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linkxsc

Horizontal Lunar (atmosphereless) Landinges / Arrestor Hooks?

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Hi ladies and gents. I didn't really know which forum I should ask about this in, though it has some bearing on the real world so I figured it might get some discussion in here.

So, what are your thoughts on landing spacecraft horizontally (perhaps with aid of an aircraftcarrier style arrestor system) on the moon (or other atmosphereless body)?

Sorry may be a bit confusing.

Heres where the question comes from though, might shed some light.

Last night, playing some kerbals, I was messing around with putting a network of satellites around minmus (I have KSP interstellar, RemoteTech, TAC LifeSupport, and Kethane, among a few other mods). For my comm satellite network I was using a SSTO I threw together previously for placing some in KSO (refueled it in space a bit, made for minmus) Arriving at minmus and completing most of the mission I came to the realization that, oh snap, I don't have fuel to get back to kerbin, OR enough to make a proper landing. (The craft has VTOL rockets and retros cause it should land normally, but well, we all waste fuel from time to time)

So I was struck with an idea. Right on the surface of one of the frozen lake sits the largest kethane refinery I have. So for my landing approach, I lined up my orbit ahead of the lakebed so I would be landing on a very smooth surface. And I burned retrograde until my orbit would have brought me ~50m above the surface of minmus, however at still quite high velocity, if I had let the craft go on it would have either smashed into the mountains, or just flown back off into orbit a few 100 km and came back around.

After biding time, and spending a little more fuel lifting myself up to not smash a hill. I get just above the lakebed, burned retro for a few seconds so I'd just touch the surface, and then when about to touch I killed the vertical I had down to ~5m/s. Note I was still way over 300m/s horizontal. From there, I just applied brakes and after a time, I came to a stop, with about 70 delta-V of fuel left. (some downward thrust was used from RCS to help keep the craft on the ground, but precious little delta-V was used during the actual landing. I made a few different attempts, mixing in the retro rockets to help slow down, but on the 4th attempt I managed it without any liquid fuel past the part for the orbital maneuver.)

Brought a rover with some more fuel in it over (long trip), then vtol hopped over to the base and fully fueled the craft, went and put the rest of the satellites in orbit and all was good.

Now, how unreasonable would it be to do this kind of procedure in the real world? In many chats talking about lunar bases in particular, the problem comes of it takes fuel to get down, and fuel to get back up (then some banter of ALOX rockets, and other things).

Now I was able to bring a craft down on minmus in this manner, when I certainly wouldn't have had the fuel to land it by killing all horizontal, and then suicide burning before hitting the ground (I know, because I tried it with quicksaves about a half dozen times before being stupid, even my horizontal landing didn't work a couple times due to minmus's low gravity, the craft would bounce off the ground and head back out into orbit)

I have tried to do it on the Mun, but I can't find an area suitably flat to do it. IRL that could be solved, lunar crust could be smoothed into a form of landing strip like I used the lakebed for. And perhaps some form of arrestor cable setup could be included.

Going to experiment later with a more, focused craft design (including some upwards facing rockets, the little rockomax ones, trim the throttle just enough to stay on the ground. And a few extra sets of wheels so more braking potential.)

I just want to know if any other players have any thoughts on this type of landing. If I was able to save some fuel, I could probably save a hell of a lot more with more practice (going to try it out later but, work trips, my laptop can't play KSP). And if perfected, for example. IRL a craft could say, ferry stuff into lunar orbit with less fuel need (less fuel being brought up for the eventual landing means it can bring more materials too). The craft also doesnt need to land with a thrust to weight ratio greater than 1 (if the ferry was picking things up in orbit then bringing them down. Though this would probably never be done that close, they'd always keep a greater thrust to weight, incase of accidents)

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Your problem is that you're going to be coming in at pretty much orbital speed. Real-life orbital speeds are far greater than those in KSP, and real celestial bodies less conveniently smooth than Minmus.

Cool manoeuvre though, I tip my hat to your piloting skills!

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The moon has an orbital speed of about 1.7km/s. Building wheels that don't spontaneously disassemble when hitting the pavement at mach 5 is rather difficult. Maybe you could pull something off with a magnetic track. But really, if you have the tech to build something like that on the lunar surface you can also refine your own fuel on the surface.

It is on the other hand extremely useful for launches. A railgun could be used to propel unmanned vehicles (Astronauts don't like 500g accelerations) to orbital velocities on the moon. Much cheaper than conventional rockets.

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Your problem is that you're going to be coming in at pretty much orbital speed. Real-life orbital speeds are far greater than those in KSP, and real celestial bodies less conveniently smooth than Minmus.

Cool manoeuvre though, I tip my hat to your piloting skills!

I was looking at an approach similar to this for "lithobreaking" a Phobos cubesat- with an orbital velocity less than 7.5 meters per second (at the height of the highest mountians of phobos) it's less a matter of D/v, and more a question of landing with a TWR of less than 1.

Smoothness is even more an issue than the moon, though.

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The moon has an orbital speed of about 1.7km/s. Building wheels that don't spontaneously disassemble when hitting the pavement at mach 5 is rather difficult. Maybe you could pull something off with a magnetic track. But really, if you have the tech to build something like that on the lunar surface you can also refine your own fuel on the surface.

Well as part of my argument, you don't NEED to land at full orbital speed. 1.7km/s is a LOT of fuel needed to slow down. landing something in the ballpark of 250m/s wouldn't be anywhere near as unreasonable (I think it translates to ~500mph) Just need a long long landing strip. Although, magentizing the thing could always help slow it down and keep it from bouncing off back into space.

And ya know, I wouldn't be too surprised if we could manage some wheels that could travel at 1000m/s or so, cause remember theres only going to be ~1/6th the force on them as if it were on earth. Yeah you can't just scale the numbers and assume that if tires can land on earth at 100m/s they could do 600m/s on the moon, but hey, we've done stuff crazier than that before.

In other news, what was I smoking when I spelt the title of this thread

Edited by linkxsc

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I remember not too long ago seeing pics of somebody's single launch space plane do a tour of the Jool system and it used this method for landing on Tylo. I think it was his last stop in the system after he had ditched all of his landers that he used for the other moons. He designed it with multi directional thrusters set to action groups and scouted out a fairly smooth "landing strip" to come down on. It was a super impressive mission, but I don't remember who posted it. I believe it was in the screenshots thread.

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Well as part of my argument, you don't NEED to land at full orbital speed. 1.7km/s is a LOT of fuel needed to slow down. landing something in the ballpark of 250m/s wouldn't be anywhere near as unreasonable (I think it translates to ~500mph) Just need a long long landing strip. Although, magentizing the thing could always help slow it down and keep it from bouncing off back into space.

If you save 250 m/s out of a 2500 m/s descent burn at 300 s Isp (If you have a runway on the moon, I assume you also have refueling infrastructure), your fuel fraction decreases from 57.2% to 53.4%. That's not that much, and landing gear is heavy.

And ya know, I wouldn't be too surprised if we could manage some wheels that could travel at 1000m/s or so, cause remember theres only going to be ~1/6th the force on them as if it were on earth. Yeah you can't just scale the numbers and assume that if tires can land on earth at 100m/s they could do 600m/s on the moon, but hey, we've done stuff crazier than that before.

The problem isn't so much the weight of the vehicle as the speed of the wheels. For a wheel of a given diameter, the "centrifugal force" experienced at the outer edge is proportional to the square of the speed. For a 1 m radius wheel with a tip speed of 1000 m/s, the centripetal acceleration is approximately 100,000 gs!

Also, consider an aircraft landing on Earth. From the plane's perspective, the wheels start out stationary, with the runway flashing by at over 50 m/s (for a large airliner). When the wheels come into contact with the runway, they are spun up very quickly, but not instantaneously, and skid until their tip speed equalizes with that of the aircraft - this is what all the tire marks on runways come from. This doesn't contribute much to slowing the plane down, but is a major source of wear on the tires and stresses the rest of the aircraft as well. That's at normal aircraft landing speeds. Now imagine that process with the vehicle going as fast as a high-powered rifle round. I can't imagine any conventional wheel surviving that.

There have been proposals to "pre-spin" aircraft wheels before touchdown to mitigate this: http://www.google.com/patents/US20040104305. However, these generally take the form of a pinwheel-type device that wouldn't work in a vacuum. Instead, you would need to use electric motors or tip-rockets, both of which would add considerable weight and complexity to the landing gear. Also, the weight of the vehicle might actually prevent the wheels disintegrating. Here's a video showing what happens when RC truck wheels are spun at full speed without the ground pressing against them:

Basically, 1000 m/s wheels ain't happening.

However, there is a potential role for "horizontal landings" in space.

The idea of a wheel-shaped space station spinning to produce artificial gravity is very common, but there are major issues with docking to a rotating station. One approaching is putting the docking ports right at the center and having the spacecraft match its angular velocity with the station, but this would only allow the station to have two docking ports. The other approach is to have the spacecraft fly in circles around the station. However, controlling the ship during the maneuver would be difficult, and dV requirements would be large.

However, with a station large enough that the mass of a docking spacecraft would be negligible, a third method would be possible: place maglev or wheel-based rails on the outside or inside surface of the wheel's "rim", or a runway on the inside surface. The spacecraft would be able to float freely in and line itself up prior to landing since it would be stationary relative the the wheel's center of mass. The velocities involved would be relatively manageable (ranging from 100 m/s for a 1 km diameter wheel to 1000 m/s for an enormous 100 km diameter wheel, assuming a 1g acceleration at the tip), and if they did get too high for wheels to be feasible, the mentioned precision maneuvering ability would allow magnetic suspension to be used. Linear motors onboard the station would accelerate the visiting spacecraft up to the station's speed, at which point it could be shunted off to a docking area. They could also be used in reverse to launch the spacecraft up to the wheel's tip speed, but in the other direction, at which point it could simply disengage from the rails and push itself away using RCS.

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For an arrestor hook you could try dragging a KAS anchor over the surface. It has a mechanic with high friction when touching a surface. For a typical lander, you probably need to scale it down to be lighter and less strong.

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You'll still have to burn enough fuel to kill your entire vertical component, and almost all of your horizontal. The added weight from having to reinforce your lander to survive a tailhook landing might make the small fuel saving pretty marginal.

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I certainly wouldn't work for a manned mission. If we assume a initial lateral velocity of 1700 m/s the moons orbital velocity and a cable system generating a constant deceleration limited to 3g for the crews sake the resulting distance needed to decelerate is 48Km and 48 Km is a lot of rock to clear and a lot of cable to unspool.

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As regards celestials, it's only going to work for small bodies with low orbital speeds. The snag is that they're where theres least need to save fuel anyway.

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As regards celestials, it's only going to work for small bodies with low orbital speeds. The snag is that they're where theres least need to save fuel anyway.

I agree, but as I mentioned earlier, it would be less for saving Dv, and more a method of landing on a small airless body with a TWR too low even for said small airless body. an Ion powered phobos "lander" that touches down at less than 7.5 m/s, despite not having the thrust to land normally.

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