Reentry with MK1 Crew Cabin

[PedroHenrique_]

Hello guys, how are you doing? I have started playing KSP two days ago and I can't express how much I am enjoying the game, not only the successful missions but the unsucessful ones aswell, in which I learn the most.

At the moment I have put 2 rockets into orbit. Everything went fine with the first one, it landed perfectly (didn't explode lol). However, I am having a problem with reentering my second mission. The thing that differs the 2nd from the 1st is that the second one has a MK1 Crew Cabin for tourists. In the screenshots i've taken is possible to see my current orbit, fuel situation and the rocket itself.

Problem:

Once I lower the Periapsis to ~37km, I use the decoupler and wait for the reentry, always aiming retrograde. During the first moments everything runs ok. Though, in the middle of the process, the rocket starts to aim prograde, even with SAS on, pointing the aerodynamic part to the planet, screwing the braking which cause me to hardland and blow everything up. 

As you can see in the screenshots, I installed some wings in the rocket predicting unstable condictions during reentry due to the lenght of the last stage. However, I didnt expect it to be so tough and the wings to overheat during the process. I have already tried to not decouple and instead try to burn retrograde during the entry, but the engines tend to overheat.

What do you people suggest me to try?

Thanks for the help!

 

SCREENSHOTS:

(I don't know why, but the "Insert other media" isn't working.)

http://imgur.com/Uf2VhLh

http://imgur.com/CG83mem

http://imgur.com/FoXZ4yN

[Edited by Snark to provide inline images]

Edited January 18, 2017 by Snark
Add inline images

[Snark]

Hello, and welcome to the forums! 

44 minutes ago, PedroHS16_ said:

the rocket starts to aim prograde, even with SAS on, pointing the aerodynamic part to the planet, screwing the braking which cause me to hardland and blow everything up.

Yup.

The reason this is happening:  your rocket is very strongly aerodynamically stable prograde.

Things flying through the air want to have their heavy bits towards the front.  What you've made there is a ship that has essentially the same characteristics as a badminton birdie:  you've got the dense, heavy, low-drag pod in front; the lightweight crew cabin in the middle; and the really lightweight service bays in the back.

Trying to reenter that ship butt-first is like trying to throw a badminton birdie with the feathered end pointing forwards:  doesn't matter how you throw it, it will flip around so that the dense part's in the front.

So, things to do.

One option would be to get rid of those service bays, to make it more stable when retrograde.  (Not sure if it would be stable enough, but it's worth a try.)  What's in them?  Do you have anything really vital in there, that you couldn't just mount on the command pod or something?

Another option would be to skip the heat shield entirely, and add control surfaces and stubby wings so that your ship flies like a spaceplane instead of like a streamlined brick.  Small wings amidships and steerable control surfaces in the back (e.g. AV-R8 winglets) ought to do the trick.  The way that works is, you do a spaceplane-style reentry, in which you're entering prograde, nose-up by about 30 degrees.  The lift you generate not only provides drag to slow you down, it also keeps your altitude up so you can bleed off a lot of speed before you get down into the really dense stuff-- and you also don't end up faceplanting at high speed.

Note that the Basic Fin won't work well for either of these scenarios.  It has a super-low heat limit, much lower than most other KSP parts.  It'll almost certainly bake off during reentry.  If you need aerodynamic control during reentry, pick something else.  (I'm a huge fan of the Basic Fin, it's just so light and cheap... but it's really only good for launch, not for reentry.)

One other thing, if you have a retrograde-unstable ship and you're trying to enter retrograde:  make sure you have SAS set to surface mode, not orbit.  (Why?  Because the atmosphere is stationary relative to the surface, not relative to orbit.)  If you have it set to orbit, you're actually pointing slightly away from "true retrograde" relative to the ground, which gives those instability gremlins more of a toehold.

44 minutes ago, PedroHS16_ said:

(I don't know why, but the "Insert other media" isn't working.)

Fixed that for ya.    The problem is that you're inserting links to the imgur page, whereas what you need to be doing is to insert links to the image itself.  (In other words, a URL that ends in an image extension like ".png" or ".jpg".)

In general, the way you do this (for any image, not just imgur), is to right-click on the image in your browser, choose "copy image location", and then paste that into the forum, where it will automagically get converted to an inline image.

[PedroHenrique_]

Hey Snark, thank you very much, but I will be using all your tips in my next rocket because i managed to land the problematic one!! Wow, it was really tough and it didn't look it would work. I tried a 45km periapsis, trying to make the reentry as gradual as possible: losing speed but not that much of altitude. As well, conciliating some retrograde burn before getting deep in the atmosphere.

Here are some photos of the reentry moments:

(managed to get the images working Snark, thanks )

 

Moment of the engine explosion. Once it happened, the rocket turned prograde, comproving what you said about the bad weight balance.

 

That was the exact moment the radial parachuttes opened,

 

(chaos):

 

And then... ready to land!!!

 

Edited January 18, 2017 by PedroHS16_

[bewing]

As snark said, the way you designed your ship, it really really wants to point prograde. There are some design tricks you can do that will help with that for future launches, and he suggested some.

But I suspect that you really want to try to get this particular rocket back to the ground if at all possible. So, to prevent the fins from burning off, what you need is to reduce your orbital speed as much as possible before reentry. So I'm going to suggest that you try pointing retrograde, and burning every last drop of fuel. Hopefully that will slow you enough that the fins won't melt -- because you really need them for the next part. After you have done that deorbit burn, turn off SAS, and hold down S for a while. This will get you spinning fairly fast. It's a hard question as to whether you should decouple first or not. You may want to just try it both ways. When the rotation starts to slow, hold down S some more to keep it spinning good. Your ship will still eventually flip around nose first and you won't be able to prevent it -- but you will probably be going a lot slower. I see you have drogues on there. Activate them, and hope like heck that they open in time to get your other chute open. Once you are going nose-first, decouple -- you absolutely need to shed the extra weight at that point.

[Reactordrone]

Make sure you're coming in over water since you're going to need all the altitude you can get. Drop your periapsis down to about 40-45km and keep your engine and fuel tank on. When the ship starts to wobble away from retrograde burn off the remaining fuel and when you flip prograde decouple the engine and drop the heat shield. Wiggle the ship as much as possible to try to bleed off some more speed and pop the drogue chutes when safe to deploy. I've managed to get a lookalike ship landed a few times so it should be possible, if a little bit risky.

[Kryxal]

Also, once you do end up going nose-first, pull the nose UP and let body lift slow you and keep from dropping quite so fast.  I'm generally able to manage reentry with a similar ship (but with only one cargo bay, between the capsule and cabin) thanks to the extra weight of the ablator helping to hold retrograde, OR by keeping the nose up without ablator.

[AeroGav]

This is why I beeline Aviation tech Pedro,  non-steerable fins keep it aiming straight going up, but coming back down they are a hindrance.  That said, even if you left the fins off and tried to maintain control with thrust gimballing alone,  you still wouldn't be able to maintain a proper re-entry attitude with a crew cabin stuck on the bottom.

If I'm playing a no-restart, no respawn game i won't launch any rocket with much more than a single flea on the bottom, until i've unlocked aviation, because the risk of killing astronauts is so high.

I did come up with a workaround however :

This will try to come in nose first

3 way small fins will ensure it re-enters heatshield first

Four way winglets keep us pointing space during launch. Obviously, decouple that propulsive stage and its fins before re-entry

Fully assembled, with boosters

Edited January 18, 2017 by AeroGav

[PedroHenrique_]

Nice AeroGav, i will give it a look right now! 

[Foxster]

This is my favourite way to get a mk1 crew cabin through re-entry...

...and you can get to both hatches

[Spricigo]

1 hour ago, Foxster said:

This is my favourite way to get a mk1 crew cabin through re-entry...

...and you can get to both hatches

As a bonus it looks happy.