Need help with a design

[Foxster]

Before I head out to drop a load of unmanned science probes around the Jool system I'd like to make the mothership returnable ready for 0.24 and I don't want to throw away resources on the probes where I don't have to.

So my idea is to make the probes really minimal, no RCS, no engines etc. But I still want to get them down. Not sure how I'm gonna manage that on the no-atmosphere moons but those with atmosphere I could land with 'chutes.

So, my two questions:

1. How can I land a probe on the Jool moons with no atmosphere without engines?

2. I don't want to de-orbit the mothership, throw off the probe and re-orbit. Wastes too much fuel. I fancy some kind of re-useable railgun system that can launch probes towards the surface. Any thoughts on a design for that?

I'm open to using any major mod parts. Landing-balloon-thingies on the probes might be going a bit far but I'll consider it if necessary.

[zxczxczbfg]

There isn't really a way to deorbit something without engines on an atmosphereless planet, short of just nudging it gently retrograde . . .

[Greenfire32]

The thing is though if these probes are going to be landed on the surface, they'll have to have some kind of engine to slow their descent. Especially if the target moon has no atmosphere.

The rail gun method would only make your situation worse...exponentially...unless the target moon has atmo. Then it's a simple case of point, fire, deploy chute.

[RoboRay]

1. How can I land a probe on the Jool moons with no atmosphere without engines?

Lithobraking would appear to be your only option.

2. I don't want to de-orbit the mothership, throw off the probe and re-orbit. Wastes too much fuel. I fancy some kind of re-useable railgun system that can launch probes towards the surface. Any thoughts on a design for that?

It would probably mass more than the fuel you don't want to waste, and it would definitely cost a lot more. You'd end up burning as much fuel as you save just hauling it out there. And don't forget that "equal and opposite reaction" business... you'll be propelling your ship when you fire it.

Edited July 17, 2014 by RoboRay

[Foxster]

There isn't really a way to deorbit something without engines on an atmosphereless planet, short of just nudging it gently retrograde . . .

That's kinda where the railgun idea came from.

I could orbit fairly low. Then fire a probe retrograde from the mothership with something reuseable. Giving just enough kick to get the probe into a deorbit path.

Catapult? Railgun? Rocket sled?

[Pecan]

What else will the mothership be carrying? Probes themselves can be pretty minimal anyway so it's possible that the engines they'll need for landing will be enough to push all the transfer fuel needed to get to Jool in the first place. Since you don't seem to have any plans for the probes to re-orbit I assume the mothership only has extra fuel and, presumably, engines - but since you don't need much TWR for interplanetary transfer those engines probably aren't worth the mass.

That leads to a design where each lander has the engines and fuel required for their target and the 'mothership' is just the transfer fuel. Bringing those empty tanks back wouldn't be worth the components it took to do it.

ETA: Railgun - you'd have to be REALLY low, and that means really fast. Your gun would have to kill all that horizontal speed and the probe would still have no way to kill the final vertical speed before that killed it.

[Foxster]

It would probably mass more than the fuel you don't want to waste, and it would definitely cost a lot more. You'd end up burning as much fuel as you save just hauling it out there. .

Would that matter if I made it recoverable?

[RoboRay]

The extra fuel you burned getting it out there (and now back) won't be recoverable.

Edited July 17, 2014 by RoboRay

[Pecan]

Anyway, 'recovery' only applies to things designed to land on Kerbin. Why not just make your mothership reusable by leaving it in space, ready to go somewhere else? Eg; see the tugs in Chapters 7/8 of my tutorial and http://forum.kerbalspaceprogram.com/threads/83368-A-reusable-transfer-stage-a-good-idea, which is one of the more recent discussions of transfer vehicles.

[RoboRay]

That leads to a design where each lander has the engines and fuel required for their target and the 'mothership' is just the transfer fuel. Bringing those empty tanks back wouldn't be worth the components it took to do it.

That's a very efficient kind of design. Take a big fuel tank and put a ring of small probes around the edge, all with their engines pointed in the same direction. Use fuel from the tank to get to the destination, then start dropping off probes at each world you visit. You're losing engines over time but you're pushing less mass as well. In the end, you just drop the near-empty tank onto the last world, or leave it in orbit for eventual salvage and reuse. Or maybe as your shared interplanetary comm relay if you're running RemoteTech.

[Foxster]

I'm getting torn between two things here.

Yes, I started with the idea of saving resources. Then I got into the whole launching unpowered probes towards the surface thing as an engineering challenge.

As usual, the one has overtaken the other. I'd really like a way to fire probes at a moon and get them down - and worry about the cost later!

[Pecan]

That's a very efficient kind of design. Take a big fuel tank and put a ring of small probes around the edge, all with their engines pointed in the same direction. Use fuel from the tank to get to the destination, then start dropping off probes at each world you visit. You're losing engines over time but you're pushing less mass as well. In the end, you just drop the near-empty tank onto the last world, or leave it in orbit for eventual salvage and reuse. Or maybe as your shared interplanetary comm relay if you're running RemoteTech.

Yep, that's the idea. Short of an infinitely reusable 'space-tug' it's the cheapest suggestion I have:-)

Edited July 17, 2014 by Pecan

[Foxster]

Here's the kind of nutty thing I have been playing with...Just a test bed. Needs a lot of other stuff. There's a free-spinning docking washer as an axle. Idea is to spin it up and then decouple the probe at just the right moment somehow and momentum deorbits the probe. I'm being silly, right?

[Pecan]

Right - unless you're going to wrap them in balloons/footballs to cope with the impact.

[Jarin]

Maybe a framework of ablative girders could handle impact if arranged properly? We are talking about very low-gravity objects, so orbital velocity should be fairly slow.

Edit: girders with landing gear on the ends; that should absorb shock a bit better. And again, the word is 'ablative, assume you're going to be repeatedly smacking these extensions into the surface, sacrificing them to bleed off speed.

Not efficient in the least, but good for proper "kerbal" quotient.

Edited July 17, 2014 by Jarin

[Foxster]

Maybe a framework of ablative girders could handle impact if arranged properly? We are talking about very low-gravity objects, so orbital velocity should be fairly slow.

Edit: girders with landing gear on the ends; that should absorb shock a bit better. And again, the word is 'ablative, assume you're going to be repeatedly smacking these extensions into the surface, sacrificing them to bleed off speed.

Not efficient in the least, but good for proper "kerbal" quotient.

Sounds interesting. I'll have a play around with that idea tomorrow.

[Idrryl]

Have you thought about RCS for Bop and Pol? It's light and cheap. If you're not overcomplicating the design of the probe it should be sufficient to land slowly and safely.

[Pecan]

Unfortunately OP says "So my idea is to make the probes really minimal, no RCS, no engines etc." (my underlining)

[Idrryl]

Oh, my bad then. So the plan for now is structural girders with landing legs? Unconventional, but Kerbal enough

[Foxster]

Oh, my bad then. So the plan for now is structural girders with landing legs? Unconventional, but Kerbal enough

Thats what I have been playing with. No luck so far - the speed of landing just causes the probes to crash.

One way forward might be very large probes with disposable structure but thats messy and big. So back to the drawing board.

[Idrryl]

Do the probes have to be all the same? Because you could do the girders thing plus a parachute for Laythe, but that certainly won't work on Tylo...