FAR/DRE: Stable entry vehicle design

[mic_e]

Hi,

so I recently decided to go all the way for realism, with Ferram and Deadly Reentry (this one).

I've tried to design a (complex) lander capable of landing on Kerbin from orbit, using the 6.25m Inflatable Heatshield. However, the design becomes violently unstable at any significant dynamic pressure levels, as high as 50km, and will always turn in the wrong direction (the heatshield will act as a parachute, not a shield...).

Only the most unrealistic, extreme modifications to the craft were capable of stabilizing it in its retrograde orientation (e.g.: A set of four delta-wings mounted tens of meters above the craft).

Are there any general guidelines on how to design a stable atmospheric entry vehicle in FAR, especially with the inflatable heat shield?

[KerbMav]

Hmm... change the drag/mass of the heatshield?

If I remember correctly the values for the inflatable heatshield made the whole assembly turn heads down.

[Hodo]

When dealing with inflatable heat shields I found it best to add your weight near the bottom of your craft, and make sure it is not top heavy.

If possible add more drag producing objects near the top, I know it can be a pain but droag chutes are the way to go. They can help keep you upright.

The other option is place it on the top and go in nose first.

[mic_e]

Going in nose-first is pretty much a no-go with DRE, as is changing drag values with FAR. Drogue chutes will burn up at hypersonic speeds, so I'm pretty much out of options... except positioning the COM as close as possible to the COD.

The main problem appears to be that FAR does not notice the growth rate of the heat shield on inflation.

[Hodo]

Going in nose-first is pretty much a no-go with DRE, as is changing drag values with FAR. Drogue chutes will burn up at hypersonic speeds, so I'm pretty much out of options... except positioning the COM as close as possible to the COD.

The main problem appears to be that FAR does not notice the growth rate of the heat shield on inflation.

I use DRE and FAR, and have for a long time. I can say one of the first things I learned was the need for slower re-entry speed. I think now most of my pods come in at less than mach 6. And actually have hit the lower atmosphere at much lower speeds. My best was a mach 5.2 re-entry, and I had a 10 second burn and the rest was gliding through the atmosphere till I could deploy the chutes.

[mic_e]

Limiting entry velocity sounds great, but there is only so much you can do when attempting to aerocapture at Kerbin or Jool...

For realism reasons, I don't wann deploy my Drouge chutes above Mach 2, even though I know that they won't burn up below ~2km/s.

[KerbMav]

Limiting entry velocity sounds great, but there is only so much you can do when attempting to aerocapture at Kerbin or Jool...

For realism reasons, I don't wann deploy my Drouge chutes above Mach 2, even though I know that they won't burn up below ~2km/s.

Aim for a moon first to get a planetary orbit?

[NathanKell]

To get FAR to recalculate your vessel aerodynamics, separate a part from it, that will force recalculation.

Instead of using the 6.25, try with the 3.75 maybe?

Also, why not use the latest DRE Continued? I've fixed a few bugs from the last release by ialdabaoth.

[mic_e]

NathanKell: I didn't think i'd reach _You_ with this post, but thanks for replying.

I wasn't actually awaere of the new DREC version; I just downloaded what google suggested. Unfortunately, the new version does not seem to fix the stability problem.

IMHO, this is a pretty massive gameplay issue; the only designs where the 6.25m shield is currently any use are extremely unrealistic ones (shields on both ends of the craft, delta-wings 20 meters behind the craft, ...)

Most ships (such as the normal command pods) have two stable orientations, prograde and retrograde, with at least a 5 degree AOA stability range (i.e., as long as I keep the AOA lower than 5 degrees, it automatically rotates back to the stable position). None of my realistic 6.25m ships had _any_ stability range whatsoever.

IMHO, the slightly conic shape of the 6.25m shield should cause a certain pitch and yaw stability, just as it works with all the command pods and other shields. Is there any difference in the implementation?

[ferram4]

Part of the problem is that FAR doesn't currently model the lifting effect from heat shields properly; when it does, that should help add some stability to those vehicles, but I've had some trouble implementing it properly and working nicely with other aerodynamic properties.

Another problem is that your reentry vehicle shouldn't be taller than it is wide; when it is, that leads to the CoM being way to far back, no matter what configuration you manage to come up with.

[Lukaszenko]

When using the inflatable heat shield, I always add LOTS of control authority, usually with a lot of reaction wheels. For realism you could replace that with RCS or with the new vernor motors.

[weeble]

There aren't any stability flaws in FAR, it's usually design flaws if anything. You shouldn't spam SAS or need to use RCS. Actually with FAR/DRE you should be keeping your SAS turned off or it'll throw you off your periapsis!

What you need to do is make landers that are short. I guarantee your lander is just too tall and you're not using drogue chutes. If you make short landers using the MK1 landing can (octagon pod) and keep your fuel tanks on the side and use drogues you'll never flip.

I have a lander with a heat shield and I couldn't make it unstable or flip out of control if I tried! Once you get good at designing the landers, you can even angle it so you coast horizontally in case you come in too steep.

[DaMichel]

Or, when you just cannot get the CoM low enough, add some fins

... and get rid of them later.