Rocket Design rules of thumb...

[GregA]

I am building a rocket that is fairly large and complex to do a noteworthy Jool mission.  At one point in the mission I have two largish tanks that I wish to dispose of, but I am having trouble deciding if it is worth it, because of the length and complexity of the mission.  On one hand I have these two 16 ton empty tanks that are only going to be kept for the remainder of the JOI burn.  On the other hand I can dispose of them by adding 1.6 tons of equipment to my rocket, but now Ive carried those two parts all the way into orbit, and for the first third of my JOI burn.

This might be an oddly specific question, but it basically takes a day to test out either possibility, and Im wondering if there is some easier way to calculate this or some experience that someone can relate...

[Francois424]

Well if you have Kerbal Engineer you can save your ship to another name and modify it to see how much Delta-V each configuration gives you.

ie: just remove the fuel from your 2 tanks and check how much you have.  then remove them entirely and add the weight to your main ship, check again.

Edited January 16, 2016 by Francois424

[suicidejunkie]

If you make your JOI burn with some slight aerobraking help, could you not just jettison them normally while your PE is still in atmo, and let Jool deorbit them for you?  (presuming the moons don't throw the path off)

With KAS, you could also just bring some C4 and detonate the tanks when you have some coasting time.

[GregA]

Suicidejunkie,

Well thats the thing.  That is a joi rocket only.  Once I am in Jool, I drop that tank and rocket stack, and the next statge plunges deep into jool (165k/m) depending on where I want to go in Jool system.  I am slowing adding bits to the payload section until I get down to just a few deltaV left in the stage.

I am just about there for a Jool 5 mission...  But I still have to add a Tylo descent stage, so I am getting to the every meter counts.  Sometimes science is more art than science.

Just blowing up the tanks with c4 wont work because it happens in the middle of the burn.

[GoSlash27]

GregA,

 I have a different rule of thumb that covers this situation: If it isn't going to deorbit somewhere, I don't jettison it. I don't like having debris floating around in space.

Best,

-Slashy

[GregA]

Slashy, 

I feel you there.  Back during my first space station there was a space junk yard in a 20km halo around it.  Big cloud of stuff, just floating there.  I used to go on missions to scavenge it for extra fuel and thruster juice.

[GregA]

Ok, so I did it and flew it and picked up like 450 deltaV...  So question answered.  Yes, loosing 16 tons of tank in the middle of a burn is worth it even though it adds to the launch weight and requires that I burn JOI insertion fuel during launch...

[Vanamonde]

How-to question moved to the how-to subforum. Carry on. 

[Snark]

18 hours ago, GregA said:

I am building a rocket that is fairly large and complex to do a noteworthy Jool mission.  At one point in the mission I have two largish tanks that I wish to dispose of, but I am having trouble deciding if it is worth it, because of the length and complexity of the mission.  On one hand I have these two 16 ton empty tanks that are only going to be kept for the remainder of the JOI burn.  On the other hand I can dispose of them by adding 1.6 tons of equipment to my rocket, but now Ive carried those two parts all the way into orbit, and for the first third of my JOI burn.

Just out of curiosity, why does it take 1.6 tons of equipment to jettison a couple of tanks?  Seems like you could manage it on a cheaper mass budget.

In general, dead mass is a Bad Thing and you get more dV by dumping it as soon as it's no longer needed.  It's why we have multi-stage rockets. 

Regarding the orbital-debris discussion:

I've never worried too much about orbital debris, space is big.  And in any case, lots and lots of debris takes care of itself (either on suborbital or escape trajectory), or else ends up in an oddball orbit far from everything else, so it's not much of an issue for me.

I did try one career where I followed a self-imposed rule of "no orbital debris allowed," just to see how it would affect the challenge of the game.  It made for an interesting playthrough, but ultimately wasn't interesting enough for me to stick with it and I haven't bothered myself about debris since.

Ultimately:  debris simply isn't a practical problem at all in KSP, so it basically just boils down to "does it bother you?" for the individual player.

Edited January 17, 2016 by Snark

[Plusck]

Just to add my B00000010 cents here: I get rid of mass as soon as possible.

In early career I was bothered by debris. Zooming in on Kerbin looked like it was wearing a wedding band. Or some sort of Centic equivalent to a boxer's pinky ring. Since then, I've given up on excessive role-playing: if I can crash it on a suborbital trajectory I will do so, but if not then space is really plenty big enoug to just ditch the mass.

[Snark]

29 minutes ago, Plusck said:

Zooming in on Kerbin looked like it was wearing a wedding band.

...you had debris display turned on in the tracking station?  Turning that off makes debris a lot less intrusive. 

[GregA]

13 hours ago, Snark said:

Just out of curiosity, why does it take 1.6 tons of equipment to jettison a couple of tanks?  Seems like you could manage it on a cheaper mass budget.

In general, dead mass is a Bad Thing and you get more dV by dumping it as soon as it's no longer needed.  It's why we have multi-stage rockets. 

Regarding the orbital-debris discussion:

I've never worried too much about orbital debris, space is big.  And in any case, lots and lots of debris takes care of itself (either on suborbital or escape trajectory), or else ends up in an oddball orbit far from everything else, so it's not much of an issue for me.

I did try one career where I followed a self-imposed rule of "no orbital debris allowed," just to see how it would affect the challenge of the game.  It made for an interesting playthrough, but ultimately wasn't interesting enough for me to stick with it and I haven't bothered myself about debris since.

Ultimately:  debris simply isn't a practical problem at all in KSP, so it basically just boils down to "does it bother you?" for the individual player.

 

Snark,

Having trouble bring up Kerbal to get game name of parts.  They are the the 10k liquid fuel tanks feeding 21 nuclear engines.  The big separators are 0.8 tons each.

 

[HebaruSan]

14 hours ago, Snark said:

Regarding the orbital-debris discussion:

I've never worried too much about orbital debris, space is big.  And in any case, lots and lots of debris takes care of itself (either on suborbital or escape trajectory), or else ends up in an oddball orbit far from everything else, so it's not much of an issue for me.

I did try one career where I followed a self-imposed rule of "no orbital debris allowed," just to see how it would affect the challenge of the game.  It made for an interesting playthrough, but ultimately wasn't interesting enough for me to stick with it and I haven't bothered myself about debris since.

Ultimately:  debris simply isn't a practical problem at all in KSP, so it basically just boils down to "does it bother you?" for the individual player.

What about lag? I thought the game had to update the locations of every craft every tick, including debris, so they could be checked for SOI changes, etc. I know that when I switch from my current relatively empty save back to one with 66 active flights, there's a very noticeable increase in latency that makes piloting quite difficult. I've started ensuring that all my debris gets de-orbited for that reason.

[Snark]

3 hours ago, GregA said:

Having trouble bring up Kerbal to get game name of parts.  They are the the 10k liquid fuel tanks feeding 21 nuclear engines.  The big separators are 0.8 tons each.

What "big separators" are you talking about?  The only "big separator" I can find that weighs 8 tons is the 3.75m stack separator, but why would you use that?  Just attach the drop tanks radially, you can use the little radial separators that are only 0.025 tons each.  Maybe with a .05-ton strut to keep them from wobbling, if it bugs you.

(A screenshot of your ship would help.)

2 hours ago, HebaruSan said:

What about lag? I thought the game had to update the locations of every craft every tick, including debris, so they could be checked for SOI changes, etc.

Lag, shmag.  Anything that you're not currently controlling (or close enough to be in its physics bubble) is running on rails.  That means that no physics simulation whatsoever is needed for them.  They're following a precisely mathematical curve.  The game doesn't need to calculate them at all unless someone is looking at them; there's a perfectly simple mathematical formula that gives the position as a function of time t.  And it doesn't need to "check each frame" whether there's an SoI change:  the planets and moons move on rails, too, so it knows exactly where and when the SoI will change over.

I've run a game where I set myself a self-imposed rule of "no orbital debris allowed" and did a career playthrough.  And I've also done a game where I was intensely profligate with debris (for example, sending very high-dV ion ships that dropped dozens of xenon tanks as it emptied them one by one) and racked them up by the hundreds.  Result?  No perceptible effect whatsoever on the game.

2 hours ago, HebaruSan said:

I know that when I switch from my current relatively empty save back to one with 66 active flights, there's a very noticeable increase in latency that makes piloting quite difficult.

Dunno what's going on there.  I've never experienced anything even vaguely like that.  The only thing that gives me bad framerate is when I have very high-part-count ships (or lots of ships) within the current physics bubble, or if I'm somewhere so graphically intense that the graphics card can't keep up with it.

However, if it bothers you that much, and you really want to keep debris down, there's a game setting for that-- you can set a limit to how many pieces of debris the game will allow (when the number goes over, the game starts auto-deleting them).  By default it's set to something like 200 or 250.  You can lower that down to a couple of dozen, or whatever makes sense to you.

Edited January 17, 2016 by Snark

[HebaruSan]

38 minutes ago, Snark said:

Dunno what's going on there.  I've never experienced anything even vaguely like that.

I think we're getting dangerously close to off-topic for this thread about the efficiency of drop tanks, so I'll note that I've found a number of other forum threads reporting and/or measuring performance degradation proportional to number of active flights and leave it at that. Maybe I have a mod you don't that's adding extra processing per ship (PersistentThrust sounds particularly suspicious, now).

42 minutes ago, Snark said:

However, if it bothers you that much, and you really want to keep debris down, there's a game setting for that-- you can set a limit to how many pieces of debris the game will allow (when the number goes over, the game starts auto-deleting them).  By default it's set to something like 200 or 250.  You can lower that down to a couple of dozen, or whatever makes sense to you.

Adding retrograde Sepratrons is easy enough (not to mention fun to trigger). And my 66-flight save is mostly SSTO spaceplanes orbiting Kerbin, not debris.

[Streetwind]

23 hours ago, GregA said:

Snark,

Having trouble bring up Kerbal to get game name of parts.  They are the the 10k liquid fuel tanks feeding 21 nuclear engines.  The big separators are 0.8 tons each.

...good lord. My Moho rockets are less than half that size, and Moho is the highest dV destination in the stock system. What are you hauling, an entire space station?

One of my rules of the thumb is: downsize, downsize, downsize! A 10 ton rocket that is 50% fuel has the exact same dV as a 1,000 ton rocket that is 50% fuel. Now I don't know your payload, but I wager you could probably drop down to 8 nuclear engines and still fly your mission just fine... and once you realize how much fuel you can remove now that you lost such a giant amount of dry mass, even the TWR reduction won't be nearly as big as you think it is.

[GregA]

On 1/17/2016 at 0:47 PM, Snark said:

What "big separators" are you talking about?  The only "big separator" I can find that weighs 8 tons is the 3.75m stack separator, but why would you use that?  Just attach the drop tanks radially, you can use the little radial separators that are only 0.025 tons each.  Maybe with a .05-ton strut to keep them from wobbling, if it bugs you.

 

The TR38-D  and it weighs 0.8 tons, not 8 tons...   And wobble bugs me more than a little bit on a craft with 400+ parts and a launch weight of 2500 tons...

7 hours ago, Streetwind said:

...good lord. My Moho rockets are less than half that size, and Moho is the highest dV destination in the stock system. What are you hauling, an entire space station?

One of my rules of the thumb is: downsize, downsize, downsize! A 10 ton rocket that is 50% fuel has the exact same dV as a 1,000 ton rocket that is 50% fuel. Now I don't know your payload, but I wager you could probably drop down to 8 nuclear engines and still fly your mission just fine... and once you realize how much fuel you can remove now that you lost such a giant amount of dry mass, even the TWR reduction won't be nearly as big as you think it is.

Jool 5 mission, with 5 kerbals on each moon.  And just to put a point on it, yes, I am hauling the laboratory around with me.

[Snark]

2 hours ago, GregA said:

The TR38-D  and it weighs 0.8 tons, not 8 tons...   And wobble bugs me more than a little bit on a craft with 400+ parts and a launch weight of 2500 tons...

Jool 5 mission, with 5 kerbals on each moon.  And just to put a point on it, yes, I am hauling the laboratory around with me.

Yes, I meant 0.8, that was a typo.  Anyway, it stands:  you don't need that much mass.  A lightweight radial separator, with a strut or two to stop bending, will be far more serviceable, at least for the ship designs I'm used to seeing.

It's entirely possible that you're right and that radial attachment simply makes no sense for your ship, but I'm suffering a crisis of imagination here because I don't think I've ever launched anything that big.  Even a full Jool mission for hitting every moon, back in the old days before ISRU, my launchpad weight was nowhere near 2500 tons.  I'm really having trouble picturing your ship.

Note that I'm not criticizing your design choices, just trying to wrap my head around the scale here.  Especially with a thread topic like "rocket design rules of thumb"-- because normally one of my rules of thumb is "don't launch things that are 2500 tons". 

Normally, a super-massive ship is one of the warning signs that you may be Doing It Wrong™.  Not necessarily, it could be that you have a particular mission profile that really requires it.  But it may be worth a review of the overall design.

Anyway, if you're looking for design advice, then a screenshot (along with desired mission parameters) would really help!

[GregA]

2 hours ago, Snark said:

Yes, I meant 0.8, that was a typo.  Anyway, it stands:  you don't need that much mass.  A lightweight radial separator, with a strut or two to stop bending, will be far more serviceable, at least for the ship designs I'm used to seeing.

It's entirely possible that you're right and that radial attachment simply makes no sense for your ship, but I'm suffering a crisis of imagination here because I don't think I've ever launched anything that big.  Even a full Jool mission for hitting every moon, back in the old days before ISRU, my launchpad weight was nowhere near 2500 tons.  I'm really having trouble picturing your ship.

Note that I'm not criticizing your design choices, just trying to wrap my head around the scale here.  Especially with a thread topic like "rocket design rules of thumb"-- because normally one of my rules of thumb is "don't launch things that are 2500 tons". 

Normally, a super-massive ship is one of the warning signs that you may be Doing It Wrong™.  Not necessarily, it could be that you have a particular mission profile that really requires it.  But it may be worth a review of the overall design.

Anyway, if you're looking for design advice, then a screenshot (along with desired mission parameters) would really help!

Alright.  I have some time.  (no work, no snow) I spent this afternoon working on documenting my mission (im learning how to use vegas video again), give me a couple of hours and I'll show an iteration of my rocket.  Right now, I can, launch, get to Jool, visit three moons and come home.  With objectives like...  get on the leaderboard and make a video with whisky tango foxtrot qualities. (ahh the life of  construction worker in deep winter with no snow)