Drain tank without accelerating

[RoninFrog]

I have encountered a bit of a problem while designing how the re-entry of a large rocket will work.  I have a 3-2 converter at the top of my rocket under my command pod.  It would be the heaviest thing on the rocket with a mass of almost 17 tons while full.  During reentry, the rocket would almost certainly flip and burn up.

Question: Is there any way to drain all of the fuel off of a tank without accelerating or moving?  If I could, it would reduce the weight to just under 2 tons which should be easily manageable.

[AHHans]

7 hours ago, MunsterIII said:

Question: Is there any way to drain all of the fuel off of a tank without accelerating or moving?  If I could, it would reduce the weight to just under 2 tons which should be easily manageable.

Not that I know of. But why not use the excess fuel to decelerate before / during reentry? If you point at the right direction between retrograde and radial-out then you can reduce your speed without lowering your periapsis.

P.S. Maybe this fits more into the Gameplay Questions forum...

[RoninFrog]

8 hours ago, AHHans said:

P.S. Maybe this fits more into the Gameplay Questions forum...

Is there a way to move it there?

[Gargamel]

30 minutes ago, MunsterIII said:

Is there a way to move it there?

Perhaps.... there just might be :).

Welcome to the Forums!

If you see a thread that needs moved, please just use the Report Thread to the upper right and explain what's going on.

That, and other wonderful tips on how things work, can be found in this thread:

 

[Geonovast]

34 minutes ago, MunsterIII said:

Is there a way to move it there?

If you hover over your post, you'll see a link on the upper right called "Report Post".  You can report your post to the mods and request it to be moved, although one of them is bound to see it anyway. (who ninja'd this!)

 

I, too, wish there was a way to do this in stock that didn't involve just jettisoning a tank.  It would make building shuttles much easier.

If you're into mods, there's a handy mod called SmartParts that includes fuel drain valves.  They do provide a very small amount of thrust, but it's usually negligible.  If even the small amount it provides is too much, you can put two valves on opposing each other to their thrust cancels out, or put the valve close enough to a part on your ship that it actually provides no movement.

 

Edited August 12, 2019 by Geonovast

[Aegolius13]

You could include two engines or thrusters  pointing in opposite directions.   Firing them both would use fuel but not move you.  

[RoninFrog]

53 minutes ago, Aegolius13 said:

You could include two engines or thrusters  pointing in opposite directions.   Firing them both would use fuel but not move you.  

How would I place them on the craft though, since if I'm off by even 1 pixel then the rocket would begin to spin, which is undesirable.  Is there any trick to perfectly center an engine?

[vossiewulf]

What @Geonovast said. I am recovering huge boosters in a 2.5x game (8m-10m, 40m long or so) with the use of Smart Parts to drain remaining fuel, Space Y for the 10m inflatable heat shield, and Ablative Airbrakes to stabilize the booster because they really like to tumble, and they also add a strong braking force.

 

[bewing]

2 hours ago, MunsterIII said:

How would I place them on the craft though, since if I'm off by even 1 pixel then the rocket would begin to spin, which is undesirable.  Is there any trick to perfectly center an engine?

He was talking about radial mounted engines, and the way you place them is to use 2X symmetry. You are familiar with using symmetry for building rockets in the VAB?

[RoninFrog]

7 minutes ago, bewing said:

He was talking about radial mounted engines, and the way you place them is to use 2X symmetry. You are familiar with using symmetry for building rockets in the VAB?

Yes, but the round adapter's connection surface isn't quite perfectly circular.  If I was off by a tiny bit, the engines would not quite be even, and the rocket would slowly begin to revolve (which is a pain in the neck on re-entry).

@vossiewulf, I prefer to not use mods if I can help it.  I would like to see whether there is a way to do this with stock parts before using mods.  The reason is that mods become obsolete after enough updates and I don't want to have to delete old mods, reinstall new mods, and fix my ships if I can do what I want with stock parts.

Edited August 12, 2019 by RoninFrog

[Aegolius13]

4 hours ago, MunsterIII said:

Yes, but the round adapter's connection surface isn't quite perfectly circular.  If I was off by a tiny bit, the engines would not quite be even, and the rocket would slowly begin to revolve (which is a pain in the neck on re-entry).

You wouldn't have to put engines on the round adapter itself; if you have any surface on the the ship with parallel sides, that would work. But if all you have to work with is curved sides, then yeah, this would not keep you stationary.

[VoidSquid]

If you're playing on PC, you can use this mod

to dump excessive fuel.

[Pecan]

On 8/12/2019 at 2:14 AM, MunsterIII said:

Is there any way to drain all of the fuel off of a tank without accelerating or moving?

If it's just a matter of *that* tank staying full - pump it to another tank (select two tanks and use the in/out options).

[Superfluous J]

Do what NASA does and throw the whole tank and engine away. That's what I do.

[Geonovast]

13 hours ago, MunsterIII said:

Yes, but the round adapter's connection surface isn't quite perfectly circular.  If I was off by a tiny bit, the engines would not quite be even, and the rocket would slowly begin to revolve (which is a pain in the neck on re-entry).

Are you building with angle snap off?

[pincushionman]

Posting a screenshot of your craft (just the re-entry portion) would help us help you determine where you could or could not easily place engines, or where parts could be moved to re-balance.

Though to be fair, if you have a full top tank on re-entry - and having it empty would help - that sounds to me like you packed too much fuel. What’s your mission profile and the stats (read: more screenies) of your vessel?

[bewing]

15 hours ago, MunsterIII said:

Yes, but the round adapter's connection surface isn't quite perfectly circular.  If I was off by a tiny bit, the engines would not quite be even, and the rocket would slowly begin to revolve (which is a pain in the neck on re-entry).

There's something called "absolute mode rotation". If you use symmetry and absolute mode, it's literally impossible for the mounting or orientation of the engines to be off at all. Even if you are attaching to a round surface.

[RoninFrog]

5 hours ago, Pecan said:

If it's just a matter of *that* tank staying full - pump it to another tank (select two tanks and use the in/out options).

Would I need to run fuel lines to the other tank? I have a stock nuclear reactor (800 PB-NUKs) between the adapter and main tank which might interrupt fuel flow.

4 hours ago, 5thHorseman said:

Do what NASA does and throw the whole tank and engine away. That's what I do.

My command pod is on top of it, and I have multiple hitchhiker pods attatched below it, so it wouldn't work without losing some kerbals.

2 hours ago, Geonovast said:

Are you building with angle snap off?

Angle snap is on.  But round surfaces are more octagonal, so angle snap would probably not fix the problem.

2 hours ago, pincushionman said:

Posting a screenshot of your craft (just the re-entry portion) would help us help you determine where you could or could not easily place engines, or where parts could be moved to re-balance.

Though to be fair, if you have a full top tank on re-entry - and having it empty would help - that sounds to me like you packed too much fuel. What’s your mission profile and the stats (read: more screenies) of your vessel?

The rocket is not completely built yet, but I will post a screenshot when it is.  It is meant for extracting ore from a moon, re-fueling a space station, and repeating until said station is full.  I guess a solution would be to have two docking ports (one for the main tank and one for the adapter) and jettisoning all the fuel when docked with the space station, but I am a n00b at docking and the less complicated the better.

[Pecan]

7 hours ago, MunsterIII said:

Would I need to run fuel lines to the other tank? I have a stock nuclear reactor (800 PB-NUKs) between the adapter and main tank which might interrupt fuel flow.

Nope, all you need to do is open the menu for any two (or more) tanks on the same vehicle and you can pump to your heart's content.  If you do select more than two and then click 'out' on the one you want to empty the fuel will go to all the others equally.  Similarly, if you want to drain tanks evenly in the future, you can click 'in' on one and draw from all the others.  Have fun.

[Superfluous J]

8 hours ago, MunsterIII said:

My command pod is on top of it, and I have multiple hitchhiker pods attatched below it, so it wouldn't work without losing some kerbals.

This is why most of my time is spent in the VAB instead of space

[pincushionman]

21 hours ago, MunsterIII said:

…It is meant for extracting ore from a moon, re-fueling a space station, and repeating until said station is full…

So why would such a craft ever have to return to Kerbin? That sounds like a perfect type of utility vessel to leave in space with the station while crew is returned by other, simpler means.