Ersatz landing gear

[farmerben]

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.

 

 

[Superfluous J]

For small ships, I found the Making History structural tubes to be WONDERFUL landing gear. Just slap one around the engine on the bottom of your lander.

Never tried it with the larger ones (ships or tubes) but I suspect they may work there as well. Though you never know. The Square Cube law can be finicky.

[Mahnarch]

Now, try using asteroids as landing gear!

You see; asteroids are made of rock. And the planets are, presumably, made out of rocks = they can't destroy each other!

 

Disclaimer: using asteroids as landing gear voids any warranty of your craft, either explicit or ambiguous. Void where prohibited. One per customer.

[klesh]

2 hours ago, farmerben said:

Scott Manley demonstrated an Eve launch with 12 asparagus aerospike stages.

 

Are you talking about this one? 

https://www.youtube.com/watch?v=ReuemZV3ZcM

 

Be careful, that's from 2012 and I am willing to bet the atmospheric situation at Eve has changed since then.

Edited August 6, 2018 by klesh

[farmerben]

yep that one

 

[StrandedonEarth]

3 hours ago, farmerben said:

So I don't understand why they would discard their elevons before launching from Eve.

Rule for stability: Draggy at the back, but not at the front. Descent is butt first, so they need drag at the nose to keep it that way. But drag at the front would make it flip on ascent, so they discard the elevons.

The center of mass will want to be in front of the center of pressure/drag. So you need to make it draggier behind the center of mass, relative to the direction/orientation you want to travel. Or you need a lot of control authority to keep it going straight in an unstable configuration, and any angle of attack (not going straight) will increase the forces trying to flip the craft, if it is not aerodynamically stable.

[Gargamel]

6 hours ago, farmerben said:

So I don't understand why they would discard their elevons before launching from Eve.

Like @StrandedonEarth said, stability.  But you are also doing it in the wrong order.  Eve missions go Landing then launch.  Kerbin missions go Launch then landing.    If you are testing an Eve vessel on Kerbin, you gotta remember the flight modes are reversed. 

7 hours ago, farmerben said:

Are legs worth it at all? 

Very much so, especially if you are landing on terrain that might not be flat and level.  Spreading out your footprint using legs to keep your CoG within the footprint will keep the ship upright.    Otherwise, the ship will FDGB. 

[Mad Rocket Scientist]

Aerospike landing gear? Pu-lease.   Place anywhere linear-7 RCS ports are the best ersatz landing gear. 50m/s impact tolerance means the launch pad will break before vernor engines do. They are also aerodynamic, light, and can survive reentry, which the 80m/s tolerant structural parts do not do as well with. Note that around 25m/s, they will phase through the ground, so you either need to stack them or land slower than that. However vernor engines work quite nicely as skids, since they have the same impact tolerance, and even look like skids.

Edited August 6, 2018 by Mad Rocket Scientist

[farmerben]

5 hours ago, Gargamel said:

 

Very much so, especially if you are landing on terrain that might not be flat and level.  Spreading out your footprint using legs to keep your CoG within the footprint will keep the ship upright.    Otherwise, the ship will FDGB. 

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.  

[Gargamel]

2 hours ago, farmerben said:

That principle is plain

You'd be surprised at the number of people this is not obvious for.  Not a KSP player for the most part, but the populace in general. 

[Zeiss Ikon]

16 hours ago, Gargamel said:

You'd be surprised at the number of people this is not obvious for.  Not a KSP player for the most part, but the populace in general. 

Yeah.  These are the same people who wonder why leaning way, WAY out on a ladder while cleaning their gutters lands them in the ER, and then pull a nearly identical stunt the next summer trying to adjust their satellite dish.  The ones who'll give their social security number and mother's maiden name, as well as their own full name, driver's license, and bankcard number(s) to a random caller who claims to be from their bank (without actually saying which bank).  The ones who'll activate remote desktop when "Microsoft Support Services" calls them out of the blue.