Mission planning

[Long Finger]

I was wondering how you guys are planning your missions in terms of fuel? I've completed a Mun landing and rescue since I ran out of fuel, and then a rescue of the rescue as it in turn ran out of fuel, and then that ran out of fuel and relied on aero-braking to land back on Kerbin. My initial calculation was as follows:

Mun landing and return dV requirement

Launch to Orbit = 2350 + 1000 (orbital velocity + rough atmospheric/launch loss estimate from experience)

Munar Transfer = 900 (rounded up but I used the figure for Kerbin escape velocity in the tracking station and then subtracted orbital velocity)

Munar Capture = 450 (I got this from an unmanned probe)

Munar Landing = 600 (again from an unmanned probe's orbit velocity)

Mun Return = 800 (Mun escape figure from the tracking station)

Total Estimate = 6100

I stupidly assumed I could escape the Mun and get an atmospheric capture at Kerbin, but I failed to account for the gravitational acceleration toward Kerbin which turned the 800 m/s escape velocity into over 3km/s. If I had to guess I think I would've needed 7500 dV to complete the mission with one rocket. I assume there are much better ways to calculate this, but what do you do?

[Leafbaron]

I calculate my mun landing missions like this.

LKO (70-75K): 3400 m/s Remember the Oberth effect. the deeper you are in a gravity well the more efficient each unit of fuel is.

Mun encounter: 860 m/s, i try to get my Perimun to around 10km to 14km

Mun capture burn: 310 m/s (remember Oberth effect again) 

Landing 580 m/s add in a little extra just in case so maybe 650 m/s

mun orbit: 580 m/s again 

mun escape: 310 m/s this should get you a periapsis on Kerbin of roughly 32 km

Total ~6100 ms 

we get the same answer basically

[MiniMatt]

Your estimates look in the same ball park as I would use, the difference seems to be in the return from the Mun - if you eject opposite to the mun's direction of travel, so ~90 degrees if on the Kerbin side, ~180 degrees if on the far side, you should get those brave green pioneers back home safe and at the dv costs you planned. edit: ignore me, I misread, as leafbaron says below, 3kms aerocapture is doable.

Edited October 18, 2016 by MiniMatt

[Leafbaron]

Aerocapture at 3km/s is perfectly acceptable. that is typically the velocity I return from the mun at as well. If you've got extra parts you're bringing back and the re-entry vehicle is less than suitablee for re entry you can always raise your perikerbin from 32 km to 50km and brin your apopasis to 100km and then return to kerbin at orbital velocities ~2300

[Long Finger]

Okay so I guess it was my piloting or rocket staging that was the problem. My staging worked out like this:

Stage 1    = ISP x 9.8 x ln (Mass Full / Mass Empty) = 195 x 9.8 x ln (69.034 / 30.034) = 1590.466 (SRBs)

Stage 2    = 320 x 9.8 x ln (20.85 / 9.85) = 2351.631 (LTV45)

Stage 3    = 345 x 9.8 x ln (6.225 / 3.225) = 2223.482 (Poodle)

 

6165.579

[Guest]

51 minutes ago, Long Finger said:

Launch to Orbit = 2350 + 1000 (orbital velocity + rough atmospheric/launch loss estimate from experience)

Looks good.

Quote

Munar Transfer = 900 (rounded up but I used the figure for Kerbin escape velocity in the tracking station and then subtracted orbital velocity)

I always create a node for 853 m/s from an 80km orbit. i can then adjust the node time on the orbit (by dragging it around) to get the desired Mun insertion trajectory, whether free-return or not. You can do it on less, IIRC, but 853 is my standard.

Quote

Munar Capture = 450 (I got this from an unmanned probe)

Should be less, about 350 or so to achieve a 12~15km orbit.

Quote

Munar Landing = 600 (again from an unmanned probe's orbit velocity)

Roughly, I usually bring along some extra margin because I can never do a perfect suicide burn and I may want to adjust my landing site as I get closer.

Quote

Mun Return = 800 (Mun escape figure from the tracking station)

600 to get back into orbit plus about 250 for the burn to escape Mun SOI and bring your Kerbin periapsis to about 25km for reentry.

E: My estimates above are exactly that, they are not perfect numbers because I build in margins and may make mistakes.

Edited October 18, 2016 by regex

[Jhawk1099]

I let certain websites/apps do the math for me. And if the Δv is the bare minimum I either multiply the requirements by 1.2 or add a mono propellant/rcs system with at least 50 m/sΔv when all the main propellant is included in the calculations. That way when the main propellant is used to make the maneuver the craft will be lighter for the correctional burns conducted by the mono prop.    

[septemberWaves]

I usually use a delta-v chart, and I tend to give myself a reasonable amount of room for error. That being said, I often go to planets from mods, which do not always have detailed delta-v requirements. In this case, experience playing the game allows me to judge from the basic planetary details (gravity, radius, atmosphere, etc) approximately how much I need (I'm usually accurate to within 200m/s without doing any calculations). I'll then design a lander with about 500m/s more than my educated guess (and aerodynamic if it needs to be) and hyperedit it to my destination to test it. I do this with some stock locations too (mainly Eve and Laythe), but I'm fairly used to the stock system by now.

[Long Finger]

1 hour ago, eloquentJane said:

I usually use a delta-v chart

That's very interesting... and also cheating!

2 hours ago, Leafbaron said:

Aerocapture at 3km/s is perfectly acceptable. that is typically the velocity I return from the mun at as well. If you've got extra parts you're bringing back and the re-entry vehicle is less than suitablee for re entry you can always raise your perikerbin from 32 km to 50km and brin your apopasis to 100km and then return to kerbin at orbital velocities ~2300

I think I was maybe a bit conservative going for a high 50km periapsis. It took 5 orbits to slow enough. I'll try some lower returns and see how hot things get!

[septemberWaves]

7 hours ago, Long Finger said:

That's very interesting... and also cheating!

Do you think NASA cheats by using existing data that has been accurately calculated in the past?

And in any case, I can usually judge fairly well how much delta-v I need. The chart just helps out for a bit of extra accuracy.

Edited October 19, 2016 by eloquentJane

[Atlas2342]

14 hours ago, Long Finger said:

That's very interesting... and also cheating!

Pointing it out as it has been again and again and again, one cannot cheat in a single-player game (e.g. KSP).

Unless you think you can  want to cheat yourself though.

Edited October 19, 2016 by Atlas2342
Rephrased it

[Boris Kerball]

LOL..LOL I to had a rescue to rescue the rescuer, I never went back until the big orange tanks become unlocked, having said that last night with a brand new clear out of all saves I started again and went to the MUN and back with out the Big orange tanks, FIRST ATTEMPT But I don't trouble myself with Delta -V I just add another tank until I make it, I even done my version of Apollo 11 decoupling in Kerbal orbit turning around and docking with fresh engine to go to Mun, One kerbal in mother ship orbiting MUN while 2 kerbals transferred into lander, decoupled landed < flag, sample took off for mother ship leaving landing legs and small engine behind. Docked decoupled from lander and then got home.lol and I was just as excited as when I first done it 3 weeks ago.  Delta -V  far to complicated..lol,  my new name Numpty Kerbal.  lol

[Long Finger]

3 hours ago, eloquentJane said:

Do you think NASA cheats by using existing data that has been accurately calculated in the past?

I don't know how NASA does its work. I think in my mind it would be cheating simply because it saves me from having to switch my brain on, even if it is entirely possible that a Kerbal on my game version of Kerbin could've calculated that chart for me. I certainly don't expect everyone else to play that way though.

3 hours ago, Atlas2342 said:

Unless you think you can cheat yourself.

Yes.

[RX2000]

5 hours ago, Atlas2342 said:

Pointing it out as it has been again and again and again, one cannot cheat in a single-player game (e.g. KSP).

Unless you think you can cheat yourself.

You can definitely cheat yourself. The difference there isnt anything necessarily "wrong" with doing it. 

[Atlas2342]

15 minutes ago, RX2000 said:

You can definitely cheat yourself. The difference there isnt anything necessarily "wrong" with doing it. 

Fair point. There's really nothing wrong with it. I guess it's a matter of "if you want to cheat yourself" than "if you could cheat yourself " though? 

[Guest]

Let's pour gasoline into the fire pit.

Mechjeb is cheating.

[RX2000]

2 hours ago, Aperture Science said:

Let's pour gasoline into the fire pit.

Mechjeb is cheating.

Hmmm. I would say thats a true statement. But then again, it doesnt really matter to anyone else if you are only cheating yourself.

And other people would say its not a true statement, & there is no objectively "right" answer to that really, heh.

Edited October 19, 2016 by RX2000

[Pecan]

7 hours ago, Aperture Science said:

Let's pour gasoline into the fire pit.

Mechjeb is cheating.

Already been done: http://forum.kerbalspaceprogram.com/index.php?/topic/150273-do-you-use-mechjeb-anymore/

While we're at it, all of you "pure" types who also think a dv map is cheating, like Long Finger: do make sure you don't use log tables in your dV calculations, won't you.  Unless you work everything out, from first principles, with nothing more than paper and pencil (pen is cheating. unless you make it yourself), you will never get to heaven.

*eye roll*

Edited October 19, 2016 by Pecan

[Tyko]

21 hours ago, Long Finger said:

I think I was maybe a bit conservative going for a high 50km periapsis. It took 5 orbits to slow enough. I'll try some lower returns and see how hot things get!

I was nervous about 3200m/s aerobraking too..turns out that the capsules can easily handle it with a heat shield. Furthermore the heat shields use less than half their ablator - so there's a little weight savings for ya.

The key is to dive in a bit more deeply. A PE of 30Km seems to work great for reentry from any orbit within Kerbin SOI.

The thinner high atmosphere does a great job of creating heat, but not so good at slowing you down. By lowering your PE you get through that upper atmosphere more quickly and into the thicker air that actually slows you down. I've done many reentries using the 30Km PE / Half Ablator profile and have yet to have a problem.

Edited October 19, 2016 by tjt

[Guest]

16 minutes ago, tjt said:

The key is to dive in a bit more deeply. A PE of 30Km seems to work great for reentry from any orbit within Kerbin SOI.

I use a 25km PE and about 40% of the ablator on any given heatshield for anywhere from within Kerbin SOI. Your Kerbals will never black out and you will still have a good 20% of the ablator left, even on 120% heating.

[Tyko]

 

1 hour ago, regex said:

I use a 25km PE and about 40% of the ablator on any given heatshield for anywhere from within Kerbin SOI. Your Kerbals will never black out and you will still have a good 20% of the ablator left, even on 120% heating.

LOL, lower PEs and Suicide Burns are really high on the pucker factor. I guess I'm still being more conservative than I need to.

[Leafbaron]

suicide burns are difficult in 1.2. I was trying to do one the other night and normally when you max out the velocity on a closed orbit your d-v would stop going up and flicker back and forth. Now it seems in 1.2 if you pass the suicide burn velocity your orbit is projected opposite the way you came. So it doesn't seem that easy to set them up anymore. any one else had the same experience?

[Long Finger]

It seemed when I started that there wasn't really a need for heat shields on a pod, but I did manage to blow a few up with steep descents and stopped doing them altogether. It sounds like with a heat shield it's okay, but have you ever found your pod won't slow down in time to get the parachutes out and pancaked in?

[Aegolius13]

2 hours ago, Long Finger said:

It seemed when I started that there wasn't really a need for heat shields on a pod, but I did manage to blow a few up with steep descents and stopped doing them altogether. It sounds like with a heat shield it's okay, but have you ever found your pod won't slow down in time to get the parachutes out and pancaked in?

I don't think this is a problem as long as you set your periapsis high enough, usually around 30km.  As stated above, the worst case  (from the mun at least) of aiming high is you head back into space and make another pass, which is good for heat control anyway.

Even coming back from Moho or Jool at huge speeds and with a heat shield, heat has been the limiter for how low I can aim, not hitting the ground.

[Nich]

A perfect suicide burn with 15 TWR and you can get away with 560 dv for a Mun landing.  However if you kill your horizontal velocity at 10km and then sink down at 50 m/s  you could spend up to 1000 dv in a low twr lander.  Lots of variability.  Also a very good gravity turn and you can get away with 3250 to LKO but 1.25m parts with a low twr and lots of draggy parts and you may need upwards of 3800-4200 dv to get to LKO.  I like to plan for the worst and hope for the best.  If you bring to much dv in your lander you can always hop to the next biome and get some more science.