Landing legs

Ahh, this is sort of what I was trying to get at in my first post - I was just describing one leg of a tripod (should have thought of that analogy) - I pressume a self operating one uses a spring to push down a centre ferrule which pushes out the leg which then locks in place by going past 90degs. Sort of like a self operating umbrella as well.

James

Alot of nice suggestions. It appears to me that if you still had the current fixed landing gear on the vehicle the stablizing gear would only have to deploy on landing, either durring or just before, and wouldn't need to be deployed at take off.

Some sort of "umbrella" type gear, as was mentioned, might be easiest to make. One that was fired with a button push and needs to be retracted manually would be more than enough.

In this version it takes off with the current fixed gear and the addional gear pops out only at the end of the run. Some weight gain but probably not too much, since most of the weight of the vehicle is handled by the fixed gear. The addtional gear dosen't have to be very heavy, just enough to keep it from tipping over.

Well, for what it's worth, our X Prize Cup flight WAS on an airport - we launched in a triangle of dirt between two taxiways and and a runway. Why didn't we launch from the pavement? The taxiways and runways at that airport are asphalt; we would have basically melted our launch point. Plus, there is always a chance of spilling LOX, and LOX on asphalt can be dangerous. But even if the taxiways and runways had been concrete, our engines chew that up pretty nicely, and I don't think the city of Las Cruces would have been pleased (not to mention the pilots who use that airport).

We would have preferred launching from a concrete pad that isn't used by planes, but the X Prize people weren't able to get one laid by the time the event started.

Well, maybe this belongs in the "Was Space Expo a Bust?" thread, but it seems to me if the organizers had provided a proper landing area the accident could have been avoided. Maybe not pavement but at least hard ground. Of course the weather could be blamed too. I bet that triangle of dirt is usually hard, just not after a rain.
On the other hand, if all four legs had missed the metal plate, it may not have tipped over. So maybe the really easy fix is just to lose the plate.

If we had been able to get that second flight in that day, we were thinking of doing exactly that. At first we were concerned about how much dust it would kick up and how much of a hole it would dig, but after that first flight, launching off of the dirt seemed like it would have been just fine.

I had another go at it, and since I hadn't tried out the latest version of Inkscape yet I figured I'd make a proper drawing this time:



The leg is attached by a rail. I imagine an aluminium rail, shaped as shown in the image (blue). This is just a simple curved bar, with a groove in the middle. The leg would have two pegs that fit in the groove, shown in light green. On top of that would be a second, identical rail (not shown, so you can see the leg), so that the leg is sandwiched between them and can not get out.

Attached to the top of the leg, there would be a spring, possibly with a shock absorber. It pushes the leg downward along the rail.

Aerodynamic pressure during the descent will probably push the leg upwards a bit, but because the bottom part of the rail is straight, the leg will not splay outward. When the vehicle touches down, the leg is pushed up (relative to the fuselage) further and swings out. Depending on how big the aerodynamic pressure is the straight part could be longer or shorter (thanks nihiladrem for the heads-up there). If the leg should splay out at a larger angle, the curved part of the rail could be lengthened.

At the bottom of the legs I've drawn the "curl" I described earlier, I don't think it's actually needed with this design, because the rail forces the legs to splay anyway when they're being pushed upwards. If you have legs that are on a hinge and pulled against the craft by a spring, then if the craft lands perfectly vertically, the legs may remain vertical. And only splay when the pilot is climbing out and the CG shifts . So moving the point of contact outwards helps a bit there, because it results in a small outward force on the leg to help it splay.

I've drawn the whole construction sticking out the side of the fuselage right now, but if there is room then the leg could sit inside, flush with the outer shell. If you leave out the fins/curls, there would be no additionaly frontal surface at all. As for extra weight: the leg is there anyway, and a shock absorber/spring seems to be nice to have on a commercial manned vehicle, so it's just the rails that are extra.

One question I have is how much friction there would be between the leg and the rail. Are ball-bearings needed? Perhaps some oil or fat will do the trick? But will that stay on in a vacuum?

Remember the ME109? Fantastic airplane, several years ahead of its time. Unfortunately, the gear was a bit too narrow: the airplane was notoriously unstable during landings. Even through to the end of the war, a notable percentage of German pilots KIA were lost due to landing accidents, and not enemy fire.

Remember: make it so that even if all hell breaks loose on the flight, at least the pilot can get 'er down safely and smoothly.

Yes.
I didn't know that.
Didn't the ME109 have retractable gear? Isn't that basically what we are talking about here, retractable gear, rocket style? Fixed gear is always safer. It can't fail to extend. The only issue is how much of a performance penalty are you willing to pay for that level of safety. My advice is to stay with the fixed gear, wider if needed for ground stability. After you have finished inventing the rocket then you think about adding the retractable gear, leather seats and DVD player.

Yep, it was retractable -- and I was thinking along roughly the same lines.

The DC-X was destroyed on its final flight when the flight crew forgot to attach a hydraulics line to one of the landing legs. It set down perfectly on three legs (the fourth failed to extend because it wasn't connected), slowly toppled over, and exploded.

Now obviously overworked flight crews shouldn't be normal, but it does show that the more things there are that can go wrong, the bigger the chance that one of them will.

Also keep in mind Murphy's Law: "If a thing can be done two ways, and one of those will lead to disaster, then someone will do it that way."

So why not do it both ways at the same time? Retractable, with fixed legs for backup? Also, the vehicle's center of gravity is above the landing gear. why not change that, by connecting the landing gear to the top of the vehicle, instead of the bottom. Make the rocket 'Hang' from the middle of the landing gear tripod (or quadrapod, or whatever).

Uh, remember, this thing's gotta get off the ground, or else the "landing" part of "landing gear" is kinda irrelevant. The desire for redundancy has to be balanced with the need to reduce weight.

Okay, first off, I must say that I agree that the KISS principle should be followed at the beginning. However, coming in at a close second, would be the need for stable landings. So, soon after the first 'steps' have been taken, it would be very useful to get a wider base.

If the current leg base is a sutible failsafe, then why not have the legs be able to splay from the currend width to a wider configuration? What would be needed, though, would be a way to have an all-or-none splay response. Having only one or two legs splay could be equally disasterous (ie, if one leg had siezed up due to temperatures).

This would be the design challenge. One very imperfect example would be an alluminum ring that goes around the legs. this ring would be moveable up and down, with some sort of loops through which each leg would slide. When the ring is down, the legs are least splayed, and when it is up, they are most splayed. If one leg does not move, ideally the ring wouldn't move up and splay the rest of the legs. I know this idea has many problems, I only mean it as an example of how there can be a 'closed' position that still could work as a failsafe landing configuration.

Stable landings are important, and drag during flight would prove costly, so it seems as if a 2-in-one solution is necessary. Unfortunatley, this is where the design doesn't exactly KISS.....

Anyhoo, just my two 'chotto' bits.

-Ben

Duuuuuuuuuuude, (not anyone in specific, that was a general Duuuuuuuude),
The rocket's got a freakin' computer onboard! It's also got alot of . . . like . . . what shall we call them . . . moving parts! A good tripod is an incredibly simple, reliable device compared to a rocket engine. When's the last time you heard of a tripod blowing up?
1) Lift the vehicle up and down on a tether to test it until you know the landing gear works.
2) The extending landing gear don't have to to extend all the way down. If all goes well, the vehicle lands on the main gear. If it doesn't, the vehicle falls onto 2 of 3 (or more) legs extending to the side.
Also, don't JUDGe aN Idea on its own meRits = JUDGNIR