Question

So, I just did my first mission to the moon and landed successfully. I took of and was only able to get my AP to around 200,000 meters above the moon. I've tried using monopropellant and that got me a few thousand meters extra and I've gotten out and pushed with my kerbals jetpack. But, nothing is helping that much. I've ejected my engine and the rest of the dead weight of my lander but still it's not looking good for Jeb, Bob, and Bill. Any advice?

SCREENSHOTS:

https://imgur.com/a/sQr6YoK

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Posted (edited)

Welcome to the forums. Fortunately, all is not lost for Jeb, Bob, and Bill.

If you kept a savegame when you were landed -- you clearly had enough fuel to make it back to orbit. So if you try again and thrust in a different direction, at least your kerbals will be in a ship in orbit (which makes them very easy to rescue).

Alternately, your kerbals are very close to being in orbit. If they each EVA and use a little bit of jetpack fuel, each of them can get into orbit for themselves, before their capsule crashes. And then you will have 3 kerbals in orbit to rescue individually. It is also possible to get your kebals about halfway back to Kerbin. However, it's much easier to rescue them from a lowish orbit around the Mun, than from an extremely high orbit around Kerbin.

 

Edited by bewing

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200k is way overkill, you can use your fuel better by going to 10k-25ish and escape westbound to a kerbin orbit from there.

 

As far as rescuing, fuel-wise you're overall better off with a high, Mun-like orbit than an orbit around the actual Mun - that's terrible advice. If you leave your craft around the Mun you have to account for capture burn, rendezvous burn, escape burn and back into kerbin atmosphere. Waste of a fuel.

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You could also, as bewing said, try the liftoff again, but this time try to keep your apoapsis as low as possible - 10km is enough. The rest of your fuel/EVA fuel you can spend on circularisation.

I shouldn't mention that you're going in terrible direction for a return/rescue, but that's another story.

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Do a rescue mission. Aka a apollo spacecraft. Transfer science and you kerbals, then fly home.

But to do that, you need a stable orbit.

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Posted (edited)
6 hours ago, ZootinZack said:

As far as rescuing, fuel-wise you're overall better off with a high, Mun-like orbit than an orbit around the actual Mun - that's terrible advice. If you leave your craft around the Mun you have to account for capture burn, rendezvous burn, escape burn and back into kerbin atmosphere. Waste of a fuel.

Terrible advice? It's very easy to test.

I put Bill in a 10 800km orbit around Kerbin which is a bit closer than the Mun. Bob I put in a 50km orbit around the Mun. Jeb is in a rescue craft with plenty of fuel in a 100km Kerbin orbit. Let's see how much dV is required to rescue both Bill and Bob and how much fuel is remaining for a braking burn in Kerbin's atmosphere after setting a 55km periapsis.  Both Bill and Bob are in orbits with 0° inclination and zero eccentricity. To eliminate bias from my own clumsy handling of maneuver nodes, I used Mechjeb and in both cases it seemed to do a stand-up job. If anything, Mechjeb penalizes the  Mun rescue because when setting the initial transfer node, I would not be aiming at the center of the Mun as Mechjeb does.

Bob (in 10 800km Kerbin orbit)

Hohmann transfer - 837.1m/s

Match velocities - 367.2m/s

Close to 80m - 17.9m/s

Match velocities - 17.9m/s

(Bob transferred)

Change periapsis to 55km - 373.1m/s

Total dV - 1613.2m/s

Fuel Remaining - 1820

Oxidizer Remaining - 2224

Bill (in 50km Mun orbit)

Hohmann Transfer to Mun - 841.6m/s

Fine Tune to 118km Mun orbit - 19.5m/s

Circularize at 118km periapsis - 236.4m/s

Hohmann Transfer to Bill - 27.7m/s

Match velocities - 29.1m/s

Close to 80m - 2.4m/s

Match velocities - 2.1m/s

(Bill transferred)

Return from Mun into Kerbin orbit with 55km Pe - 254.9m/s

Total dV - 1413.7m/s

Fuel Remaining - 2063

Oxidizer Remaining - 2521

It looks very much like it costs about 200m/s less dV to rescue Bill from the Mun orbit than Bob from the Kerbin orbit. I would assume that any variation from perfect 0° inclination and no eccentricity is also going to favor the Mun.

Edited by mystifeid
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Posted (edited)
On 8/12/2018 at 8:30 PM, The Aziz said:

I shouldn't mention that you're going in terrible direction for a return/rescue, but that's another story.

Interestingly, and as an addendum to the above, it took about 11m/s more dV to reach and return from a 50km Mun orbit with a 180° inclination than the 0° inclination used in the example above - but again, that might have just been me.

Edited by mystifeid

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Hi @FlexT_ape

A tip for getting a stable orbit if you find your Ap is way high: don't continue pushing prograde :targetretro:, actually push inward :antiradial: (yes, toward the surface).  This will lower your Ap and raise your Pe but retain a good proportion of the excess kinetic energy you've already expend3d.

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46 minutes ago, Vexillar said:

Hi @FlexT_ape

A tip for getting a stable orbit if you find your Ap is way high: don't continue pushing prograde :targetretro:, actually push inward :antiradial: (yes, toward the surface).  This will lower your Ap and raise your Pe but retain a good proportion of the excess kinetic energy you've already expend3d.

Or push against the horizon.

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2 hours ago, mystifeid said:

Terrible advice? It's very easy to test.

Beyond these very nice numbers -- a kerbal in a 10.8M orbit around Kerbin is going to get ejected from the system fairly shortly. So you have a very limited time to rescue him, if you put him there. And three kerbals in 10.8M orbits are going to drift apart quite soon, which will make them much more annoying to rescue.

 

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2 hours ago, Xd the great said:

Or push against the horizon.

Yeah, @Xd the great, that's often going to work in these situations, but the orbital anti-radial direction covers more cases and is something you can lock the SAS to (assuming sufficiently experienced Kerbal pilots or advanced probe cores).  Just saves eyeballing a bit.

(And apologies, the icon for prograde should have been :prograde:.)

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In the future, use a more aggressive angle for your ascent. You don't need an orbit of 200km at the Mun. At lot of that fuel spent gaining altitude would be better spent raising velocity. More sideways, less up and down.

As for what you can do right now: if you can, reload the save. You should be on the surface before liftoff. If you can't reload the save, get out and push. You probably wont have enough time to get a nice orbit, but if you can get Pe above 12km, you'll survive (I believe anything lower and you risk striking one of the Mun's more prominent mountains).

So get them to 12km on Pe and then send a rescue crew to pick them up. Don't leave your capsule behind unless there's absolutely nothing you can do to get it to a safe altitude. You'll need it to refuel your EVA packs.

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On 8/11/2018 at 7:27 AM, FlexT_ape said:

I took of and was only able to get my AP to around 200,000 meters above the moon.

If you're limited in dV, don't go for a high orbit like that-- as others have commented here, it's not efficient.

Instead, go for a very low Mun orbit instead.  It's more efficient.  Try this:

  1. When you lift off the surface, immediately crank it over so you're thrusting almost horizontally-- you want your nose to be just slightly above horizontal, no more than 10 degrees at most.
  2. Set SAS to "hold :prograde:" and keep your throttle at 100%.
  3. Wait until your Ap rises to about 10 km, then cut thrust.  Don't raise Ap any higher-- the lower it is, the more fuel you save.  10 km is low enough to conserve dV, but high enough to avoid smacking into any mountains.
  4. Coast to Ap, then burn :prograde: to circularize.

How much dV does your ship have, before it takes off from the Mun's surface?  (Or if you don't know the answer, we can calculate it for you if you can give us three numbers:  the ship's mass as it sits on the surface, the amount of fuel it's carrying, and what kind of engine it's running.)  As long as you have 600 m/s or more of dV, you can get to low Mun orbit.

(Fun fact:  An EVA kerbal standing on the surface of the Mun actually has enough EVA thruster propellant to get to a very low Mun orbit-- barely.  I mean a really low orbit, as in "under 10 km", and you have to nail it just right-- the margin is razor-thin.  But it's doable.  Just something to be aware of.)  ;)

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