Re-entry question for interplanetary shuttle pilots

[Geonovast]

I could use some advice while fine tuning my 3rd generation shuttle design.

This is the first time I'm planning on taking these things out of LKO.  Just to the Mun and possibly Minmus for now.

I'm curious about re-entry methods.  Given that the main idea is to return with little to no fuel, what's the best approach?  I've gotten pretty good at building shuttles that are stable on re-entry from a 80x80 - 100x100km orbit with 30 - 40 degree AoA.  Is it best to come straight in, and try to design the vehicle to withstand re-entry at much higher speeds, or should I focus on a several-pass airbrake method?

Re-circularizing would take quite a bit of fuel, and just getting to the Mun with a standard return is taking enough as it is.  I'm also trying to be tidy, so I don't want to leave a bunch of spent tanks in orbit.  The main external fuel tank is jettisoned after launch, at around a 100x50km orbit.

I have a shuttle right now on its way back from the Mun, with ~500 m/s left in the tanks, but it went there with a completely empty cargo bay.

Any words of wisdom on where I should direct my efforts?

Also, if a direct descent is preferable, are there any methods that would help me get as close to the KSC as possible?  It's capable of atmospheric flight, so just needs to be close-ish.

[bewing]

Well, the difference between an aerobrake pass and a direct reentry is just a few km of altitude. So you drop down to 50km or so, at 90 degrees AoA, and scrub off those extra 850m/s as fast as you can. When your Ap drops to 70km, you go prograde. When you are about 400km from KSC, you go to 90 degrees AoA again, until you are slow enough that you will fall on top of it. If you don't seem to be slowing down fast enough, you go into maximal drag configuration -- maybe opening the bays & extending the wheels.

 

Edited September 19, 2018 by bewing

[Geonovast]

Thanks @bewing

I'm just now coming in, I'll see if she can hold a 90 deg AoA.  This specific design seemed to want to naturally hold a 20-40 deg AoA on its own during re-entry testing.  Not sure how holding 90 and switching to prograde will go!

Here goes nothin'

[bewing]

3 minutes ago, Geonovast said:

Thanks @bewing

I'm just now coming in, I'll see if she can hold a 90 deg AoA.  This specific design seemed to want to naturally hold a 20-40 deg AoA on its own during re-entry testing.  Not sure how holding 90 and switching to prograde will go!

Here goes nothin'

 

You've got 200 units of monoprop. Do you have RCS jets? Turn them on! And you may want to be in Surface mode on your Navball.

 

Edited September 19, 2018 by bewing

[Geonovast]

5 minutes ago, bewing said:

You've got 200 units of monoprop. Do you have RCS jets? Turn them on!

 

Sure do, but I don't want to rely on them, especially if I end up coming back with little to no MP left.

I'm impressed.  It held Radial Out with zero problem until I told it to go prograde, and she dipped down to prograde as if she were still in orbit.  Re-entry was easily survived.  I got a few temp bars, but nothing to be worried about.

Not sure I can get to the KSC though.  I did come in a bit steep, PE was roughly 40km when I hit atmo, and even though I hit the prograde button at an AP of around 100km, it was down to 35-40ish when the nose finally hit prograde.  I got a little extra distance by pulling the nose up some more, but not a lot.  The airbreathers may get me back to land, but I doubt I'll get back to the KSC.

Looks like I have some practicing to do!

[bewing]

Yeah, a little practice, but you got pretty close. Turning on the RCS just helps the Radial Out/Prograde attitude adjustment happen much faster. You don't get any points for landing at KSC with a full load of monoprop still onboard.

 

[Geonovast]

3 minutes ago, bewing said:

Yeah, a little practice, but you got pretty close.

I'm... attributing that to luck.  I think I just happened to come in at the right time to basically get in the right spot.  Doing this on purpose is going to take the practice.

 

3 minutes ago, bewing said:

Turning on the RCS just helps the Radial Out/Prograde attitude adjustment happen much faster

I used to use RCS for re-entry stuff, back when I first tried to build a shuttle (which was a bit of a tumbler, as I had no idea what CoL even was).  So I'm a little gunshy of it being an Over-Reaction Control System.  This ship is pretty well balanced, thanks to SmartParts' fuel dump valves, which let me re-enter in nearly the exact same condition every time.

 

6 minutes ago, bewing said:

You don't get any points for landing at KSC with a full load of monoprop still onboard. 

Oh, I know, but again don't want to count on it.  I've finally gotten to a point in the game where RCS is for docking, and not much else!  I have no shortage of reaction wheels!  These can't run out on re-entry, as I have fuel cells and fuel canisters dedicated for re-entry.

 

As always, you're very helpful!  Thank you @bewing.

[Laie]

3 hours ago, Geonovast said:

I'm just now coming in, I'll see if she can hold a 90 deg AoA.  This specific design seemed to want to naturally hold a 20-40 deg AoA on its own during re-entry testing.  Not sure how holding 90 and switching to prograde will go!

You may want to turn her on her back, with your AoA forcing you into the atmosphere.

The idea is not to get to a certain depth and then (possibly) out again for another round. That is for pods. With a plane you can control your climb rate, after all, and achieve level "flight" somewhere in the upper atmosphere around 50km, for a comparatively gentle slowdown that may take you halfways around Kerbin. Then do the 90-degree screeching halt when the KSC mountains come into view.

[AeroGav]

The biggest factor in heat tolerance is how far back the crewed part is in the stack.   Thus,  your mk3 shuttle cockpit is basically right at the front of the ship (the nose cone only covers a small part of its area) so is much more prone to overheating than a mk1 inline cockpit that's a few parts back in the stack .

I haven't really explored the limits, preferring to play it safe,  but my mk1 inline cockpit ships are ok for direct descent from an AP of 350km or less.    From  Minmus,  I prefer to do a few aerobraking passes with PE not lower than 45km for the initial pass.  Depending on how hot things got, i may lower PE further on subsequent passes.

Duna is not too much different, but Jool is a challenge to say the least.

For aerobraking,  holding radial out is fine, but i'm not a fan of using that for the final re-entry or below 45km altitude where aero forces are getting strong.    A plane that's aerodynamically stable will be fighting the RCS hard to bring its nose down, the plane will seesaw wildly with oscillations in pitch, roll and yaw,  and you'll waste huge amounts of electrical power and monoprop that you might be needing later.  If your plane has negative stability when empty, it will flip and go backwards.      Radial out deep into the atmosphere is only possible if you got really strong RCS and a perfectly neutral airplane that is neither stable nor unstable, something that would pull off awesome aerobatic manuvers lower down.

 

The other problem with high angle of attack for the final re-entry is that you're giving up control over where the plane comes down.    You can't predict how long the tumbling will last,   even if you hold perfect radial out you can't increase or decrease rate of descent.

If heat is an issue i prefer using 20 degree AoA on final re-entry, since that gives almost max lift (keeping you out of the lower atmosphere till slow as possible),  and also high drag.

If you're not in dire straits,  i prefer to enter with 10 degree AoA,  and watch your trajectory on the blue line in the map mode screen.     If you look like falling short ,   lower the nose a bit so you're closer to optimum glide angle (5 degree aoa when  above mach 1)  .   If I'm overshooting,  I pitch up above 10 degrees, since high AoA adds more drag than lift.        I usually retro burn to ground level in the desert continent west of the space centre,   if your craft is reasonably aerodynamic you'll find the blue line moves steadily downrange as you encounter the atmosphere , since the blue line is calculated for wingless capsule re-entry vessels,  so you'll need to pitch up  and increase drag to prevent overshoots.

 

I did an old youtube video demonstrating this - it has annotations explaining all this, but i don't know if your player is still able to show them (they don't work on smartphones, for sure)

   Of course, this thing has the same size wings as your shuttle on a much smaller fuselage, so it glides a lot more..

[michal.don]

Well, from my shuttling experience, there is not a certain, fool-proof way to get any shuttle to land where you want it to land. It depends on the design, on the possibility of maintaining high AoA (you seem to have this one covered), and on your knowledge of consequences of each maneuver. And the last thing comes from flying your shuttle, a lot. Generaly, if you are well-experienced with your machine, I'd go for a direct descent which will put you on the right hemisphere, and if you have enough wing surface (which it seems you have), you'd be amazed how much you can influence the trajectory. After flying several dozens of missions in my shuttle, I could land it on the runway, even from high, inclined orbit, about 80% of my attempts, with the rest not being too far off. Even in the high atmosphere, you should be able to change the inclination by several degrees, and by changing the angle of attack the length of your glide.

By the way, if you want a few missions to practice, and maybe a few ideas what to do next, there happens to be a shuttle challenge, which I happen to run,  and I don't recall you flying there - you might find some of the stuff there interesting, and get some pointers from other shuttle maniacs that hang around there

Michal.don

[Geonovast]

3 hours ago, michal.don said:

By the way, if you want a few missions to practice, and maybe a few ideas what to do next, there happens to be a shuttle challenge, which I happen to run,

This shuttle is being developed specifically for your challenge, actually.  I wanted it tested and refined before I entered, as I don't want to make any major changes to the design after I begin.  And the changes I do have to make, I want to be a planned customizable option of the design.  Unfortunately I didn't have time to do any flights last night. 

 

[michal.don]

2 minutes ago, Geonovast said:

This shuttle is being developed specifically for your challenge, actually.  I wanted it tested and refined before I entered, as I don't want to make any major changes to the design after I begin.  And the changes I do have to make, I want to be a planned customizable option of the design.  Unfortunately I didn't have time to do any flights last night. 

Oh, that's nice to hear then  You don't have to wait until the design is 100% finished though, I'm sure the folks there would appreciate some WIP look at your shuttle - it looks really well!

Anyway, good luck with your orbiter, looking forward to seeing you over there,

Michal.don

[Geonovast]

5 minutes ago, michal.don said:

You don't have to wait until the design is 100% finished though

Oh, I realize the challenge doesn't require it, that limitation is self-imposed!

You should see me in a few days.