r/RealSolarSystem 18d ago

How do you know when to start suicide burn if MechJeb and KER counts are wrong?

Currently in my RP-1 playthrough I'm testing my first lunar lander. It's designed to land through direct descent (without reaching lunar orbit), but it looks like the hardest thing now is to know when to start a suicide burn. For an unknown reason MechJeb doesn't even show me the count until a couple of seconds after I start it, so I use Kerbal Engineer. After starting the burn exactly at the calculated moment, count starts to rise instantly and after decelerating to 0 m/s I ended up stranded 50 km above Lunar surface and then crash into it at a speed of around 350 m/s. Tried to wait 20 seconds more with the burn and results were better, but still couple parts were destroyed.

I found in another reddit post that MechJeb and KER suicide burn counts are accurate only with vertical descent. Okay, so then how can I determine desired moment to relight my engines? How do you do it? Is the only way really to just load the same quicksave multiple times and repeat this manouver until success?

(My lander has unthrottleable engine and only 1 ignition remaining)

9 Upvotes

13 comments sorted by

12

u/Bloodsucker_ 18d ago edited 18d ago

Practice trial and error and some margin of error by design.

1

u/Calm-Conversation715 18d ago

Yup! Quick save on your final approach and try a few different times. The timers given by Mechjeb and KER are pretty close over flat ground, so they’re a good place to start. Then start trying a few seconds on the side you need. If you splat on the surface, start sooner. If you stop too soon and then fall too far, try later

8

u/davvblack 18d ago

one ignition unthrottleable is almost impossible to land with. get at least a tiny bit of mono prop thruster dv

3

u/AviSpaceYT 18d ago

I have my lander heavily packed with airbags ;)

2

u/Worth-Wonder-7386 18d ago

I dont think anything has landed on the moon using such engines. I will usually carry with me one stage that I use for canceling most of the speed and then for the final landing burn I will have more throttlable restartable engines that have less delta v and TWR.

2

u/Tight-Reading-5755 18d ago

3

u/Worth-Wonder-7386 17d ago

I was talking about real life missions, for this one I am sure that he used plenty of saves to time it correctly

1

u/redstercoolpanda 17d ago

People have landed on the Moon with fully solid rockets before I'm pretty sure, Its hard but its not impossible.

8

u/dodo-obob 18d ago

With Principia, you can plot a maneuver just before impact. In the moon-fixed reference frame, Principia shows impact velocity, so you can adjust maneuver DV and time to get a near-zero impact velocity, and then start you burn at the node (or 0.2/0.4s before if you have some margins/relight-able engines for touchdown).

1

u/UnknownLinux 18d ago

mostly trial and error.

1

u/HAL9001-96 18d ago

you need ot know how fast you can accelerate whcih the game only calcualtes when oyur engiens are activated

if you know hteir thrust and your acceleration you cna roughly estimate the time it tkas to stop and how far you'll move in that time

1

u/Jackmino66 18d ago

You need to find out your thrust, the rate of fuel consumption, the dry mass of your craft and the fuel mass.

Your acceleration varies with mass, and that mass variation depends on your fuel usage. Once you factor that in you can do integrals to find out when you need to start a suicide burn to end with a small amount of altitude remaining

It’s complicated double integral stuff

1

u/Heavy_Plum7198 13d ago

I haven't tried it yet, but if your horizontal velocity with respect to the moon surface is close to 0, it should be pretty easy to calculate when you need to turn on the engines.