter lunar orbit. Just aim for the center of the moon with just enough energy for its gravity to pull us out of earth orbit. Then brake as needed for soft landing. Is that an option?

Bill

You don't want to aim straight for the Moon, but for a near miss. Put the lander in a minimum energy transfer orbit that leaves it just short of the Moon, but within its impact parameter. The Moon will deflect the lander into a hyberbolic trajectory that will intersect the surface. I'm working on such a trajectory now. I 've got an early estimate of the delta v needed for such a trajectory from LEO. I've still need to work out what kind of delta v we need to kill for a soft landing.

[Pyrotex]

Originally Posted by TheBigDog

What about the Ralph Kramden method of straight to the moon. ...

Janus is right.

There are four things that the human mind was simply not evolved to understand:

Compound interest
Probability
Orbital Mechanics
Women

[TheBigDog]

Originally Posted by Janus

You don't want to aim straight for the Moon, but for a near miss. Put the lander in a minimum energy transfer orbit that leaves it just short of the Moon, but within its impact parameter. The Moon will deflect the lander into a hyberbolic trajectory that will intersect the surface. I'm working on such a trajectory now. I 've got an early estimate of the delta v needed for such a trajectory from LEO. I've still need to work out what kind of delta v we need to kill for a soft landing.

Excellent!

[Kayra]

I would still go with this type of a landing system((My post in other thread)

The smaller rockets act as a parachute. Not only as decelerators but they would also have the same effect as a parachute in that they keep the bottom facing the moons surface. This means that only the bottom of the lander would require inertial absorption material.

Question: How would we get rid of this material after landing so that the rovers would not get tangled in it?

[Qfwfq]

Originally Posted by Pyrotex

The conclusion that a low, fast Lunar orbit makes landing more difficult than a high, slow orbit --- does that include the Potential Energy of the two orbits? You need delta-V to reduce your velocity, of course, but you ALSO need delta-V to reduce the "virtual velocity" that you would have built up from "falling" from your orbital altitude --- your Potential Energy.

Of course I was counting potential energy! As you say:

Originally Posted by Pyrotex

We should keep horizontal residual velocity to a minimum, just at the Mars Landers did. Rolling really fast can build up rotational accellerations and rates way too high to maintain structural integrity. Ideally: a straight vertical drop to the surface.

Which means that from a circular orbit grazing the crater tops, you either increase your altitude (cheating, it makes it equivalent to starting from higher) or you need to come onto a good enough landing strip for your undercarriage, or you need plenty extra fuel to maintain altitude until you've burnt off angular momentum. The altitude above which this ceases to be a problem depends on thrust/mass but of course, the less margin you have, the more exactly you need to adjust as you reduce tangent velocity.

Of course from high enough altitudes you can zero angular momentum long before crashing so of course the extra potential energy only adds requirement of fuel. From 100 km however, I would never use the method Janus suggested. You'd want to just kill angular momentum and then drop, breaking just a bit before it's too late.

Originally Posted by Pyrotex

Janus is right.

There are four things that the human mind was simply not evolved to understand:

Compound interest
Probability
Orbital Mechanics
Women

The trick is to get there with as little angular momentum as possible, according to the moon's coordinates. We can agree that lunar rotation is unimportant ( times a couple 1000 km/28 days, a bit more than 23 km/h, well within corrections on approach) so it's basically a matter of using the moon's centre as your cartesian origin.

[CraigD]

Originally Posted by TheBigDog

What about the Ralph Kramden method of straight to the moon. Never enter lunar orbit. Just aim for the center of the moon with just enough energy for its gravity to pull us out of earth orbit. Then brake as needed for soft landing. Is that an option?

I’d certainly say it’s an option.

1966’s Luna 9 used a “Low orbit around the Earth and then a direct landing trajectory”, though it’s landing wasn’t soft, being a roughtly 15 m/s impact absorbed by its “inflatable balloon skin” landing system.

In the case of Luna 9, the semi-soft landing was a concession to the shortcomings of 1960s Soviet (or the world’s, for that matter) automation technology – they just didn’t trust an automatic landing system enough to let it handle the final, precise maneuvers necessary for a very soft (~1 m/s) landing. Luna 9’s final maneuver was to eject the lander “ball” from the rest of the spacecraft, to assure the latter didn’t collide when the former when crunch/splat.

1970’s Luna 16 used a “Low orbit around Earth, translunar trajectory, then lunar orbit followed by landing”, and a soft landing via an automatic radar guided system. It even launched a sample return vehicle, which was successfully recovered back on Earth.

The only obvious advantage I can see of a lunar orbit vs. a direct landing trajectory is that the former allows for a free return trajectory, so in the event of a major system failure, your spacecraft can return to near earth with minimal use of its rocket motors. This was key to the survival of the crew of Apollo 13, as it allowed their return to Earth to be accomplished using only the LEM’s motor and the CM’s maneuvering thrusters, avoid use of the badly damaged SM. In a one-way unmanned mission, of course, there’s no need to program for a free return trajectory.

Ultimately, the best approach will have to be determined by detailed calculations and simulations.

I hope to have my GRAVSIM* enhancement up to the task of providing a realistic simulation of a continuously mass-changing/fuel burning, multi-stage vehicle, using real motor data, this weekend - this post’s simulation run was, as it’s title states, highly unrealistic, though truly a strait shot to the moon (in under 20 hours!), with no Earth or Moon orbits. I’ve not bothered counting its net delta v, but am sure it’s far beyond the capability of any existent hardware.