Aerobraking?

[Gilead]

Hi everyone! I have played KSP a long time ago (before career mode was released), and stopped playing after a while, cause my PC wasn't powerful enough to do what I wanted to do. But at that time, I could do most things I wanted to, mainly navigating Kerbin system (going to Mun, docking in orbit, building space stations, that kind of things).

Now I have a much more powerful PC, so I started playing again, and I encountered a new problem: Landing on Kerbin with aerobraking. Sometimes, my crafts are solwed enough by the atmosphere to safely deploy drogue chutes and everything goes fine, and I land safely. Sometimes the atmosphere doesn't slow me enough and I crash on the surface at 800 to 1200 m/s without being able to deploy my chutes... Usually, I change the angle of my re-entry and try again and again until it works.

But today, I sent 5 Kerbals in orbit in a ship that I had previously sent in orbit and safely recovered with the full crew, for a ferry mission. I remember landing this exact same craft a while ago, but today, I can't do it. I tried different entry profiles. I did 6 tries to get from orbit to the surface. In each one, I burned retrograde to get my periapsis in the atmosphere. One try with a periapsis of 10 km, one with a periapsis of 20 km, etc until one with periapsis of 60 km. None of them worked, I crashed everytime at at least 750 m/s... I managed to land this ship multiple times without problems a while ago and today, it's proving impossible ! What am I doing wrong ?

My last 2 stages (the ones i re-enter with) are made of 1 Mk1 command pod, 2 Mk1 crew cabin, 2 drogue chutes, 3 chutes, a decoupler, a FL-T200 fuel tank ~40% full (before re-entry burn) and a LV-909 Terrier engine (321m/s dV left according to Kerbal Engineer). 5 Kerbals inside.

 

Thank you for reading me. I hope you can provide me with answers.

Edited January 23, 2018 by Gilead

[Cpt Kerbalkrunch]

Welcome to the forums!

Try setting your Pe between 30-35km. You definitely have enough chutes to do the job. With the proper entry angle, you should be able to land safely (though it certainly wouldn't hurt to stage away the Terrier and fuel tank, as well). Also, check the settings on your chutes to be sure they open when you need them to. And most important of all, make sure you stage them. 

Edited January 23, 2018 by Cpt Kerbalkrunch

[Gilead]

Hi!

Thanks for the answer! 

I usually set my periapsis to 30-40 km, and that's what I tried first. I did get rid of the engine and tank once my deorbiting burn was complete. My staging seems fine (here's a screen of the ship in the VAB, so you can see it: https://ibb.co/gsDojG ).
Concerning the settings of the chutes, I have 2 settings : Min Pressure and Altitude. I think I know what they do, but I have no idea what to set the first at? What does the number represent? Currently my drogue chutes are set to 0.02 and the others to 0.04.

 

[GoSlash27]

Gilead,

 Are you rotating nose- first? If so, that's your problem. Your ship needs to be stable reentering backwards. All mass needs to be as close to the heat shield as possible, and it helps if the nose is draggy.

Best,
-Slashy

[Cpt Kerbalkrunch]

1 hour ago, Gilead said:

Hi!

Thanks for the answer! 

I usually set my periapsis to 30-40 km, and that's what I tried first. I did get rid of the engine and tank once my deorbiting burn was complete. My staging seems fine (here's a screen of the ship in the VAB, so you can see it: https://ibb.co/gsDojG ).
Concerning the settings of the chutes, I have 2 settings : Min Pressure and Altitude. I think I know what they do, but I have no idea what to set the first at? What does the number represent? Currently my drogue chutes are set to 0.02 and the others to 0.04.

 

The pressure setting is when the chutes will fully open. For the regular chutes, turn 'em all the way up to .75. The altitude is the height above the ground when they pop. Set it to whatever you like. I usually set it for 2,500 or 3,000. As GoSlash said, make sure you're coming in rear end first. The flat side of the ship will help slow you down. Whereas nose first, you'll cut through the air like a bullet.

When I say to make sure you stage the chutes, I mean hitting the spacebar once the chute icons go gray. That means they're safe to deploy. I think we've all forgotten at least once. 

Edited January 23, 2018 by Cpt Kerbalkrunch

[IncongruousGoat]

11 minutes ago, Cpt Kerbalkrunch said:

The pressure setting is when the chutes will fully open. For the regular chutes, turn 'em all the way up to .75. The altitude is the height above the ground when they pop. Set it to whatever you like. I usually set it for 2,500 or 3,000. As GoSlash said, make sure you're coming in rear end first. The flat side of the ship will help slow you down. Whereas nose first, you'll cut through the air like a bullet.

When I say to make sure you stage the chutes, I mean hitting the sidebar once the chute icons go gray. That means they're safe to deploy. I think we've all forgotten at least once. 

Chutes can also be set to Deploy When Safe, which allows you to stage them whenever. The game then waits until it's safe to deploy the chutes. This is what I usually do-the cases when it doesn't work due to odd re-entry dynamics are few and far between.

[Cpt Kerbalkrunch]

11 minutes ago, IncongruousGoat said:

Chutes can also be set to Deploy When Safe, which allows you to stage them whenever. The game then waits until it's safe to deploy the chutes. This is what I usually do-the cases when it doesn't work due to odd re-entry dynamics are few and far between.

A good point. I got into the habit of waiting 'til they're gray in the 1.0 days. You'd get a message saying your chutes were "destroyed by aero forces and heat". Quite a few of my early missions went down the crapper that way. 

[Spricigo]

My suggestion is to change the upper stage just a bit: nosecone, crew cabin, crew cabin , FL-10 adapter(if you have it, may also be a FL-A5), command pod, decoupler.

That make it a bit less stable on the way up, but (I think) not enough to cause issues. On the way down, after dropping the engine/fuel tank, it will be stable inthe retrograde direction (command pod first). That way you will have a blunt surface exposed to the airflow, resulting in more drag to slow you down.

 

Another trick is adding a service bay to your stack, and using the doors as improvised airbrakes. Works better than supposed to do. 

 

[bewing]

Reentering backwards is preferable. Then you can burn the fuel to slow yourself to parachute speed, if you want.

If you can't seem to help coming in nose first, then a smart solution is to not forget aerodynamics. A couple of tiny fins just behind your CoM will allow you to fly right down, kill all your speed, and land on the runway if you like.

Force/encourage your RV to tumble. You need drag. Let your ship help you slow down.

Additionally, don't stage off your lower stage after your retroburn. That stage  generally provides you lots of rear-first stability, plenty of heat protection, and lots and lots of drag to slow you down.

 

Edited January 23, 2018 by bewing

[Spricigo]

2 hours ago, bewing said:

Additionally, don't stage off your lower stage after your retroburn. That stage  generally provides you lots of rear-first stability, plenty of heat protection, and lots and lots of drag to slow you down.

 

High drag from an open node and low mass from empty tank will not help to provide rear-first stability. I'd expect it to point nose first even with half tank. And while it may add drag, it also adds inertia to resist it. 

 

 

[Guest]

Looking at your craft, I'm quite sure your problem is aerodynamics -- your craft is too slippery and goes through the atmosphere like a greased cat. Great for a plane, not so great for a re-entry vehicle.

Recommendation:

1. Get rid of those crew compartments and make a one-kerbal re-entry vehicle.
2. Make that work.
3. Add crew compartments one by one, with aero tweaks to make the new design work.

Personal experience: Those crew compartments are tricky to get to work on a classic rear-first re-entry vehicle. It's possible but not easy, so I usually don't bother. Instead I wait until I have the four-person Hitchhiker Can and 2.5 m heat shield, and build my multi-kerbal re-entry vehicle from that.

[Gilead]

Thanks for all your answers!
First, I'd like to say that I did 3 or 4 missions with this exact ship previously, and it worked fine. I don't think the problem is the craft.

 

11 hours ago, Cpt Kerbalkrunch said:

When I say to make sure you stage the chutes, I mean hitting the spacebar once the chute icons go gray. That means they're safe to deploy. I think we've all forgotten at least once. 

I know that, thank you, but I'm going too fast, the chute icon is still red when I hit the ground.

 

Well, since you seem to recommend modifying the craft, that's what I'm gonna do. But it's sad, cause I know I used this one without problems multiple times.

Edited January 23, 2018 by Gilead

[Gilead]

I did it! I landed my craft safely. I just did it without physical time warping... Took a long time, but it worked \o/

[Snark]

1 hour ago, Gilead said:

First, I'd like to say that I did 3 or 4 missions with this exact ship previously, and it worked fine. I don't think the problem is the craft.

It's absolutely the craft, if what you say here is true:

14 hours ago, Gilead said:

I have played KSP a long time ago (before career mode was released)

Am I correct in understanding that this ship is one that you flew before KSP 1.0?

Because the rules COMPLETELY changed in KSP 1.0.  Aerodynamics is completely different.  A ship that previously reentered just fine in the old days of the "souposphere" (i.e. before 1.0) can easily have problems post-1.0, and vice versa.

The problem is that  what you've built is basically a lawn dart:

14 hours ago, Gilead said:

1 Mk1 command pod, 2 Mk1 crew cabin, 2 drogue chutes, 3 chutes, a decoupler, a FL-T200 fuel tank ~40% full (before re-entry burn) and a LV-909 Terrier engine

My friend, that is the poster child for a ship that is going to go slamming into the ground at hypersonic speed.

Long.  Skinny.  Pointy.  No maneuvering ability to speak of.  Really high ballistic coefficient.  It's not a reentry vehicle, it's a kinetic-kill weapon.

The good news is that it's trivially fixable.    You have a few options.

• You could put airbrakes on it.  Don't use them when you're reentering at 2000+ m/s, because they'll melt.  But once you have slowed below 1200 m/s or so, deploy the airbrakes and you will slow down in a hurry.
• Another option:  Just put a couple of AV-R8 winglets on the back, mounted horizontally like the flukes of a whale.  These are steerable fins.  As you get lower in the atmosphere, you can use them to pitch your nose up-- doesn't have to be a lot, just 20 degrees above or so.  This has the double benefit of significantly increasing your drag (to slow you down), and generating a lot of "body lift" from the cylindrical body of your ship (reducing your rate of descent, so you have more time to slow down).

Either of the above will take care of things in a jiffy.

[EDIT] Hah, you ninja'd me.  Just now saw what you posted right as I was posting my own message:

3 minutes ago, Gilead said:

I did it! I landed my craft safely. I just did it without physical time warping... Took a long time, but it worked \o/

Congrats!  Glad it worked out. 

[Gilead]

8 minutes ago, Snark said:

Am I correct in understanding that this ship is one that you flew before KSP 1.0?

It's a ship I flew 2 weeks ago. I don't remember the ships I flew back in the days, before KSP 1.0, on my old laptop.

And, as I just said (like, just before your comment, what a timing), I managed to land it, by doing it without time warping.

But I will take all your advices into account for my next ship, thank you all !

Edited January 23, 2018 by Gilead

[Gilead]

Well I added 2 AV-R8 on my last stage, and I have to say, you were right, re-entry is so much easier now with just that little difference. Although it is spinning like crazy, I can deploy my chutes safely at about 10km, so, nothing compared to the close call it was when it worked without the AV-R8. Thx!

[Snark]

1 hour ago, Gilead said:

Although it is spinning like crazy

Huh?  Why is it spinning?  Fins on the back should make it more stable, not less.  Got a screenshot?

[Gilead]

Here are pictures of the craft in the VAB with the additional fins, and some screenshots of the re-entry.

https://imgur.com/a/rs31r

 

Edited January 24, 2018 by Gilead

[Gilead]

For the high atmosphere spinning, I figured it out, I need to re-enter nose first (and pitch a little up, but you told me that). No spinning, nice atmosphere slowing.

Concerning the spinning when I deploy the drogue chutes, it's still there until the drogue fully deploy. Weird but not a big issue, I think.

Edited January 25, 2018 by Gilead

[Spricigo]

Oh well..Steerable fins on a upper stage make me cringe in disgust (not rational, just an idiosyncrasy). But, if that is working for you, enjoy. 

 

Since you are adding wings to your craft you may consider the idea of make it so you can use it for a nice landing at KSC grounds and, as you confidence and skill improve, eventualy a true spaceplane landing in the runaway.

[Gilead]

Ok, I'm back with re-entry problems.

I replaced the second cabin with a material bay, and the material bay exploded upon reentry. So I thought it was overheating. I replaced the parachute on the nose of the ship with a 0.625m heat shield and put a 1.25m heat shield under the material bay, so I'm protected wether I enter atmosphere nose up or the other way around. I activated thermal highlights and tried a reentry with that modified ship. At some point, the command pod starts to heat, but nothing serious, then at around 30km my material bay explodes, without having shown the "progress bar" that show you how hot the part is.

I do not understand why my material bay explodes without warning! I guess I'm doing something wrong... Again.

Man KSP was so much easier a few years ago...

[Cpt Kerbalkrunch]

2 hours ago, Gilead said:

Ok, I'm back with re-entry problems.

I replaced the second cabin with a material bay, and the material bay exploded upon reentry. So I thought it was overheating. I replaced the parachute on the nose of the ship with a 0.625m heat shield and put a 1.25m heat shield under the material bay, so I'm protected wether I enter atmosphere nose up or the other way around. I activated thermal highlights and tried a reentry with that modified ship. At some point, the command pod starts to heat, but nothing serious, then at around 30km my material bay explodes, without having shown the "progress bar" that show you how hot the part is.

I do not understand why my material bay explodes without warning! I guess I'm doing something wrong... Again.

Man KSP was so much easier a few years ago...

I guess you are doing something "wrong", so-to-speak, but not really what you're thinking. It's not so much the design as the idea. The science jr. is basically made of tinfoil. It is not very hardy, and will often explode on reentry (or be crushed on touchdown, with its puny 6m/s impact tolerance; a quick tip, if you do bring one home, aim for a splashdown). Luckily, you don't need to bring it home. Just EVA one of your Kerbals, right-click on the Jr. and hit "Take Data". Now you have the full science from it and can just dump the dead weight. No need to bring it back to Kerbin.

Edited January 31, 2018 by Cpt Kerbalkrunch

[Spricigo]

As @Cpt Kerbalkrunch says: take the science and dump the instrument.

 

Also, your design is flawed. the 0.625m heatshield is too small to cover the 1.25m parets behind it. And to add insult to injury the material bay is notorious for taking heat even behind a 1.25m heatshield sometimes.

[Snark]

14 hours ago, Gilead said:

I replaced the second cabin with a material bay, and the material bay exploded upon reentry.

Yes, that's pretty common.

14 hours ago, Gilead said:

I do not understand why my material bay explodes without warning!

Because it's getting too hot.

Here's the thing to understand:  Science instruments have much lower temperature tolerance than most other parts.  Most KSP parts are able to withstand 2000K or more; quite a few of them go up to 2400K.  But science instruments can only tolerate 1200K.

That's crucial, because you're almost guaranteed to break 1200K when reentering Kerbin's atmosphere from orbit.  Depending on your ship design and how fast you're going before hitting atmo, the temp may or may not get above 2000 K (which is why it's often possible for a ship to reenter without a heat shield)... but it's certainly going to get above 1200 K.

This matters a lot for the science bay, which is big and bulky and therefore trickier to protect.  First of all, remember all that advice before about "put a couple of steerable fins on the back of your 1.25m ship and then you can use body lift to slow down"?  That's out the window.  Because if you're using body lift by angling your ship above while reentering, it means that the airflow is hitting the side of the ship (that's how body lift works), which means the entire ship heats up, not just the front.  Which is fine when the entire ship has a 2000 K heat tolerance; but not when you've got a part that's limited to 1200 K, like the materials bay.

In practice, if you've got a materials bay, then, you have to be strictly throughout the hot part of reentry, with a heat shield in front of it the whole time, so that it's completely shielded from the incoming airflow.  So that's problem #1.

14 hours ago, Gilead said:

I replaced the parachute on the nose of the ship with a 0.625m heat shield and put a 1.25m heat shield under the material bay, so I'm protected wether I enter atmosphere nose up or the other way around.

Actually, no.  It means you're not protected, regardless of which way you enter the atmosphere.

First, the 0.625m heat shield is not going to protect the materials bay, because it's 1.25m.  You're trying to clothe a hippo with a fig leaf.  You need more coverage.

The reason why it's not protected in the other direction is a bit more subtle.  You've mounted the heat shield directly to the materials bay, right?  That's a problem.  Why is that?  Because of thermal conduction.  Heat shields get really hot.  They don't get as hot as they naturally "should", because they have ablator that cools them off and keeps them from getting so hot that whatever's attached to them will explode... for most KSP parts.  But the materials bay can only take 1200 K, and the heat shield does get a lot hotter than that.  Not hot enough to blow up a 2000 K part (i.e. "just about any part except the materials bay"), but plenty hot enough to melt your science instrument as soon as you've been in reentry long enough for the heat to conduct from the shield.

Fortunately, this latter problem is easily solved:  just don't mount the materials bay directly to the heat shield.  Put something in between them that can take 2000 K, such as a 1.25m battery or reaction wheel or something.  That way, what happens is that the heat shield warms up the thing-attached-to-it, and that thing warms up the materials bay.  And since a typical reentry sequence is over within a minute or two, the intervening part doesn't have time to get hot enough to warm up the materials bay above 1200K, and so you're fine.

 

And, finally, of course, the simplest solution is just to eliminate the problem before you even hit atmosphere:  as @Cpt Kerbalkrunch suggests, take the science and ditch the bay before reentry.

 

 

[Gilead]

Thanks for all those explanations, you guys are great!

So, I'm gonna take your advice and choose the easy solution, ditching the bay.