farmerben

I've made at least two totally gamey discoveries recently. 1) Kerbals can splashdown safely using only their jetpacks. So I was deorbiting a vessel of some kind in career mode, when I realized my vessel had no parachutes or other provision for landing, it did manage to aerobrake to a reasonable speed. OK no problem, I'll go EVA and the kerbonaut will land on his personal parachute. Oh crap, he doesn't have one, must be a rookie. Using the jetpack he managed to hit the water at just under 50m/s and survived. 2) Aerospike engines can fire safely through large numbers of toroidal fuel tanks and tiny decouplers. Many players have probably tried putting toroidal fuel tanks and decouplers below their spark engines, nothing gamey about that. Well I decided to try it with the Aerospike and it worked. It worked too well. I've added dozens of extra stages of fuel tanks that pop off the bottom when depleted. I've never caused an explosion through overheating. Aerodynamic instability... and even wobblieness in vacuum eventually becomes a problem, but autostrut goes a long way toward fixing it. The exhaust flames are definitely bigger than the hole through which they go, but it seems 100% efficient, not like what normally happens when some part of the craft is below an engine.

Outstanding post! I'm slowly figuring out the answers to my first few questions. I'll have to reread the entire thing again sometime. Thanks.

The advantage seems to be that electromagnets are among the longest lived components. Presumably the power generation system, batteries, or computers would fail first. If using primarily chemical thrusters, the fuel could be made to last longer. Compared to a gravity gradient tether, the electromagnets are probably lighter, and offer deorbit capability.

I'd like to know why this very simple idea is not used on satellites. Three solenoid electromagnets powered by solar panels could give any satellite its own directionally controllable magnetic field. The magnets on the satellite would interact with the Earth's magnetic field. This would give useful orientation control, the entire satellite could slowly orient like a compass needle. If the satellite starts to spin along an axis parallel to the Earth's magnetic field, this would be difficult to control, but I think through careful wobbling it would be possible to eventually cancel the spin. The satellite could also gain some component of prograde or retrograde thrust, while usually changing its inclination at the same time, because it can accelerate with respect to the Earth's poles. I can see two drawbacks. 1) the effect may be very weak. 2) controlability is poor or non-existent in some directions from some positions. Despite these limitations, a triple electromagnet seems like a very useful thing to have. Reaction wheels or thruster fuel are often the first things to fail on man-made earth satellites. The electromagnets are much more long lived. Electromagnets would allow many satellites to remain pointing in the proper direction much longer. It would also give them a capability to deorbit themselves when their mission is over, without depending on moving parts or finite propellants.

And I meant to say the maximum amount of aluminum vaporized is 5*10^-5 m3 not 5*10^5 m3.

Check your units. I was supposing all the energy is concentrated at the point of impact. Thus neglecting the total mass of the target.

Suppose all the energy went into vaporizing aluminum. How much aluminum would be vaporized? m = E/S = 1.8MJ / 12 MJ/kg = 0.15 kg of aluminum vaporized. volume of cavity = 5*10^5 m3. Which corresponds to a cavity 20cm long and only a couple of millimeters across. Since the target is large compared to the cavity it is likely to penetrate all the way through before the target cracks all the way across. This plasma does some work on debris fragments scattering them radially out from the point of impact. About 1/3 of this work is done along vectors undesirable to us. I'm not saying the problem is negligible, I just think the work done on fragments is a small fraction of the total energy. 1/10 seems high to me. So more like 1/30 of the debris cloud remains a hazard. That said, these massive energy collisions can create problems. So the ice pellet gun is probably more suitable for applications where the relative velocity is much smaller. Going for shots with half the momentum change, gives us a quarter the energy...

When a very light thing collides inelastically with a very heavy thing, the light thing rebounds with nearly equal and opposite velocity. A pellet that vaporizes is about as close as you can get to an inelastic collision. Of the energy gained by the debris only a fraction of it will be kinetic energy of fragments. Most of it will become heat. Part of the target will vaporize at those energies. So most of the stuff moving at high speed is gas.

Rendezvous with debris sounds like the slow and expensive solution. But it may be key for certain applications. The simplest lightest way to deorbit very large debris is to attach an electro-magnet with a solar panel. The electo-magnet must be capable of changing the direction of its field according to program. It can pump against the Earth's magnetic field. If necessary it can increase inclination toward the Earth's poles, and then retrograde.

The figure of ~100 m/s is overkill. As long as the debris touches atmosphere we don't mind if it skips off dozens of times. A simple gun with ice pellets should be expected to shoot 1 km/s or less with respect to its launcher. But that doesn't matter much. Orbital velocity is 30 km/s. So our cleaner will pass debris anywhere up to 60 km/s. In other words a .01 kg pellet hits sputnik 10 kg at 60 km/s. Sputnik changes velocity 60 m/s and grazes the atmosphere.

Maybe mercury or gallium or something would work. It would have to fragment to micron size every time or be used only on trajectories which will aerobrake most of the projectile. The atmospheric pollution is minimal... but the globule threat is bad. Lasers are good for cleaning up debris as part of a hunter killer network. Lasers might be the best way to clean up loose bolts and things like that. One problem is that it may not be possible to fully deorbit junk in a single pass. Pellets offer a much stronger and well timed impulse. If you shoot junk with lasers and don't bring it down before losing line of sight, then you have a lot of paperwork to do...

One solution to the space junk problem is to target debris with a pellet gun. This is far more economical than capture solutions because the relative speed of the target can be very large. If a piece of debris is large enough to see, its orbit can be tracked. From this we derive the set of all impulses which would lower the perigee of the debris to reach atmosphere. A subset of those will match the set of what we call a good shot or a safe shot. Our cleaning satellite has many pellets to shoot at debris. The satellite will have a high inclination low altitude orbit. It will not maintain a fixed orbit, but must guarantee that a high proportion of its shots move the satellite into a higher orbit or change its inclination. A good shot also requires that we know where our misses will end up, and we cannot add to the debris problem. In very low orbits a set of good shots will have our misses burn up in the atmosphere. At higher orbits this becomes less feasible. Collisions between pellets and debris have the possibility of fragmenting into smaller debris. Very flimsy debris may make poor targets for this countermeasure. However lots of space junk would be suitable if the projectile is much flimsier. This gun is designed to impart momentum, which is different than the goal of almost every other gun. The safest thing then is to make the projectiles out of a phase changing material, such that solid pellets become gas some hours or days after they are fired. We can also design such that pellets will instantly vaporize (or at least pulverize) on impact with the target. This will reduce fragmentation of targets. An example of suitable pellet material is dry ice CO2. Most other types of ice, and perhaps some types of wax or plastic could be used. It could be advantageous (lighter) to have electrical conductors in each pellet so that they can be launched electrically. What materials could be used as conductors so that they dissipate or otherwise become safe as debris hazards for other spacecraft?

That principle is plain. The question is do other parts accomplish this as well or better than legs. I like the swept elevon quite a bit. I had not messed with girders and vernor engines too much, but I will. I'm still fairly new to the game. When I was first learning to land on the Mun I flipped on the bounce of landing legs several times. That was mostly newbie lack of skill, not a problem now. But I usually use rover wheels or engines for that now and have no problems, so I'm not sure why I'd want landing legs. I've also got some boar and mammoth launchers that parachute to Kerbin and tend to break landing legs or not need them at all.

yep that one

While trying to develop a vessel to recover rocks from Eve I became very interested in alternative landing gear. To the extent I understand it (not well) the key metric is impact tolerance. The aerospike engine has remarkably high impact tolerance. This is matched by a few other launch engines, and exceeded only by some wheeled landing gear and some structural components. Testing shows that landing on Kerbin or the mun with 4 aerospikes works great. Leg style landing gear only has an impact resistance of 12m/s, which is low compared to lots of things, lower than almost all the aerodynamic parts for example. I'm not sure how to take into account the spring distance these parts have. But, it seems to me elevons have some flexibility built in so why not? The experimental vessel shown below can single stage to orbit on Kerbin and has excellent controllability at all airspeeds. It can glide to KSP quite well. To land I pitch vertical and deploy a parachute. This photo was an attempt to land on the VAB that almost worked, but I'm not perfect at the pitch up and parachute maneuver. Questions: What are your favorite landing parts? Are legs worth it at all? Should I mount legs so that they only partly compress before the engine hits the ground? Also, a couple problems I've having with designing my Eve mission. Scott Manley demonstrated an Eve launch with 12 asparagus aerospike stages. My attempts to launch a similar craft on Kerbin become aerodynamically unstable and flip out. I can sort of overcome this by adding lots of parts and mass, which hurts my dV. Matt Lowne and others have Eve missions with lots of elevons on the nose of their craft to hold retrograde on descent which they discard for launch. I tend to need stability parts to remain stable during launch, especially if I'm asparagus staging with aerospikes. So I don't understand why they would discard their elevons before launching from Eve.

I'm not a huge fan of putting super heavy and expensive turbo machinery on a spacecraft if you don't need it, and its really only super useful for the first 500 m/s. Some kind of flying stovepipe has got to be a better deal if it will actually work. Carrier planes compare to directly rocket sleds, and are obviously better for some applications... The higher the mass of the final stage the more favorable a launcher looks.

A 10 km launcher throws humans at 5g up to about 1000m/s. That is potentially a dV of 1000m/s more than you could possibly achieve without a ground assist. In practice though the best thing to shoot is fuel. The launcher will shoot consistent orbits... very useful for fuel depot rendezvous. If aluminum foil can be used to hold vacuum this becomes much easier than I thought. There is talk about a plasma gate.. but that’s fairly unproven at scale.

A 10 km launcher throws humans at 5g up to about 1000m/s. That is potentially a dV of 1000m/s more than you could possibly achieve without a ground assist. In practice though the best thing to shoot is fuel. The launcher will shoot consistent orbits... very useful for fuel depot rendezvous.

I thought the tone of the page was amateurish, but those points are somewhat defensible. If there is a more serious look at the topic, I haven't found it yet. Escape capsule is more than just a seat, that system might take more dry-mass than others. A straight rail is simpler than a curvy roller coaster. More precise perhaps, but simpler. Inside a tube at supersonic speeds air pressure builds up on the nose of the vessel. On the surface this would be no more of a problem than in the air.

Please correct me if this is way off. A gas gun with between 300-400 psi has in the ballpark the thrust of a falcon 9 or similar. Supposing you can maintain that pressure through the length of a barrel. Three huge problems arise. 1) You need reasonably tight gas bushings on the vessel. 2) You need an exponential increase of gas behind the vessel inside the barrel. 3) the barrel must be maintained with absolute precision. Possible solutions: 1) The vessel wadding is made up primarily of CO2 dry ice. It could have fibers to control cracking, have hex tiles,... It could have pneumatic pistons like self adjusting brakes to push the shoes against the barrel with constant force and diameter as the dry ice ablates. The shoes can be discarded at the muzzle of the barrel and will evaporate before they hit the ground 2) Rather than use foul timed explosives or sit on the wrong end of a rocket nozzle, we could use N2 as the driving gas. The heat and driving energy can all be kept external to the barrel. We can dump an exponential amount of heat into liquid nitrogen and get an exponential volume of cold gas. This is very clean and beneficial to the system compared to hot corrosive gasses. 3) To make the barrel absolutely precise we construct it out of H20 water ice. Specialized Zamboni robots can machine an entirely fresh barrel surface in only a few hours. Reserves of liquid N2 help ensure the special glacier remains stable throughout the year.

Two inflatable heat shields make a cool looking flying saucer. I've almost managed to land on the VAB without breaking something using the twitch engine. And to plummet a crap probe to eve that broke. Who has the coolest flying saucer?

One huge advantage is that at launch you can get away with low thrust relative to fuel mass onboard. TWR = 1 gets you a long way. With radially mounted external fuel tanks, a single aerospike vehicle is practical. External fuel tanks are stages, but quite a contrast with engine stages. Dropping a tank into the sea isn't as big a deal.

My error I forgot to account for the lander fuel. Spark tends to win on thrust now. Two serious errors in one thread, my apologies.

And a TWR of 1.03

Here is a simple proof that there is something to the concept. Each image has 5 stock parts and a mechjeb. Image one: TWR = 1.31, dV = 3257 Image two TWR = 1.33, dV = 3262