advanced lithobraking possible?

[king of nowhere]

the difference between lithobraking and advanced lithobraking is that with the latter, your ship remains whole. at least the parts that matter.

i am planning robotic missions to remote worlds. i collect science. yay! but i loathe to transmit it. it gets small returns. but i can't take my whole robotic probe and bring it back; that would require a probe bigger than i want. so, i designed a tiny probe that can return on kerbin with my precious samples.

this probe must be as small as possible, of course. so, an ant engine (20 kg) and an oscarB fuel tank (230 kg, of which 30 kg dry mass). of course the core of it all is a sample canister (50 kg), and i can't go below 100 kg with a HECS probe core (i could use a OKTO2, but i need reaction wheels, so i'd have to add them aside; i'd only save 10 kg, for a massive extra cost). add in an antenna, a battery (as much as i want to be light, the internal battery of the HECS won't last long, and i'm not going to risk a multi-year mission on the chance that my probe will miss a critical manuever because it was being eclipsed by a planet. not if i only need 5 kg to fix this) and a couple light solar panels, and we're at 220-230 kg of dry weight.

and then there's the parachute. i need a parachute to survive reentry. and the smallest parachute is 100 kg. it increases my dry mass by almost 50%.

this is why i was considering skipping the parachute entirely.

the probe is so light, air slows it down almost to the end. it slams on the ground at only 70 m/s. i only need to lose that much.

first i considered rocket braking, but it's not possible. the ant engine has no efficiency in air. i would need to use a spark, which weights 110 kg more than the ant. a parachute is lighter.

and this is why i started experimenting with putting some structural pieces around my probe and deorbiting it. but without much luck. out of three attempts, only once some pieces survived the impact, and the science canister was not among them.

I also considered large, flat surfaces that would create drag and slow the probe further, but alas, they are all too heavy.

so, before spending hours to a task that may be impossible, and without much notion of what i'm doing, i want to ask: is there the chance to make a probe as i described that can survive impact against the ground without parachute, while adding armor that weights less than a parachute?

(i used octagonal struts because they are light. other structural parts are more resistant, but they weight more than the parachute would)

 

[James M]

i'd try a single Pocket Edition I beam as an impact leg with a couple basic fins used specifically for flaring upon final approach.That would come in at 80kg. 

Wait no. Math is hard. Anyway that's the process I'd go along with. There's no way you can slow down to landing speeds through impacts alone with less mass that a mk1 parachute. Flaring on wings is probably the most weight efficient way of going about pulling off a lithobrake landing. 

(That is until we get airbags if ever) 

Edited July 10, 2020 by James M

[king of nowhere]

35 minutes ago, James M said:

 Flaring on wings is probably the most weight efficient way of going about pulling off a lithobrake landing. 

(That is until we get airbags if ever) 

the ant engine at low altitude has a TWR of less than 0.1. flaring won't help much. flying on wings like a glider is another option i considered, but i don't have much knowledge of flying in this game to know if it can work

[James M]

14 minutes ago, king of nowhere said:

the ant engine at low altitude has a TWR of less than 0.1. flaring won't help much. flying on wings like a glider is another option i considered, but i don't have much knowledge of flying in this game to know if it can work

Well you'd have to have the wings placed more rearward from your center of mass. Once you're falling at terminal velocity, you'd wait until the last second and flare up using your reaction control systems. The wings would provide some lift and then you'd do this a couple more times until you've slowed down enough to land. The only issue is if you flare too much and actually stall out. At that point, you'd nose down and just slam into the ground. Obviously not ideal.  What you're trying to do is really challenging but I believe it still could be done if done right. You're basically trying to land a mini space shuttle with no wheels. 

Guess I should also mention, the engine of your craft would need to act as the front of the probe upon landing as it would be the first thing hitting the ground. Activating them won't provide any thrust but they would take the impact. Also, I'd recommend using all of the fuel in the oscar-b tank before attempting landing as lighter crafts are affected by aerodynamics more. 

Edited July 10, 2020 by James M

[AHHans]

7 hours ago, king of nowhere said:

flying on wings like a glider is another option i considered, but i don't have much knowledge of flying in this game to know if it can work

Oh, it definitely can work. The major question is if it can be made to work with significantly less mass than a single parachute. Unfortunately the lightest aerodynamic surface - the Basic Fin - is very temperature sensitive, so I don't think you'll be able to get that to survive reentry. I think that your best chance might be something like the Elevon 1, all the good candidates (e.g. like the Delta-Deluxe Winglet) are pretty heavy...

[Spricigo]

5 hours ago, AHHans said:

Unfortunately the lightest aerodynamic surface - the Basic Fin - is very temperature sensitive, so I don't think you'll be able to get that to survive reentry.

Such light craft pointing radial out will slow down pretty quickly. I'd be surprised to even get heating bar.  

Edit:D'Oh! was thinking about a return from LKO when gave that brainfart. Yes, the basic fin will fry and so will the probecore.

 

Edited July 10, 2020 by Spricigo

[Zhetaan]

14 hours ago, king of nowhere said:

I also considered large, flat surfaces that would create drag and slow the probe further, but alas, they are all too heavy.

so, before spending hours to a task that may be impossible, and without much notion of what i'm doing, i want to ask: is there the chance to make a probe as i described that can survive impact against the ground without parachute, while adding armor that weights less than a parachute?

I'm suggesting this without having tried it, but have you considered autorotation?  Arrange winglets or fins (AV-T1 Winglets mass 37 kg and Basic Fins mass 10 kg) around your probe body to convert some of your linear velocity to angular velocity.  If that isn't enough, then use the reaction wheels to counter-rotate and lift against that air.  I'll leave it to you to determine the best angle and placement, or to decide whether this will work at all.

[king of nowhere]

i managed to touchdown at 25 m/s gliding with two basic fins. probably can be improved too, if one is expert about flying. with lift, even the small push from the ant is enough to slow down significantly.

that's slow enough that the small armor i made could have survived that, had i not fallen on water.

this is the design i used; in the end i went for OKTO+wheels not for the 10 kg saved, but because it gave me much more manueverability in air

 

I am now sure gliding down to a slow speed and surviving the crash is possible.

however, it's complex enough to make it work that for my actual probes i'll stick with a parachute. this is now more of a feasibility concept than anything else

 

Edited July 10, 2020 by king of nowhere

[Spricigo]

2 hours ago, Zhetaan said:

I'm suggesting this without having tried it, but have you considered autorotation?  Arrange winglets or fins (AV-T1 Winglets mass 37 kg and Basic Fins mass 10 kg) around your probe body to convert some of your linear velocity to angular velocity.  If that isn't enough, then use the reaction wheels to counter-rotate and lift against that air.  I'll leave it to you to determine the best angle and placement, or to decide whether this will work at all.

Tested with 4 basic fins and it works. But the heat will be a problem if reentrying from anywhere but LKO. 

 

17 hours ago, king of nowhere said:

the ant engine has no efficiency in air.

When testing Zhetaan's idea I noticed that TWR was about 0.4 (with full tank). Swaping to a spider engine may give just enough oomph, given the better atmospheric ISP, if one is not enough a pair will be.

Still, the problem is heat you will face when returning from another planet. You need to either shield your probecore from it or have a lot of fuel to reduce the speed you enter the atmosphere.

[AHHans]

32 minutes ago, king of nowhere said:

i managed to touchdown at 25 m/s gliding with two basic fins.

So, you do survive reentry with them. Good to know!

And, yes, with that it should be possible to land without breaking the important parts. (But on land, not water...)

36 minutes ago, king of nowhere said:

however, it's complex enough to make it work that for my actual probes i'll stick with a parachute. this is now more of a feasibility concept than anything else

That's what I though, too. But part of KSP is to try out crazy - or crazy sounding - ideas, so it's always good to try.

[king of nowhere]

20 minutes ago, Spricigo said:

Tested with 4 basic fins and it works. But the heat will be a problem if reentrying from anywhere but LKO. 

 

When testing Zhetaan's idea I noticed that TWR was about 0.4 (with full tank). Swaping to a spider engine may give just enough oomph, given the better atmospheric ISP, if one is not enough a pair will be.

Still, the problem is heat you will face when returning from another planet. You need to either shield your probecore from it or have a lot of fuel to reduce the speed you enter the atmosphere.

reentry is not a problem. i always perform some aerobraking first, gradually reducing my speed in multiple orbits. in fact, some of my crafts proved quite vulnerable to reentry and i had to spend 10 orbits or so to slow them down carefully enough. it should be possible to reduce speed enough for capture.

as for the spider engine, i hadn't unlocked it yet and forgot it exhisted. it should be enough for the final brake!

[Bej Kerman]

Depends on the speed and how you've set your landing gear.If you come in with a very low vertical speed, you can touch down with wheels and brake.

[Spricigo]

50 minutes ago, king of nowhere said:

reentry is not a problem. i always perform some aerobraking first, gradually reducing my speed in multiple orbits.

I'd say that it still is a problem, just you have your way to deal with.

Still worth mentioning that you may need more than "some" aerobraking in the first pass depending where you are coming from or you will just get back to interplanetary space. Hope you have it figured out.

 

[James M]

10 minutes ago, Spricigo said:

I'd say that it still is a problem, just you have your way to deal with.

Still worth mentioning that you may need more than "some" aerobraking in the first pass depending where you are coming from or you will just get back to interplanetary space. Hope you have it figured out.

 

Wonder if he's got fuel left over in the oscar b after interplanetary flight?

[Spricigo]

1 minute ago, James M said:

Wonder if he's got fuel left over in the oscar b after interplanetary flight?

The probe he describe have 1.9km/s of deltaV. if that is not enough an extra Oscar-B raise it to 2.8km/s

[James M]

27 minutes ago, Spricigo said:

The probe he describe have 1.9km/s of deltaV. if that is not enough an extra Oscar-B raise it to 2.8km/s

An extra oscar b weighs more than a mk1 parachute so it's pointless to do that. 

[Spricigo]

49 minutes ago, James M said:

An extra oscar b weighs more than a mk1 parachute so it's pointless to do that. 

I get what you are mean but I'd not call it pointless if the fuel it holds is actually needed.

Edit: also, it will not weight as much after burning that fuel. 

Edited July 10, 2020 by Spricigo

[Klapaucius]

I love the fact that the Wikipedia entry for Lithobraking mentions KSP.

 

https://en.wikipedia.org/wiki/Lithobraking

 

[James M]

1 minute ago, Spricigo said:

I get what you are mean but I'd not call it pointless if the fuel it holds is actually needed.

Well with the spider engine it might work, but the whole idea was to reduce mass. Fuel is one of the heaviest mass per volume things in the game. 

1 minute ago, Klapaucius said:

I love the fact that the Wikipedia entry for Lithobraking mentions KSP.

 

https://en.wikipedia.org/wiki/Lithobraking

 

Omg I didn't even see that 0.o That's awesome.

[Spricigo]

5 minutes ago, James M said:

Well with the spider engine it might work, but the whole idea was to reduce mass. Fuel is one of the heaviest mass per volume things in the game. 

Unfortunately we can't burn the parachute for thrust/deltaV the same way we do with fuel.

[James M]

4 minutes ago, Spricigo said:

Unfortunately we can't burn the parachute for thrust/deltaV the same way we do with fuel.

haha just had a funny vision of a flea booster being used as a parachute cannon meant to smack into the falling probe to slow its velocity mid fall.