Reentry question

[jonpfl]

All,

I am trying to take two people into space so I use a capsule (with a pilot) and that two pod for the passengers.  I cannot get back into reentry without either burning up (on occasion) or crashing into planet (most of the time).

What the is best way to approach Kerbin?  Do I want to come out of orbit and have my periapsis around 60k or 65k and then shut off engines and release them?  Or do I want to keep burning my engines until my periapsis is below the surface?  Or near the surface?

Thx
jonpfl

[FullMetalMachinist]

11 minutes ago, jonpfl said:

I am trying to take two people into space so I use a capsule (with a pilot) and that two pod for the passengers.

I'm guessing that you're trying to reenter with the Mk1 pod on top of the Mk1 crew cabin?  Let me guess, it wants to fall with the pointy end first?

If that's what you're seeing, it's a pretty common problem. What happens is that the pod is heavier than the crew cabin, so it wants to fall first. Think of a throwing dart or arrow. The heavy bit is at the front, and the light draggy feathers/fins are at the back. Which means that for reentry your ship has it's pointy end first, allowing you to cut through the atmosphere easily. That's the opposite of what you want. You want to reenter back first, so that the blunt bottom of your ship slows you down more. 

One way around this is to put some fins on the side of the pod towards the top, next to where you (probably) have your parachute. That combined with the reaction wheels in the pod is usually enough to keep you pointed retrograde during reentry. Just make sure that the pod has electric charge when you start to reenter. 

Also, from low orbit most people set their periapsis to something around 20-35 km. The reason is that reentry heat is cumulative. It's better to endure higher heat for a shorter time than it is to bake at low temps for a long time at 65 km without slowing down much.

Also, if none of that sounds like it is the problem that you're seeing, a screenshot of your ship during reentry (and maybe one of it in the VAB with the CoM marker showing) will help to diagnose problems better.

[Reusables]

I found that Pe of 38km~42km generally works well. Provides enough deceleration while not giving too much heat burning up. Heat tolerant parts(2000K) can definitely handle the reentry even when they are placed directly to the airstream. Also entering parachute-first is helpful as the parachute have excellent heat tolerance. (2600K, it's next to the best)

Besides, does it slow down well under 10km? If it don't want to decelerate on transonic speed, then you're definitely in trouble. The spaceship won't land safely, if you don't have drogue parachutes. (i.e. try drogue parachutes if this is the case)

[Spricigo]

 

Roughly speaking too high and you fry, too low and you crash. But every vessel will have different limits for too high and too low. For the craft you described, reentering with apoapsis 80km and periapsis around 20km should be a good ballpark.

Since you are relying on drag to slow down, as soon you setup your trajectory release the fuel tanks and engines, the added mass is a disvantage.

 

Edit: duh! Read as 1 pod for pilot +2 pods for passengers.As  @FullMetalMachinist suggest, just add some draggy parts to make it slow quickier. It may be basic fins, a open service bay or even some radial atached part are usualy enough. And whille its not the most elegant design a crew cabin on top of the command pod work like a charm.

 

Edited May 24, 2017 by Spricigo

[jonpfl]

36 minutes ago, FullMetalMachinist said:

I'm guessing that you're trying to reenter with the Mk1 pod on top of the Mk1 crew cabin?  Let me guess, it wants to fall with the pointy end first?

If that's what you're seeing, it's a pretty common problem. What happens is that the pod is heavier than the crew cabin, so it wants to fall first. Think of a throwing dart or arrow. The heavy bit is at the front, and the light draggy feathers/fins are at the back. Which means that for reentry your ship has it's pointy end first, allowing you to cut through the atmosphere easily. That's the opposite of what you want. You want to reenter back first, so that the blunt bottom of your ship slows you down more. 

One way around this is to put some fins on the side of the pod towards the top, next to where you (probably) have your parachute. That combined with the reaction wheels in the pod is usually enough to keep you pointed retrograde during reentry. Just make sure that the pod has electric charge when you start to reenter. 

Also, from low orbit most people set their periapsis to something around 20-35 km. The reason is that reentry heat is cumulative. It's better to endure higher heat for a shorter time than it is to bake at low temps for a long time at 65 km without slowing down much.

Also, if none of that sounds like it is the problem that you're seeing, a screenshot of your ship during reentry (and maybe one of it in the VAB with the CoM marker showing) will help to diagnose problems better.

Should I put a heat shield on the bottom of the crew cabin to lower the CoM?

I will try your suggestions and see if it helps.  If not, I will post a pic.

Thx

jonpfl

[FullMetalMachinist]

37 minutes ago, jonpfl said:

Should I put a heat shield on the bottom of the crew cabin to lower the CoM?

I wasn't sure if just the heat shield would be enough, so I just did some tests. I made a simple craft that was just a Mk16 parachute -> Mk1 command pod -> Mk1 crew cabin -> heat shield. That was the reentry vehicle. Under that I put a decoupler, a fuel tank, and a terrier engine. Then I put it in orbit using the debug window. Ap was 85 km, I burned to get a 25 km Pe, then decoupled the tank & engine. As soon as I hit the atmosphere I set SAS to hold retrograde. Worked like a charm. Only issue was that once the 'chute fully deployed all the weight made splash down a little harder than I normally like. So I tried setting the heat shield to jettison when the 'chute deploys. Nice, gentle 8 m/s touchdown. 

In case you're not familiar with that last bit, you first have to go to the settings menu and turn on Advanced Tweakables (pause menu -> settings -> toward the bottom, right above the UI section). Then in the VAB right click on the heat shield and click on "HS Jettison Not Staged" to change it to "HS Jettison Staged". This will add a decoupler icon in the staging list on the right. Then just drag that so that it's in the same stage as your parachute.

[Cpt Kerbalkrunch]

12 minutes ago, FullMetalMachinist said:

In case you're not familiar with that last bit, you first have to go to the settings menu and turn on Advanced Tweakables (pause menu -> settings -> toward the bottom, right above the UI section). Then in the VAB right click on the heat shield and click on "HS Jettison Not Staged" to change it to "HS Jettison Staged". This will add a decoupler icon in the staging list on the right. Then just drag that so that it's in the same stage as your parachute.

You could also just right-click on the heat shield and hit "jettison heat shield" when you no longer need it.

[FullMetalMachinist]

Just now, Cpt Kerbalkrunch said:

You could also just right-click on the heat shield and hit "jettison heat shield" when you no longer need it.

True, or you could set it to an action group. But I loath having to click on parts to perform actions during flight if I can assign a keypress to do it instead. That's why in my career games I set the difficulty option to allow actions groups from the start.

[Spricigo]

Shameless self promotion time:

https://kerbalx.com/Spricigo/Orbiter-Zero

Its actualy for 4 passengers, if you really want just 2 remove one crew cabin.

AVOID LANDING ON SLOPES!!

[Cpt Kerbalkrunch]

3 minutes ago, FullMetalMachinist said:

True, or you could set it to an action group.

Never thought to do that before. I think that would be better than enabling staging on it; at least for me. I'm never too sure if I'm gonna need to dump the shield or not. If I'm going a bit too fast, I dump it. Otherwise I keep it to get my money back (cheapskate-style).

[FloppyRocket]

I had no idea you can jettison the heat shield. That's a helpful tip.

[bewing]

Heatshield have very very low drag. So if you have one on, generally you won't slow down very much. So they often make you crash harder, or fry even worse. I consider heatshields to be deathtraps myself.

If I'm having a reentry problem like this, I find that mounting two fins at the dry CoM works wonderfully. It allows you to steer while you slow down. So not only do you not fry or crash, you can land right at KSC, too.

And in super-cheapskate fashion, I never decouple or jettison anything. It takes less effort to land the whole thing and get all your money and science back, than it does to carefully decouple just a few pieces.

 

[Aegolius13]

If you just need two (not three) people, and do not have CommNet problems, I like using just a Mk 1 passenger cabin (no command pod), with a probe core, reaction wheels and batteries stashed in a service bay on top, and a heat shield below.  I find this less prone to the lawn-dart effect since it has no especially pointy end.  And you can open the service bay to use it as a makeshift airbrake, which will keep it and the back, and therefore keep your heat shield on the front where it belongs.  I usually also throw on a couple drogue chutes.  Those may not be needed with the service bay airbrake, but passenger cabin reentry modules scare me, so might as well err on the side of caution. 

[Spricigo]

@bewingI think the idea behind the heatshield is to shift the CoM away from the pod+parachute. It's not a idea I like,  but under certain circumstances it works.  

@Aegolius13 stuff-inside-service-bay & crew cabin works fine for 2, 4, 6 and 8 passengers according to my experience.  However it requires a bit higher tech,  specially if one wants SAS available. 

 

[Reusables]

Besides, if you unlocked aerodynamics, 2 deployed eleven 1 works really well with reentry issues. It lets the craft decelerate quite fast, and prevent it from getting in streamlined attitude.

[jonpfl]

8 hours ago, Spricigo said:

Shameless self promotion time:

https://kerbalx.com/Spricigo/Orbiter-Zero

Its actualy for 4 passengers, if you really want just 2 remove one crew cabin.

AVOID LANDING ON SLOPES!!

Thanks, but I don't think that will help me as I still have problems getting into orbit and back with delta V 3.4k.

I might try the 2 man capsule on top though, I just figured that would make it much harder to get into orbit.

jonpfl

1 hour ago, Abastro said:

Besides, if you unlocked aerodynamics, 2 deployed eleven 1 works really well with reentry issues. It lets the craft decelerate quite fast, and prevent it from getting in streamlined attitude.

What does this mean?

Thx
jonpfl

8 hours ago, FullMetalMachinist said:

True, or you could set it to an action group. But I loath having to click on parts to perform actions during flight if I can assign a keypress to do it instead. That's why in my career games I set the difficulty option to allow actions groups from the start.

I haven't messed around with action groups but I guess I need to read up on them (or watch a Scott Manley video).

Thx
jonpfl

[Reusables]

21 minutes ago, jonpfl said:

What does this mean?

Strap 2 'Elevon 1' on the craft in mirror symmetry. Deploy them before reentry, and you won't hit the ground before parachute gets deployed.

They are basically early-game heat-tolerant airbrakes.

Also, it might be better to set parachutes to deploy when risky.

Edited May 24, 2017 by Reusables
Typo

[LoSBoL]

Slap a heatshield on top of the MK1 capsule and just dive in nose first, that's how I did most tourist contracts early career with a Mk1 pod and 1 or 2 crew cabins behind that.

A bit more drag on the way up, but no issues coming down.

 

[Reusables]

Usually there's no need of headshield on the top of typical command pod, because those parachutes have great heat tolerance.

[jonpfl]

9 hours ago, FullMetalMachinist said:

I wasn't sure if just the heat shield would be enough, so I just did some tests. I made a simple craft that was just a Mk16 parachute -> Mk1 command pod -> Mk1 crew cabin -> heat shield. That was the reentry vehicle. Under that I put a decoupler, a fuel tank, and a terrier engine. Then I put it in orbit using the debug window. Ap was 85 km, I burned to get a 25 km Pe, then decoupled the tank & engine. As soon as I hit the atmosphere I set SAS to hold retrograde. Worked like a charm. Only issue was that once the 'chute fully deployed all the weight made splash down a little harder than I normally like. So I tried setting the heat shield to jettison when the 'chute deploys. Nice, gentle 8 m/s touchdown. 

In case you're not familiar with that last bit, you first have to go to the settings menu and turn on Advanced Tweakables (pause menu -> settings -> toward the bottom, right above the UI section). Then in the VAB right click on the heat shield and click on "HS Jettison Not Staged" to change it to "HS Jettison Staged". This will add a decoupler icon in the staging list on the right. Then just drag that so that it's in the same stage as your parachute.

In regards to parachutes, should I release it when the icon is grey (I assume it provides some drag) and then use it higher than 1k?  Lately I have not been able to get to the grey icon though (like I said, I usually crash and sometimes burn up).

Thx
jonpfl

[LoSBoL]

14 minutes ago, Abastro said:

Usually there's no need of headshield on the top of typical command pod, because those parachutes have great heat tolerance.

You're probably right, but after the first flipping round and nose dive and explosion I took the 'better safe than sorry' approach, and it served me well since. 

Edited May 24, 2017 by LoSBoL

[Reusables]

5 minutes ago, LoSBoL said:

You're probably right, but after the first flipping round and nose dive and explosion I took the 'better safe than sorry' approach, and it served me well since. 

That can happen if the craft has bad glide ratio.

Reentry has changed recently(1.2), probably you were playing on 1.0.5 or 1.1 back then?

Edited May 24, 2017 by Reusables
Typo

[Cpt Kerbalkrunch]

10 minutes ago, jonpfl said:

In regards to parachutes, should I release it when the icon is grey (I assume it provides some drag) and then use it higher than 1k?  Lately I have not been able to get to the grey icon though (like I said, I usually crash and sometimes burn up).

Thx
jonpfl

If you can't deploy your parachutes in time, it's because you're still moving too fast. Until you learn to eliminate that problem with a better reentry, just add a couple of drogue chutes. They can deploy at higher speeds. This will slow you down enough that you can open your main chute. 

[LoSBoL]

Just now, Abastro said:

That can happen if the craft has bad glide ratio.

Reentry is changed recently(1.2), probably you were playing on 1.0.5 or 1.1 back then?

I wasn't playing back then, 1.2.2 is my first   reentry craft was just an MK1 + 1 or 2 inline crew cabins. 

[Reusables]

8 minutes ago, LoSBoL said:

I wasn't playing back then, 1.2.2 is my first   reentry craft was just an MK1 + 1 or 2 inline crew cabins. 

Let me test again. Probably I recall it wrong..

Will post the result in a day.