Ideal re-entry altitudes/speeds/maths

[KocLobster]

Sometimes when I'm returning larger types of vessels (typically 15+ ton payloads of the at least 2.5m variety) back to Kerbin, I experience my ship losing control, flipping around a bunch, and of course causing my heat shield unable to do its job. It only takes a second or two before I disintegrate. This typically happens around 22km, when the air starts becoming much thicker. I normally aim for about 31km PA for my re-entry because that is what I've read.

I already design my ship in such a way that the heaviest stuff is on the bottom towards the heat shield, going upwards to the lightest items. The craft is always quite aerodynamic. I'm not running out of electricity or anything else. I usually use SAS locked on retrograde. I have been adding reaction wheels to this final payload where I generally had them lower, on a part of the ship that would be jettisoned before re-entry, and it has mostly fixed the issue. However, I don't feel like this is the optimal way to solve the issue. Generally I'm entering Kerbin's atmosphere at 3100m/s (with a periapsis of ~31000m like I mentioned earlier). So I have a few questions:

1) What am I doing wrong to cause this?

2) What is the ideal altitude to return from that (ideally) minimizes how many passes through the atmosphere are necessary without causing this?

3) Does speed play a significant part?

[prykpryk]

Use bigger (wider) heatshield. Additionaly, you can lower the amount of ablator you take to make it lighter. AFAIK you can easliy survive reentry with no ablator at all.

[KocLobster]

46 minutes ago, prykpryk said:

Use bigger (wider) heatshield. Additionaly, you can lower the amount of ablator you take to make it lighter. AFAIK you can easliy survive reentry with no ablator at all.

I'm sorry but this doesn't have anything to do with what I was actually asking about... And no, you basically can't survive re-entry without an ablator (when you're going as fast as I am and lowering your periapsis as low as I am). Surviving re-entry without an ablator is typically only done with a particular command pod (Mk1) and typically only when returning from one of Kerbin's moons, neither of which apply to my situation and this thread.

[MaxL_1023]

How is your rocket shaped? You want it to be stable while pointed retrograde, meaning that a small deviation would create an opposing force. I have never had stability as long as everything behind the heatshield has a gradually decreasing diameter - a 1.25m tank behind a 1.25m heatshield with a pod on top can flip, while the pod by itself will be stable. 

If you get a stable configuration, the PE height is not a big issue - you can survive a single-shot re-entry and landing using the heatshield. Speed does play a role, influencing how low you get before you bleed off velocity. Too fast, too low will put large forces on your craft and amplify any unstable characteristics. 

Basically, your craft should be able to re-enter retrograde with no control input at all, besides starting retrograde at atmosphere-top. 

[KocLobster]

This is how I generally shape my rockets too. I have a 2.5m heat shield, attached to a 2.5m MPL, followed by a 1.25->2.5m addapter, attached to a Mk1 command pod and a chute on top. I also have a very similar payload but without the adapter, and the Mk1 command pod is actually a Mk1-2...so essentially the exact same shape and aerodynamic profile.

Are you saying that this is the problem? How would you improve upon this? Here is a picture of what I poorly tried to describe:

http://imgur.com/H4wrhwM

[prykpryk]

Well then I'll try to answer your questions.

1) Ship may be not stiff enough. Some parts may create asymmetric lift even though they're behind the heatshield (F12 to check that). Wide and low ships are better than narrow and tall, but I know very well that it is impossible to launch such spacecraft easily. Instead of reaction wheels you can use Vernors on a long arm at the top (to increase torque, proven with Eve manned landing). MechJeb autopilot is much better at re-entry than stock SAS. If it is possible, consider splitting your craft in a few parts and landing each individually.

2) Higher PE is better unless you would skip on top of the atmosphere a few times. If 31km works then it's fine. I've always used 25km.

3) It is significant but you can't always change it, I think. If you could do a retro-burn to slow your craft to ~2000 m/s that might be advantageous if your ship is only mildly unstable. Re-entry temperatures depend only on speed.

[KocLobster]

Was your earlier post suggesting I do something like this?:

http://imgur.com/RdOWYKg

[OhioBob]

That Mk1-2 command pod is pretty massive to be at the back end of you vehicle (the pod weighs more then the MPL).  I think it's likely that your CoM is just too far aft.  You need to either find a way to distribute the mass closer to the heat shield, or find a way to add drag at the aft end.

[rodrigoelp]

Hi @KocLobster, just as @OhioBob suggests, you center of mass might be too high on your return probe, also you are going a bit too low on a reentry of 3100m/s.

Try adding smaller control surfaces (such as the aerobrake) close to the top of your probe to help you stabilise the probe. Also increase your periapsis around the 40k mark (such is substantially lighter than 31k). This will give you two passes more in order to do the complete insertion, but at least gives you a smoother increase of temperature than going into the atmosphere at 2800m/s (you probably will bleed 300m/s to 400m/s speed the first time to get to 31k assuming your speed was 3100m/s at the time you got to 70k mark - your speed will increase between 70k to 60k). After that you should be able to readjust your periapsis to have less elevation and come into full atmospheric insertion.
You can also play with thrusters to control your temperature, avoiding scortching temperatures... but this is risky and forces you to have to extra fuel just to control your descent... When I send probes to Eve I tend to set it (for instance) at the 80k mark with thrusters pointing retrograde. This allows me to decrease faster my speed when the temperature starts to rise, allowing me to do a single pass or two around the planet before the insertion (sometimes without having the need of heat shields... although I like to always have it as opposed to losing my probe when worst comes to worst.

Does speed plays a significant role?

Depends on the shape of your probe. Parts sticking out of your probe will generate lift which can reorientate your probe. KSP can show you the aerodynamic forces on your probe, just turn it on and take a look at your probe while in descent (this might give you a clue as to what is going on). 

[MaxL_1023]

The MPL is very long, so with a heavy pod on the front of it your craft is likely unstable. Your best bet would be to put the pod under the MPL - I know it looks wierd but the mass balances better. You might also get away with putting the whole thing behind a 3.75m heatshield. 

[KocLobster]

1 hour ago, OhioBob said:

That Mk1-2 command pod is pretty massive to be at the back end of you vehicle (the pod weighs more then the MPL).  I think it's likely that your CoM is just too far aft.  You need to either find a way to distribute the mass closer to the heat shield, or find a way to add drag at the aft end.

I apologize, I meant the Mk1-3 I believe. I think it's from one of @Nerteas mods. It does in fact weigh about a ton less than the MPL.

22 minutes ago, MaxL_1023 said:

The MPL is very long, so with a heavy pod on the front of it your craft is likely unstable. Your best bet would be to put the pod under the MPL - I know it looks wierd but the mass balances better. You might also get away with putting the whole thing behind a 3.75m heatshield. 

Would you be giving me this same advice if I had correctly identified just which command pod I was referring to? I meant the Mk1-3 (I think that's its name, it's not stock). More importantly, it weighs about a ton less than the MPL.

[MaxL_1023]

10 minutes ago, KocLobster said:

I apologize, I meant the Mk1-3 I believe. I think it's from one of @Nerteas mods. It does in fact weigh about a ton less than the MPL.

Would you be giving me this same advice if I had correctly identified just which command pod I was referring to? I meant the Mk1-3 (I think that's its name, it's not stock). More importantly, it weighs about a ton less than the MPL.

I would still do it - the configuration you have puts the CoM well above the heatshield. In general, you want it as close to the heatshield as you can get it.

[rodrigoelp]

9 hours ago, KocLobster said:

I apologize, I meant the Mk1-3 I believe. I think it's from one of @Nerteas mods. It does in fact weigh about a ton less than the MPL.

Would you be giving me this same advice if I had correctly identified just which command pod I was referring to? I meant the Mk1-3 (I think that's its name, it's not stock). More importantly, it weighs about a ton less than the MPL.

I wonder, why do you want to bring the whole thing down? are you trying to recover cost or get the science? The science that is required for the lab to work can be transferred by a kerbal into a escape/command pod. The lab would stay in orbit around Kerbin until you can man it and continue getting science our of it. All science created in there would not depreciate during transmission.

[mrclucks]

For some reason the pictures arent loading, but based upon the posts I will assume you are returning from interplanetary space.  As long as your periapsis is below 70km and your have an apoapsis inside kerbins soi you will eventually return to kerbins surface.  Keep your periapsis above 50km if your apoapsis is around minmus's orbit.  For every entry into the atmosphere your AP will lower considerably and your PE will lower a bit, which will create more drag and lower your AP and PE even more.  Even for reentries from minmus, I keep my PE above 30km since 40km or so is when atmosphere starts to thicken considerably and I find that 35km and lower will practically guarantee rentry with that pass(assuming your craft can survive the heat and all the forces.  

To help your craft slow down, I'd try using airbrakes to help slow down your craft and orient it retrograde.

[swjr-swis]

17 hours ago, KocLobster said:

I usually use SAS locked on retrograde.

Consider that stock SAS is still suboptimal and often very wacky with minimal craft - the smaller/lighter the craft, the more it seems to want to point off the actual vector you are telling it to lock onto, regularly pointing 5-10 degrees off or more. Couple that together with it having enough torque to fight aerodynamics pretty hard on lighter craft, and it's enough to make a retrograde hold almost impossible.

What I do is test the SAS hold first for the section that needs to reenter atmosphere while still outside the atmosphere. If it shows any tendency at all to point or jitter away from exactly retrograde, I simply disengage SAS entirely during reentry. Let aerodynamics keep it pointed the way it should.

[rodrigoelp]

46 minutes ago, swjr-swis said:

Consider that stock SAS is still suboptimal and often very wacky with minimal craft - the smaller/lighter the craft, the more it seems to want to point off the actual vector you are telling it to lock onto, regularly pointing 5-10 degrees off or more. Couple that together with it having enough torque to fight aerodynamics pretty hard on lighter craft, and it's enough to make a retrograde hold almost impossible.

What I do is test the SAS hold first for the section that needs to reenter atmosphere while still outside the atmosphere. If it shows any tendency at all to point or jitter away from exactly retrograde, I simply disengage SAS entirely during reentry. Let aerodynamics keep it pointed the way it should.

SAS can be very wacky at times, but it depends on which command module is controlling it and how many SAS you have on the probe.

But again... if you want your probe to always be retrograde as you are reentering... use airbrake, if overheating retract for a bit and redeploy.

[KocLobster]

Sorry this will be short, I deleted a long post on accident and I'm on my phone. Although kind of irrelevant, I meant the Mk3-9 Orbital command pod.

Although SAS is suboptimal, it's usually good enough, and I don't want to use an autopilot like Mechjeb.

@rodrigoelp

Sounds silly I know, but it was to complete a space station around the Sun contract that required a bunch of stuff, including a lab, and also so I could ferry exactly 8 kerbals to Sun orbit at the same time. I could of probably designed a better rockets but it seemed good enough.

4 hours ago, mrclucks said:

To help your craft slow down, I'd try using airbrakes to help slow down your craft and orient it retrograde.

 

3 hours ago, rodrigoelp said:

But again... if you want your probe to always be retrograde as you are reentering... use airbrake, if overheating retract for a bit and redeploy.

People keep suggesting this for different things, but I've also read that airbrakes are not meant for re-entry, they're for planes. They have a lowered heat tolerance, won't they explode on interplanetary re-entry?

[mrclucks]

5 minutes ago, KocLobster said:

People keep suggesting this for different things, but I've also read that airbrakes are not meant for re-entry, they're for planes. They have a lowered heat tolerance, won't they explode on interplanetary re-entry?

They do, which is why I'll have them on when I enter the atmosphere and then turn them off when they start getting hot and back on when they cool down enough.  I also use KER which tells me critical temperature of parts which is highly useful.  You can typically use airbrakes with minimal heating from 70km to atleast 50km if you're not coming in too hot.  You can also try and position them so even when they're deployed, they are still behind the heat shield which will still create drag but keep them safe from most of the heating

[KocLobster]

Thanks for the clarification and suggestions, I will start giving this a try. I have elimanted this problem for the most part with reaction wheels though. Typically I don't ever reenter slower than 3100m/s, because I don't really ever bother eith UN/Minmus anymore, so I don't know how effective airbrakes would be or how much I'd be able to use them.

Wouldn't putting them further and further protected behind the heat shield be directly proportional to their decreased effectiveness? They aren't able to create as much drag if they're hiding behind the head shield because of heating issues, right?

[Violent Jeb]

People have been jogging the airbrakes with some success. It seems in 1.1 you come in a lot faster for those landings. Tweaked my parachutes and smashed into the ground.

If my craft is really top heavy, i'll let that beast come to earth prograde and fight it close to the ground to flip again. Heck, just land upsidedown, the kerbals don't seem to mind. Finally, if possible try bleeding off speed with a few extra passes, so that any forces that happen from tumbling doesn't rip apart your ship.

[KocLobster]

6 minutes ago, Violent Jeb said:

If my craft is really top heavy, i'll let that beast come to earth prograde and fight it close to the ground to flip again. Heck, just land upsidedown, the kerbals don't seem to mind.

The temperature of the vessel cares though... If I don't come down on the heat shield I will explode. I don't understand, is that what you're suggesting?

If not, then you're suggesting it doesn't matter if I flip upside down when I'm far far closer to the surface?

If so, that may be true but my problem was getting to that point. My ship was losing control and exploding well before that (although I've managed to fix this issue now with the help of the advice in this thread).

[Violent Jeb]

So you have to reduce speed to reduce heat, either skim the atmosphere or get creative.

and yes, IIRC you can survive with no heat shield if you're below 2.5 km/s

If you're moving faster than that, AND you can't keep it from going head first, then maybe try to put a heat shield on top. 
If you're going to be getting creative anyway, a tank of RCS and a pair of thrusters should also give you the control you need to stay facing retrograde while falling through the sky.

Edited April 20, 2016 by Violent Jeb

[KocLobster]

Thanks for the advice. I love getting creative with my designs, it's one of the main reasons I love KSP. I will have to give some of these suggestions a shot next time I have this problem.

[numerobis]

3100 m/s cant be right; return from Minmus is close to 3500. I regularly aerobrake from Minmus without a heat shield, with as few as two passes. I've re-entered straight up, with just a pair of small radiator panels to cool down my pod and materials bay.

The main problem a heat shield causes is that you don't have much drag to slow you down, so you get to the middle atmosphere still going very fast.

For maths, @NathanKell will probably tell me I'm horribly wrong (I'm hoping he does), but my understanding is that heat is linear in air density, cubic is speed. Air density roughly follows a scale height of 5km on Kerbin (not exactly but close).

 

That predicts that if you drop 5 km without losing speed, you gain nearly 3x the instantaneous heat flow.

Conversely, if you lose 30% of your surface speed, you can drop 5km without increasing the heat flow.

[KocLobster]

@numerobis

The last mission I did outside of the Kerbin system, for example, was a 'ferry' trip to the Sun's SOI. As soon as I was in the Sun's orbit, I turned around and headed back to Kerbin. When I started hitting the atmosphere I was going about 3100m/s. When I go to the Mun/Minmus, I typically am going far slower than that. I'm on my phone and don't have my computer in front of me, but I believe I'm typically in the 2200-2600m/s range, and certainly not anywhere near 3500m/s.

I especially appreciate those last two tidbits at the bottom of your post, I thought that was actually pretty interesting.