Landing Delta V

[Fyrem]

First, let me exclude ANY Celestial body that has an atmosphere of any kind. That leaves me with

Moho, Gilly, Mun, Minmus, Ike, Dres, Vall, Tylo, Bop, Pol, Eeloo.

Most (if not all) of these bodies have "known" Delta V values to get a ship into orbit: Kerbal Engineer Redux, MechJeb 2 (I think), and of course the Wiki have these values for Launches from the body, up to a 'normal stable orbit'.

My brain says that there SHOULD be a relationship between the DeltaV to Launch and orbit an object, and the DeltaV needed to safely Land that same object from that same orbit. Is the DeltaV needed to Launch from Mun and get up to say a 20k stable orbit, the same Delta V needed to LAND from a stable 20k orbit? Now I realize that "when" the burns occour significantly changes the math, and that Thrust to Weight (based on the gravity of the body your landing on) also changes things.

What I am looking for is "Can I use the Launch Delta V" of a rocket as the "minimum delta V I would need to Land that same rocket?

(I would likely add 20% as a safety measure).

[Kerbolnaut]

Yes, launch delta V = landing delta V. If you had a cannon on the surface of the moon and pointed it straight upwards, the cannonball would return to the surface at the same velocity that it left.

It only gets a little bit tricky when landing because you usually can't pull off a perfect landing with a single full-power burn. Most people need several small, gentle burns to keep their speed within a safe range as they approach the surface. This is less efficient than just taking off at full throttle and burning until you've used up all of the fuel allocated for takeoff. This is why you'd need some extra fuel for landing. However, if you're using some program which will let you land with a single full-power burn, then I wouldn't worry about that.

[Fyrem]

The straight up=straight down I understood would be the same. I just wasn't sure about a circular orbit. Good to hear. And yeah, I don't have any program to do the burns, that is why I would add about 20% more fuel to land with.

[drakesdoom]

Remember that you can use less delta v on take off if you land at the equator and burn with the rotation of the body, results may vary based on location.

[K^2]

Your ideal landing should be the ideal takeoff in reverse. Of course, you can't mirror it perfectly, because the rocket gets lighter as the fuel gets used up, but this is the basic reason why the two delta-Vs are basically the same.

P.S. You should budget extra for landings, though. It's very unlikely that you'll perform anywhere close to a perfect suicide burn, and if you brake a little early, you'll need extra fuel to fight gravity.

Edited April 23, 2013 by K^2

[Alistone]

Oberth effect means it is more efficient to wait as long as possible before you do your decelerating landing burn so you're going as fast as possible.

After deciding I didn't like the trial and error of I'll slow down at 30km...splat....F9....I'll slow down at 50km... too high... F9...

My preferred method now is to get my mun periapsis at about 5km above the surface. This ensures I'll be going nearly maximum velocity for my slow down burn. (If I want to change from an equatorial to a longitudinal orbit I slow down just enough to remain in very oblong Mun orbit and change my orientation at the apoapsis)

I burn at periapsis to make my orbit circular and it allows me to be tightly orbiting at still relatively high velocity (high velocity = good fuel economy for landing burns). For landing I just continue decelerating at full power along the horizon, only pointing my nose up enough to keep my vertical velocity near zero. Once I have no velocity at all I do a controlled slow descent the last 5000 feet.

This is especially useful since I often use a pair of low power high ISP atomic engines to land. For a vertical descent they would have to start slowing down very very early due to low T/W ratio. But by using a tight periapsis they can easily do most of their decelerating maneuver at low altitude and high speed without splatting my lander into the mun. Saves me the trouble of having to bring an extra rocket engine along just for the lander.

Don't forget some downward facing lights if you want to land on any dark areas.

Edited April 24, 2013 by Alistone