Why my vessel crash before reentry finishes?

[Anonymous49]

I wanted to earn some money before I start to do my mun flyby. So I accepted to launch 9 tourists to a suborbital trajectory & orbital trajectory. And it will give me over 250k in funds and over 100 in reputation. So I did my suborbital things. And now I need to do my orbital ones, which is the one which gives huge money. So my first one went smoothly and successfully landed after 17 minutes via parachutes. And I needed to do my second one. Now , everything went smooth, even reentry, but then the red things in the screen wouldn't go away and I couldn't open my drogue chutes. So it crashed in ~700m/s. I reverted that and did it again and again  and it had the same ending, I could not finish my contract. Please help. Thanks.

 

[VoidSquid]

Hi and welcome to the forum, @Anonymous49  

Chances are, your vessel is pretty heavy (a vessel for 10 Kerbals usually is), plus you came back into the atmosphere too steep, not fully retrograde and/or maybe without heat shields. 

This is upper part of my orbital tourist vessel coming back to Kerbin (1 pilot plus 8 tourists), for reentry I set the Pe to approx. 20-30km. All excess weight (engines, fuel tanks, etc.) is decoupled before reentry, to keep the orientation retrograde during reentry I have put a large reaction wheel at the bottom just above the heatshield.

 

[Anonymous49]

I only have all the level 3 parts, all the level 4 parts except for aviation. I only ship Valentina Kerman and 4 tourists there

that means I only have the small incline reaction wheel, which is close to useless i can say, but add a lot of weight

[Anonymous49]

my craft

 

from top to bottom:

Command Module

mk12 radial drogue chute (on top of pod)

mk1 command pod

mk2 radial mount chute x2 (mounted radially on the pod)

Tourist module

mk1 crew cabin x2 (one under another)

Service module (that doesn't get ditched)

service bay 1.25m

z-100 rechargeable battery bank x8 (to power the tourist module) (inside the service bay)

Stage 2

FL-T400 Fuel Tank

LV-909 engine

Stage 1

FL-T400 Fuel Tank x3 (Stacked together)

AV-R8 Winglets x4 (Mounted Radially on the bottom FL-T400 Fuel Tank)

LV-T45 engine

Boosters:

Aerodynamic Nose Cone x4

Bacc thumper srb x4

TT-38k radial decoupler (in between of the wings) (mounted radially on the bottom FL-T400 Fuel Tank)

 

What's the problem?

[VoidSquid]

Mind to share a screenshot of the reentering part of your rocket?

[Anonymous49]

How?

[vv3k70r]

2 hours ago, Anonymous49 said:

Now , everything went smooth, even reentry, but then the red things in the screen wouldn't go away and I couldn't open my drogue chutes. So it crashed in ~700m/s.

I guess You build a pencil and it will not decelerate or breaks agains aerodynamics forces with such a speed if try?

Drouge parachute have its max speed. But You can tweak it to open immeditialy, in such case it will breake but decelerate You to stage next drouge.

[VoidSquid]

1 hour ago, Anonymous49 said:

I only have all the level 3 parts, all the level 4 parts except for aviation

Without seeing the vessel in question, I assume it would be something like this:

10 minutes ago, vv3k70r said:

I guess You build a pencil and it will not decelerate or breaks agains aerodynamics forces with such a speed if try?

Just what @vv3k70r said, yes: very small diameter in relation to the mass, and the little reaction wheel won't be able to keep that stable on retrograde.

I'd suggest you make two launches, with just the Mk1-1 pod plus one passenger cabin for reentry.

[Anonymous49]

2 hours ago, vv3k70r said:

I guess You build a pencil and it will not decelerate or breaks agains aerodynamics forces with such a speed if try?

Drouge parachute have its max speed. But You can tweak it to open immeditialy, in such case it will breake but decelerate You to stage next drouge.

I know that, I am just asking how to decelerate in over 700m/s WITH ONLY THE USELESS SMALL REACTION WHEEL

2 hours ago, VoidSquid said:

Without seeing the vessel in question, I assume it would be something like this:

Just what @vv3k70r said, yes: very small diameter in relation to the mass, and the little reaction wheel won't be able to keep that stable on retrograde.

I'd suggest you make two launches, with just the Mk1-1 pod plus one passenger cabin for reentry.

I had the idea but that also means no extra funding

I mean how do you send a screenshot? im using windows

Edited January 11, 2021 by Anonymous49

[paul_c]

I am guessing 700 m/s is above the threshold for safe parachute deployment. I am not sure if your drogue chute deployed then immediately tore off; or never did. Obviously, check the "pressure" parameter for its deployment and look at it.

Whatever the speed limit for the safe chute deployment, the key would be drag vs weight. You want it to be as draggy as possible, but as light as possible, so the two forces during re-entry balance out at as low a speed as which allows the drogue chute to safely deploy. Once that happens, the chute(s) should be able to do their jobs.

Get rid of the batteries, you don't need batteries or a reaction wheel on the final stage. Make it fly retrograde naturally by good design (the heat shield is quite heavy, so it helps to keep it retrograde). 9 pax is a lot. I can't remember what level 3 buys you but if you don't have the 2.5m wide hitchhiker module (which is the light relative to the number of pax) I'd not be doing 9 at a time.

[Anonymous49]

So what I do is to launch and  immediately pitch until it is 60 degrees east. Then get a apogee of around 120km. Then wait for the perigee to rise over 70km then turn it back down to ~29km. After a while, it will reenter the atmosphere and eventually crash for some reason

Just now, paul_c said:

I am guessing 700 m/s is above the threshold for safe parachute deployment. I am not sure if your drogue chute deployed then immediately tore off; or never did. Obviously, check the "pressure" parameter for its deployment and look at it.

Whatever the speed limit for the safe chute deployment, the key would be drag vs weight. You want it to be as draggy as possible, but as light as possible, so the two forces during re-entry balance out at as low a speed as which allows the drogue chute to safely deploy. Once that happens, the chute(s) should be able to do their jobs.

Get rid of the batteries, you don't need batteries or a reaction wheel on the final stage. Make it fly retrograde naturally by good design (the heat shield is quite heavy, so it helps to keep it retrograde). 9 pax is a lot. I can't remember what level 3 buys you but if you don't have the 2.5m wide hitchhiker module (which is the light relative to the number of pax) I'd not be doing 9 at a time.

The drogue chutes never opened, it never let me to.  And also I needed to ship 9 pax altogether but I only do 4 at a time, pls check on one of my old post, it shows how my craft looks like. I have all of the level 4 except for aviation. But the hitchhiker module is level 5 (it is part of space exploration). The batteries, thanks for your advice, (I never realized it before), have almost never used any electric charge during the whole flight (however did use some). Shall I get rid of the whole service module or put 2 batteries instead?

Edited January 11, 2021 by Anonymous49

[paul_c]

Better would be to get to a 71x71 orbit (or thereabouts) then de-orbit to literally just within the atmosphere. If you had an Ap of 120km your speed it greater. Having said that, the heat shield and initial 'hot' part of the re-entry would just be longer and it would probably reach a similar terminal velocity, albeit later on.

But a better flight profile would give better margin of safety.

The other option, although it is definitely not necessary, is to retroburn during the re-entry but then you need fuel, engine, etc etc etc so best learning/knowing how to do it with the basics.

[Anonymous49]

Just now, paul_c said:

Better would be to get to a 71x71 orbit (or thereabouts) then de-orbit to literally just within the atmosphere. If you had an Ap of 120km your speed it greater. Having said that, the heat shield and initial 'hot' part of the re-entry would just be longer and it would probably reach a similar terminal velocity, albeit later on.

But a better flight profile would give better margin of safety.

The other option, although it is definitely not necessary, is to retroburn during the re-entry but then you need fuel, engine, etc etc etc so best learning/knowing how to do it with the basics.

Sorry, but I don't know how to get on a circular orbit. 120x101km orbit was the best for me. But I wanted to finish the mission fast and want to use less delta v because I am doing this for funding from the tourists not for exp or anything

And I removed the heat shield since 0 ablator was used during reentry, check it out

[KawaiiLucy]

Your craft isn't well built for atmospheric reentry. You need to consider the center of mass for having a stable profile. It should be as close as possible to the heatshield for maximum stability. Your command pod is much heavier than those crew cabins, so put them on top with a nose cone, for the ascent. Next, the thermal rating of the crew cabins isn't as high as the one of the command pod (you can see this in the vab when hovering over the parts), so they are less likely to resist high temperatures. That means either a shallower reentry trajectory ~45km, with multiple passes maybe, or slower descent, engine assisted retro-burn maybe, with a gimbal which can help keep your craft pointing retrograde.

[VoidSquid]

5 hours ago, Anonymous49 said:

So it crashed in ~700m/s

700 m/s is a lot, you definitely had your craft in retrograde orientation during the whole reentry atmospheric flight?

[Anonymous49]

3 minutes ago, KawaiiLucy said:

Your craft isn't well built for atmospheric reentry. You need to consider the center of mass for having a stable profile. It should be as close as possible to the heatshield for maximum stability. Your command pod is much heavier than those crew cabins, so put them on top with a nose cone, for the ascent. Next, the thermal rating of the crew cabins isn't as high as the one of the command pod (you can see this in the vab when hovering over the parts), so they are less likely to resist high temperatures. That means either a shallower reentry trajectory ~45km, with multiple passes maybe, or slower descent, engine assisted retro-burn maybe, with a gimbal which can help keep your craft pointing retrograde.

So are you saying that I should use the reentry command pod and place it underneath the cabins and use a aerodynamic nosecone on top. (And also remove the td-12 decoupler in between)

 

Just now, VoidSquid said:

700 m/s is a lot, you definitely had your craft in retrograde orientation during the whole reentry atmospheric flight?

uh so which marker is the retrograde one

[paul_c]

11 minutes ago, Anonymous49 said:

Sorry, but I don't know how to get on a circular orbit. 120x101km orbit was the best for me.

In this instance: at Apogee, do a retrograde burn until Perogee is 71km. Then at Pe, do a retrograde burn until Ap is 71km (ie to circularise it). Then you can choose where in the world (approximately) to land - I'd suggest water, for now.

 

11 minutes ago, Anonymous49 said:

And I removed the heat shield since 0 ablator was used during reentry, check it out

Heat shield is always recommended unless you know its going to be okay; or something unusually small or resilient or with retro firing.

[Anonymous49]

1 minute ago, paul_c said:

In this instance: at Apogee, do a retrograde burn until Perogee is 71km. Then at Pe, do a retrograde burn until Ap is 71km (ie to circularise it). Then you can choose where in the world (approximately) to land - I'd suggest water, for now.

 

Heat shield is always recommended unless you know its going to be okay; or something unusually small or resilient or with retro firing.

I know heat shield is always recommended, but no ablator was used during reentry. But every time it never burns any ablator up why shall I even bother using it? Does it slow the speed down?

Right, I had Jebediah who already have xp before the launch, and now valentina is still level zero

And also which launch site is the best. Kerbal space center?

[vv3k70r]

35 minutes ago, Anonymous49 said:

I know that, I am just asking how to decelerate in over 700m/s WITH ONLY THE USELESS SMALL REACTION WHEEL

Force tweakable of one drouge in staging to deploy immediate after triggering and it will stop You (and break) - propably enough to stage another drouge.

You popably try to go sideways to decelerate using reaction wheels and pencil breaks due to aerodynamic forces.

It is good to have more parachutes than You need and use force one of them for agresive breaking. Use advanced tweakables in menu and twek one, selected parachute to immediate deployment after excluding it from symetry and separate it staging - all this You can do in flight.

4 minutes ago, Anonymous49 said:

But every time it never burns any ablator up why shall I even bother using it? Does it slow the speed down?

You do not have this issue if You pass to 700m/s.

Specialy if You keep faling prograde or retrograde (keeping track of lowest air resistance).

6 minutes ago, Anonymous49 said:

Right, I had Jebediah who already have xp before the launch, and now valentina is still level zero

It play no role If You are falling - air resistance will set You to lowest resistance if SAS turned off.

[VoidSquid]

36 minutes ago, Anonymous49 said:

uh so which marker is the retrograde one

32 minutes ago, Anonymous49 said:

And also which launch site is the best. Kerbal space center?

Short answer: yes.

Spoiler

Long answer: the latitude of a launch site is the minimum inclination of any orbit you can reach directly from that launch site. From an equatorial launch site, you can go for a any inclination, but if your launch site has e.g. a latitude of 30 degrees, you can only directly reach orbits between 30 and 90 degrees relative to the equator.

 

Edited January 11, 2021 by VoidSquid

[VoidSquid]

48 minutes ago, KawaiiLucy said:

Your command pod is much heavier than those crew cabins, so put them on top

I wouldn't recommend that. In general, avoid any non-smooth transitions in diameter, they are killing aerodynamics and cost a huge amount of dv during ascend.

Here's a cheap (costs just 13k) little orbiter for 3 Kerbals which has no issues with reentry:

You can download the craft file here https://easyupload.io/tdfgm7

Edited January 11, 2021 by VoidSquid

[jimmymcgoochie]

When I’m trying to get tourists to orbit and back I tend to stack Mk1 command pods with radial parachutes on each, which isn’t the prettiest way of doing it but does mean that it has plenty of control to force itself into the airflow and slow down, plenty of space for drogue chutes and those pods are more than capable of taking the heat of low orbit re-entry so no heatshields needed.

Another alternative is to use the KV series pods from Making History which have 1.25m bases, are very good at re-entering and can hold up to 3 Kerbals, though they also have considerable drag during ascent and no built-in reaction wheels so can be a bit tricky to use.

Edited January 11, 2021 by jimmymcgoochie

[Anonymous49]

15 minute orbital mission complete, putted valentina kerman into orbit and came back down. Now I have the prograde and retrograde markers up, what is next?

[VoidSquid]

1 hour ago, Anonymous49 said:

I mean how do you send a screenshot?

Just upload the screenshot to an image sharing site like Imgur or Imgbb and post the link to the image here.

[Anonymous49]

oh thanks

https://imgur.com/a/fo1Gzv5

and guys i think i got why I am crashing again and again. When i was doing the burn i was just facing upside down. And prior to landing I was facing a yellow marker (I thought it was retrograde) but it was prograde. i read the wiki and only know that prograde and retrograde have yellow markers so i was facing prograde and not retrograde LOL

And btw that was the old version i already removed the service module which was underneath the crew cabins