Best way to land a 'classic' SSTO on a moon?

[Evanitis]

...by 'classic' I mean one that wasn't designed for that task. I imagine that RCS in every direction or landing legs around the engines makes it easy.

But is there a (relatively) easy way to land on (for example) Minmus with only regular gears and retrograde facing thrusters? During my MJ abusing days, I managed to land on the Mun with a vertical approach and tipping the plane to horizontal on the very last seconds. But it took me many quickloads, even with MJ.

So... any tricks I'm missing, or am I just need (a lot) more practice?

Edited July 10, 2015 by Evanitis

[Coga19000]

Yeah, of course it is possible. It might take some practice, but is perfectly doable. Alternatively, if you're landing on a flat area and feeling lucky, you may try to keep some horizontal Velocity and land horizontally... You will have to land tail first, though.

[Sharpy]

On Mun, that may be tricky, but on Minmus it's perfectly doable.

A gentle enough descent will land you on your engines. Then tip with reaction wheels and (potentially) slow the fall with RCS.

You will need a hill slope for launch though, unless you reach orbital speed while rolling over the plain

[Mic_n]

Minmus is slow enough to be able to kill your velocity just above the surface then quickly nose down 90 degrees to land on your wheels.. Mun has more gravity and so it's a little sketchy.. if you have plenty of SAS, that's do-able as well.

Way back in the day before we had legs, people used fins to land on anyway.. it depends on a bunch of things though.. if you have a relatively light craft (which it should be if you're landing it on the Mun), then you should be able to carefully sit it down on its tail end, landing gear or not. Carefully.

[_stilgar_]

There are two ways to do it. Either make your plane a VTOL, by placing engines or lots of vernors on the bottom and land normally.

Or, just land it as you would a normal lander, engine down and try to get your speed to 0 when you are 10 meters or so (depending on the size of your plane) above the ground, then rotate it to horizontal position (lots of reaction wheels or RCS helps) and land it.

[match]

If you do attempt to maintain some horizontal velocity and set down on the flats, you will need to get under 80 m/s. The landing gear will not take much more than that. (I normally touched down around 40 m/s and had little issue unless it was a "hard" landing.)

As to the "why"? Is easier to simply set down on the tail, but by keeping some horizontal velocity, I didn't have to pinpoint my landing at my refinery, I could put it down on the flats and roll it up (which is actually trickier than the landing!) + is cool

[cantab]

What exactly is going wrong in the failed attempts?

[Edax]

Landing vertically is a lot safer if you have the fuel for it. Just point the main engine prograde until you've killed velocity and are pointed upright. (though technically the spaceplane in this video isn't a SSTO, but that isn't that important)

Edited July 10, 2015 by Edax

[Wanderfound]

As the others have said above, especially for Minmus.

For the Mun, either add a handful of Vernors on the belly for low-grav VTOL, or very carefully land it all the way (until the engine bell is on the ground) before toppling forwards onto the wheels.

A bit of VTOL does make it a lot easier, though:

[ajburges]

As long as you have a certain surface TWR, you can use an altered powered decent to end on your side. It's a simple but hard technique that requires some intuition involving trig and vector algebra.

Essentially, you want to keep more horizontal velocity than normal for the final suicide burn. Then you can start your suicide burn as normal except earlier than your velocity dictates and incorporate a pitch-down below retrograde into the burn instead of following retrograde. Your horizontal thrust is related to the sine of you AoA to the horizon and vertical thrust is related to the cosine. The vertical component is used to control decent, and you use a high initial horizontal velocity so that the horizontal thrust takes you closer to zero. At 30° your vertical component is still 50% of the total thrust! Due to the need to follow a non-prograde course, this technique only works without an atmosphere. If your craft has an atmospheric landing procedure for a place like Duna, it is probably operationally simpler (if less efficient) to use that procedure if applicable.

Planning the manuver mentioned does require calculus to account for the varying pitch (on top of the rocket equation). It might be easiest to select a fixed pitch find the thrust components and calculate a ballistic launch from surface to an arbitrary time. The final conditions of that launch (altitude and velocity) can be used as the start condition of your landing burn. You can use use the simplification that acceleration values will be approximately equivalent. A few hundred of a few thousand dV for a high payload rocket won't realize much change in acceleration capacity.

Edited July 10, 2015 by ajburges

[Evanitis]

...you can use an altered powered descent to end on your side...

I feel that this is the specific solution I was looking for. I have to use the verb 'feel', for it would be an overstatement to say that I fully understand every terminus technicus you use. Long ago, I was exposed to math and English too in school, but never at the same time.

Anyways, I think I got the basics of the concept, and I trust the eyeballing proficiency that playing KSP gave me. I'm fairly certain that the gear's crash tolerance will cover my margin of error. In a few tries. Backwards landing, here I come!

[RocketPilot573]

It's not as difficult as it may seem. Once you get right above the ground, slow the plane to a hover with the rockets. Then, use the RCS to quickly rotate the plane to a horizontal position. Midway through the rotation, throttle down the rockets a bit, it's ok to fly forwards a bit (isn't that what the plane is designed to do?). Most decently built planes can survive a couple m/s touchdown on the wheels.

[ajburges]

I feel that this is the specific solution I was looking for. I have to use the verb 'feel', for it would be an overstatement to say that I fully understand every terminus technicus you use. Long ago, I was exposed to math and English too in school, but never at the same time.

Anyways, I think I got the basics of the concept, and I trust the eyeballing proficiency that playing KSP gave me. I'm fairly certain that the gear's crash tolerance will cover my margin of error. In a few tries. Backwards landing, here I come!

Further investigation reveals that factoring the pitch manuver into your burn makes for some mean integrals.

I now recommend using AoA to control decent/deceleration balance. It is easy to design a craft that can be tailstrike safe at 20°. At that angle, your vertical thrust component is about ⅓. If your surface TWR is at least 3 you can counter gravity at this pitch.

The hard part is that you are committed to a significant horizontal thrust in this landing configuration. This limits how much thrust you can use fighting gravity Here is the Kerbal approach I recommend. (Practice landing procedure at a flat site)

Burn for a low Pe on farside. The lower without impact enroute, the better.

Near Pe, kill enough velocity to guarantee impact.

At 1 km above terrain (no less than 500 m), use a vertical burn to control vertical speed. Pitch down a little to reduce horizontal speed to a controllable level (200 m/s or so). You need to give yourself a low enough speed so that you can avoid obsticals if needed. The idea is to do a constant altitude decent while keeping a certain horizontal velocity.

A few hundred meters from impact, just keep decent to -20 m/s as you approach the ground. As long as you can arrest your fall, you can spend as long as needed killing horizontal speed.

Tens of meters above the ground pitch to a landing AoA use pitch control and throttle to control horizontal deceleration and your fall.

You can tolerate landing as long as your vertical speed is less than 10 m/s and your absolute horizontal speed is less than 120 m/s (forwards or backwards), but the terrian is a bigger threat the faster you go (even the hills can be scary with gravity this low). Forward velocity will also need retro-rockets (or at least RCS) to kill above 20-40 m/s.

Edited July 10, 2015 by ajburges