Fuel efficient descent to Mun

[JMG 0]

When landing on the Mun, is it more fuel efficient to slow a bit all the way down or to burn hard at the end?

When landing on the Mun, is it more fuel efficient to get into an orbit first or just come straight at the Mun and burn before you hit?

(And if you understand the Physics, why?)

Thanks

JMG

[bewing 3,050]

When landing on the Mun, the burn is called a retroburn.

If you go into orbit first: if you try to burn all at once from orbit all the way to a standstill, then once you get to a standstill you start to fall. Depending on your altitude, you pick up some or a lot of velocity as you fall. Then you have to kill all that downward velocity before you actually land. That's the inefficient part -- it's called "gravity losses". You have to kill your orbital velocity before you can land. So all that 500+ m/s of deltaV is gone no matter what you do. But the falling part is optional and wasteful.

So you want to be as low as possible when you do your retroburn. That's all it comes down to. For a very low cost, you can lower your Pe down to a couple thousand meters above your landing site.

Burning slowly insteand of all-at-once means that your average altitude is fairly high. Which means you are still accumulating gravity losses. On the other hand, apparently there is a mathematical proof that a "constant rate of descent" landing is equally efficient to any other method.

_________________

If you land directly without entering orbit first, that is called a "suicide burn" and it is significantly more efficient. Why? Two reasons. The first is that if you do it in two steps and enter orbit first, then you have the problem of gravity losses during your final descent, as said above.

The second is the Oberth effect. The faster you are going when you do your burn, the more efficient it is.

Once you have completed your suicide burn, you are extremely low to the ground and there is very little time left for falling or gravity losses. There is a fairly clever way to use a maneuver node to discover how long you will need to burn for your suicide burn, and how many seconds you have left before impact. If you leave yourself a few seconds to spare, you will generally be fairly safe. If you do a search on the forums for "suicide burn", you will find many postings about the technique.

HOWEVER, the problem with this method is that you can't really pick your landing site, and it might be unsuitable when you get there. (If you're aiming for one of the flats on Minmus, this isn't a problem.) Aborting a landing, and then traversing terrain to find a better landing site is about the most fuel-wasteful thing that you can possibly do in a rocket.

 

 

Edited June 7, 2020 by bewing

[jimmymcgoochie 1,436]

Landing directly from a transfer is more efficient, but it's not called a suicide burn for nothing! A slight miscalculation and your rocket is a smear across the surface.

Landing from orbit, it's best to bring your periapsis down as low as you can to minimise the distance you have to 'fall' and pick up speed via gravity that you then have to get rid of again, and also to maximise Oberth effect: the faster you're going, the more efficient a rocket burn becomes so coming in faster is better.

When you're near your chosen landing site, burn retrograde in what is effectively a reverse gravity turn, scrubbing off as much horizontal velocity as you can while fighting against gravity as little as possible until you end up with only a little bit of velocity at a low altitude. Too much braking and you'll just have to land vertically which uses a bit more fuel; too little altitude and you'll crash into the ground at considerable speed. However, a longer burn with lower thrust can be better as you have a bigger safety margin to abort the landing if you have to and won't make much of a difference in terms of fuel use.

It also depends on how precise you want your landing site to be- if you just want it to be somewhere on Minmus' Greater Flats, you can focus more on being efficient, but if you want to land close to something else on the surface (like when you're adding a new module to a surface base, for example) then it's better to kill your horizontal velocity first even if you then have to drop a greater distance vertically and use more fuel, to make sure you land exactly where you want to. MechJeb's landing function uses this method to land at a specific set of coordinates with very high precision.

[Laie 1,407]

11 hours ago, JMG said:

When landing on the Mun, is it more fuel efficient to slow a bit all the way down or to burn hard at the end?

Depends on TWR.

Well no, actually it doesn't -- the ideal landing works like a gravity turn, but in reverse. That is difficult to plan and execute, though.

With lots of thrust, a suicide burn is virtually indistinguishable from a reverse gravity turn -- if you can pull it off. Braking too early may lead to you effectively hovering for tens of seconds, and braking too late is, well, suicidal. With a local TWR > 3, your timing and steering errors will have more of an effect than the theoretical difference between suicide burn and an ideal landing.

With TWR <2 you want to get into a low orbit first, and try something like a constant-altitude landing, as demonstrated by @Kosmo-not in this classic:

That video is very old, the Mun was much flatter then. He also has a very low TWR, so as to better be able and demonstrate the technique.

You can usually do better, dV-wise, by doing more of a "controlled descent rate" landing rather than "constant-altitude". The beauty of that method is that it can be flown seat-of-your-pants style, with little regards to planning or split-second timing, and with some experience and educated guesses, the controlled descent approach can come very close to the reverse gravity burn.