Is there an optimal (low delta-v) reentry path?

[tranenturm]

So the situation is as follows:

Valentina was flying a 1.25 rocket out to a Minimus polar landing. I overshot the light side by a bit and was suddenly landing in the dark, didn't know the hard cap was 3700m, and thankfully only lost the lander legs and engine. Queue rescue mission.

I take my "standard" Minimus landing rocket and replace the Material Lab with a lander can. So now I'm a half ton heavier and less aerodynamic on liftoff. My liftoffs care about this as I use 4 Kickback SRBs as my first stage which pretty much pops me out to orbit. I know this isn't the most efficient method, but its been working. I can't do the gravity turn with them as when I eject them, there's a danger of hitting the rocket. So I eject, then turn, but at that point there isn't much thrust required to get to orbit (before the circularization).

All this is to say, the rescue rocket is on the Minimus North Pole and I ended up using more fuel than I'm used to. My normal reentry involve launching from Minimus, clearing its SOI, then doing a retro burn to pull my PA to about 40,000 - 60,000. Right before I hit the atmosphere, I burn everything left in the tank to take as much speed off before I start the reentry heat. Even so, I usually end up getting several heat bars before I decelerate enough to cool off. I do use a heat shield.

I know I have enough fuel to get me back to Kerbin, but I don't know if I have enough to slow me down enough to not burn to a crisp. In standard Kerbal practice, I ask this question now with my two Kerbals committed, instead of asking when I was redesigning my rocket. Reverting to a save is not an option.

So how do I get back to Kerbin using the least amount of fuel? And is there an optimal flight path through the atmosphere I should be aiming for to minimize reentry heat?

I am willing to spend some time (but not too much) having them aerobrake through the Kerbin atmosphere with enough speed to go back to orbit, and do this until enough speed is wiped off to successfully land. However, I've never done that maneuver so I don't know the optimal altitude to aerobrake without landing. Too high, and it takes forever, too low and I reentry too early (and therefore too hot).

I've got 137 points of liquid fuel (and equal level of oxidizer), a 6.2 ton rocket, and the LV-909 engine. (currently landed at the Minimus north pole)

Thanks in advance for any help!

Pete

Edited May 16, 2015 by tranenturm

[Old Foxboy]

Hello,

I would try to make a direct ascend from Minimus into retrograde Kerbin orbit direction. And i won't wait for leaving Minimus' SOI, because this means higher delta V needed to lower your Kerbin periapsis.

The main problem is to find the perfect launch direction to make all the needed delta V in one big burn from Minimus launch. Else you'll need to orbit Minimus and plan a good ejection burn. This might be safer, but however, plan your ejection burn in a way that you don't need a big delta V later to lower your Kerbin periapsis.

If you are already near Kerbin (about 2/3 between apoapsis and periapsis) and want to lower your periapsis, it might be better to burn anti-radial instead of retrograde. You won't lose much speed, but your flight path with change significantly, allowing for a much bigger atmospheric drag.

Coming in with almost 3000 m/s, you want to aim at 25.000 m periapsis to land at Kerbin. Above 30.000, and you'll probably only get an aerobrake. For aerobraking, don't aim at anything above 40.000 - it won't brake you enough to make up for the risk of stacking heat damage.

So, if you fear the full reentry will toast you to pieces, aim at 35.000 and do a one-orbit aerobrake. This should be sufficient. Make sure you have a little extra fuel left to fine tune your second reentry at 25.000 periapsis.

Good luck, may the force be with you.

[Snark]

What Foxboy said.

Assuming that the "6.2 tons" you said is your wet mass, I make your delta-vee to be 953 m/s, which should be plenty to get home from Minmus.

The theoretical optimum would be to do one single burn from Minmus surface, but that would be pretty hard to control just right, so I'd recommend orbiting Minmus first. To take maximum advantage of the Oberth effect, you want that orbit to be as low as possible. So when you take off, go to max throttle and tip over to near-horizontal almost immediately, so you can do a surface-skimming circular orbit at really low elevation, say 6000-7000 meters. (If it's a couple of kilometers higher, no big deal-- Minmus has a pretty shallow gravity well, you don't lose all that much).

Since you're at the pole, it matters which direction you go. You want the plane of your Minmus orbit to be perpendicular to the direction to Kerbin. That is, if you picture someone on Kerbin looking at you, they should see your orbit as a perfect circle from that angle, not a back-and-forth line.

This will now give you time to plan your 2nd burn, which will take you home. You have leisure to set up a maneuver node. Your burn will be over either the N or S pole of Minmus (depending on which direction you're orbiting). If an observer on Kerbin would see you making a clockwise circle, you want N pole; if counterclockwise, S pole. (The idea is that the direction you're orbiting Minmus should be opposite the direction that Minmus is orbiting Kerbin, so that your Minmus orbital velocity will be added to your going-home burn.)

Set up your maneuver node to take you to a Kerbin periapsis like Foxboy said. If you want to minimize reentry heating (i.e. spread it out over a long time), 35km or just a smidgeon lower (I usually use 34) works well. That will give you one pass to aerobrake to LKO, and a second pass for Kerbin reentry.

You actually have a fair amount of excess delta-vee to work with-- 953 m/s should leave you a few hundred m/s left over as you approach Kerbin. So you have some additional options. One option would be to circularize into LKO after your initial 35km burn (won't take much), then launch another rescue vehicle to pick up (it would be a pretty cheap one). Another option is to burn up the remainder of your fuel as you enter the atmosphere on your second pass, to kill a few hundred m/s and give you just that much less reentry heating to deal with.

Another area of strategy is how to handle the actual aerobraking itself (e.g. orientation relative to your velocity, how heat-tolerant your lander is, etc.) Do you have any heat-shield at all? Airbrakes? Any fins/airfoils? What sort of configuration is your lander?

Edited May 16, 2015 by Snark

[Cardano]

also - with a north pole landing on Minmus that means just enough launch velocity to allow you to go into an immediate 90 degree heading and burn east ( kerbin retrograde )

-edit - snark explained the whole kerbin retrograde thing alot better than i did - but from the north pole you should be able to do an immediate kerbin retrograde burn straight out of minmus. he is correct however that with that much dV you can orbit minmus and still eject safely and slowly - giving you time to plan that manuever node and understand the physics of it. mostly - the whole LKO circularize option is excessive worry IMO since the heat shield can safely take you in even if you insertion angle is 90degrees ( straight down ) - as long as you have (2) chutes - 1 chute could very well mean disaster - so hopefully when you re-tooled that ship from single kerbal use - you added a second chute. otherwise I would rescue from LKO.

until you eject from Minmus SOI. if you were equitorial on Minmus you would wait until you were facing kerbin retrograde to launch straight 'up' which would eject you from Minmus SOI into kerbin reentry path.

As to Aerobraking - my experience in Kerbin SOI with 2 kerbal weight vessels reentering at ~3k ms is that it is often best to leave the 909 and FT-Lxxx on the ship instead of ejecting it and just using the heat shield. but in either case you will be fine for a direct reentry. I shoot for 35k PE and burn hard retro to empty the tanks as soon as atmo is observed heating the ship. you can keep the lander legs extended to increase drag and they will usually burn off ( safely ). Exciting but not deadly.

pretty much with minmus you always have enough dV to do what you want.

and pretty much with Kerbin SOI reeentries you wont 'splode.

you can take your PE up to 40k and it might take 2 or 3 aerobrake orbits to land, but that should not be necessary with a heat shield.

other neat tricks you can attempt : come in max drag oriented once the empty tank and engine are no longer able to thrust retro - they can still decelerate you with drag and you can ditch them if they get too hot.

Only issues you might run into ( since they arent burning to a crisp ) are chute deployment. aim for an ocean as best you can cause a high mtn can ruin your day. and deploy the chutes in the 300 ms range - then use SAS torque to increase drag by pushing into a higher drag shape ( anything other than what the wind pushes you into naturally will increase drag and speed your deceleration .

Finally - I also like the 909 and FT-Lxxx [and if any legs are left] since they can explode on touchdown and my precious kerbals are still safe.

Just ran a test and a 41k PE got me a three hop to insertion with a 2400 ms velocity on final - and nothing got red temp bar at all ( I was even facing nose prograde the whole time )

You got this.

Cardano

Edited May 16, 2015 by Cardano

[Cardano]

in further news - I just took a pair of Mk1 pods with a shield and (2) radial chutes in at 3300ms straight down ( blasted to edge of SOI straight up and fell back down )

it was 'dicey' but I only lost 80 ablator, got about midway into red bar on temp gauge and only had about 1500 meters elevation when I hit 300 m/s and deployed the chutes.

however, since I was over water - it was no problem on the landing.

Moral of the story - with decent chutes and a water landing - reentry can be very forgiving.

Cardano

[DoToH]

With current settings (1.0.2) you can reenter directly from Jool, without heat shield and your pod and chute will survive if correctly oriented during reentry. Tested with Mk1 pod and Mk16 chute. Works even in a Jool -> Eve direct reentry (hitting atmo at 6000m/s).

On topic: Since heat is not a true danger, choose the most fuel efficient reentry profile.

Edited May 16, 2015 by DoToH

[Aethon]

Yeah, don't worry about the heat. Kerbin is too small to do much heating damage at Minmus return speeds. As long as you can keep your ass end (engine works fine but you said you have a heat shield) pointing retrograde you'll be fine.

I've been testing some Kerbin aerobrake altitudes recently. It depends somewhat on your ships' unique drag profile. My standard 1.25 m early Mun Minmus lander is pretty standard so my results may apply to you.

From Mun, I use a 30 km periapse for breaking into Kerbin orbit. Anything lower will put you on the surface and higher will allow for another pass. Minmus will be similar (haven't tested much yet). You could probably shave a km off the 30km peri and still go back to orbit. Alt f5 for safety and try 27 km peri. You may go halfway abound the planet, but that should land you.

Welcome to the forums and good luck... We're all counting on you.

[tranenturm]

Thanks All!

It's almost a little disappointing that I don't have to worry about heat. I decided to take it seriously anyway and did the following.

I took off from Minimus and just took a hard right. (Kerbin retrograde) I still had to do a second burn to get my periapsis in, but I set it at 35,000 and decided to go one round of aerobraking. Lander legs heated up, but nothing big. On the second pass I burned the remainder of my fuel and jettisoned the tank and engine. Came in a little over 2,000 m/s with no problems on the reentry, other than the landing itself. The additional weight of the lander over the standard material bay meant I came in for landing about 8 m/s which ended up busting up my science experiments that I still took. Oh well. Both trips were still a net gain.

Thanks again everyone!