Deadly Re-entry and Ferram Aerospace Research

[Enigma179]

So I've been trying KSP out in hard mode with these two mods plus the Ioncross Life Support mod. I tried to run before I could walk and set up a Mun base, but unfortunately screwed up landing on the new Mun's more hilly terrain, and eventually gave up and sent a rescue mission to pick up my three kerbonauts.

Problem is, on the way home I discovered something nasty about FAR: drag isn't as powerful as it usually is. I used to have just deadly re-entry, and coming in at 30km periapsis would be enough to land me safely without even burning through half the heatshield on the Mk1-2. With FAR in the mix though, presumably because air resistance falls off after getting past Mach 1 (I think that's the physics behind it anyway), 30km sets me up for a high apoapsis. Luckily I forsaw that there might be a problem like this and quicksaved, so through heavy trial and error I worked it out that my best shot at re-entry from a Munshot was 23km periapsis. Anything below that, I'd either lose my heat shield and burn up (way off) or not have enough time to bleed off speed for my parachute to avoid snapping at full deployment (above the ocean mind you). Above that and I'd miss completely and have to come around for another pass, which I guess the pod could survive, but I didn't want to take that chance or the extra few hours.

Anyway at 23km I found that the temperature of the burn started decreasing again just as the heatshield ran out, so while it heated up to 1400 degrees or so it still avoided exploding, and also that I would pass my own periapsis, causing the pod to start climbing for a few seconds before again falling, reducing vertical velocity and maximizing the amount of time I had to bleed off my fall, so that by the time I hit 500 m I was travelling at "only" 120-130 m/s, which was still nerve wracking because I only had one chute to take all that force.

I'm getting to the question now: how do you go about figuring out where to drop your periapsis to using FAR? I don't know if Deadly Re-entry makes a difference, but I don't think it does. Should I just aim for 23km no matter what? I feel like from where I'd be going would make a difference but is there a guide or some way to calculate the proper re-entry angle?

[Swifty]

I think it's down to guess work with FAR as mechjeb landing guidance uses stock drag to calculate the drag. Also if your in a spaceplane lift from the wings are also a problem so my need to manage some nose down aoe to compensate.

FAR is much much more complex than stock and calculating drag on re-entry by hand is not possible as drag on each component changes with angle of the oncoming airflow. Thus to calculate re-entry height a solver would need to be written to number crunch (albeit in a clever manner) the different possibilities to find the desired solution...

[Ralathon]

It totally depends on how fast you're coming in. If you're coming down from an interplanetary Jool trip you'll have to dip much deeper into the atmosphere to lose your velocity before your heat shield runs out. Conversely, you can get away with a relatively high periapsis when coming down from low kerbal orbit.

I usually aim for 3km periapsis when coming down from interplanetary and 15km when coming down from low orbit. It is pretty much a inverse linear relation between those 2 points vs velocity for the rest.

Also, note that the Mk1-2 heat shield burns up much faster than the tiny mk1 capsule. So take that into account, what works for one might not work for the other. It might be best to just make a list based on trial and error.

[Enigma179]

It totally depends on how fast you're coming in. If you're coming down from an interplanetary Jool trip you'll have to dip much deeper into the atmosphere to lose your velocity before your heat shield runs out. Conversely, you can get away with a relatively high periapsis when coming down from low kerbal orbit.

I usually aim for 3km periapsis when coming down from interplanetary and 15km when coming down from low orbit. It is pretty much a inverse linear relation between those 2 points vs velocity for the rest.

Also, note that the Mk1-2 heat shield burns up much faster than the tiny mk1 capsule. So take that into account, what works for one might not work for the other. It might be best to just make a list based on trial and error.

I feel like this applies to only Deadly Re-entry, because from a Munar drop, when I aimed for 20km periapsis, with FAR I still had too much speed when it came time for my chute to fully deploy. Then again, under similar deployment conditions on a test run from LKO the chute managed to stay attached when I strutted it up, but I still believe aiming for a 3km periapsis is suicide with FAR in the mix.

[mizo]

By the way, with FAR, you can control how much lift you get from the capsule by controlling your angle of attack, so you can actually resist to some degree both being sent into the lower atmosphere too soon and being bounced out altogether.

You can also steer the capsule to control it's heading somewhat.

Check this video I made some days ago. It used a version of FAR which was calculating too much lift for the mk1-2 pod, but the principles are the same. You are just not going to get a performance this good from current FAR, but you can still work with it.

It was made using FAR and Deadly Reentry, and it was possible to accomplish reentry with no heatshielding whatsoever, and that with minmus reentry speeds.

[Event Horizon]

Might have to add a last stage for a powered landing. If you want to conserve fuel (and therefore, weight), you can do this with jet engines. B9 Aerospace has these really nice VTOL engines that you can adjust as well. Point em downwards when you add them in the builder, then you can turn them sideways to help cancel horizontal velocity as well.

[Frederf]

I just recently did testing with FAR/DRE on a sample unmanned Mk. I pod and chute. A probe body was along for the ride in order to provide control.

Previously I did have problems with FAR not slowing the craft for suitable recovery. With more recent versions (955 is what I'm testing right now to see if it's changed) I discovered I had a wide-ish window for reentry up to Munar energies. I attempted minimum orbit (60-70k) and Munar schemes and all had acceptable windows. I found that during Munar reentry my acceptable periapsis was 0-34km bounded by G tolerance and aerobraking limits respectively. The most shield I ever used was about 60% (I recall 108/250 left) and was never a limiting factor on these profiles. Perhaps double brake profiles or higher energies would reach the shield material quantity limits. Firstly I noticed a large difference in drag depending on orientation. Second I noticed a small ~5 degree cone of stability around the blunt attitude and another larger one around the pointy attitude. A zone of repulsion exceeded reaction wheel strength at significant Q loadings. The consequences of being on the wrong side of the fence too late into the reentry are obvious.

At 30km I had a no-bounce profile with no more than 4.5Gs. Approaching 34km I had a significant balloon in altitude to 66km which used more shield material and lessened G loading. Toying with this limit with minimal onboard battery life was beneficial but risky.

Lift (which I think FAR 9551 addresses) offered surprisingly significant control. Not much experimentation was done regarding trajectory control by lift but L/Ds of no less than 8 were recorded.