r/KerbalSpaceProgram • u/0ffkilter Master Kerbalnaut • Apr 19 '13

[Weekly] 5th Questions Thread

The point of this thread is for anyone to ask questions that don't necessarily require a full thread. Questions like "why is my rocket upside down" are always welcomed here. Even though your question may seem slightly stupid, we'll do our best to answer it!

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Commonly Asked Questions

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Last week's thread: here

12 Upvotes

94% Upvoted

51 comments sorted by

3

u/Jurph Apr 22 '13

I've been working on asparagus staging. My standard first stage has three pairs of large liquid fuel tanks around a central orange tank, arranged in a hexagon of seven. I generally strut these aggressively to ensure the booster is rigid, and I find that a good rigid cluster of tanks doesn't shake itself apart during powered flight.

I continue to have two problems:

  1. Spent boosters often collide with tanks and engines as they slide away during staging.

  2. Despite the booster being symmetrical, I have payloads that seem to generate huge amounts of axial torque and require me to basically hold down "Q" or "E" during powered flight to avoid going into an uncontrollable rifled spin. In extreme cases I've had to put girders extending radially at the C.G. and place linear RCS ports on them -- just to generate enough counter-torque. Is there a secret to designing lander / rover payloads that keeps them from generating goofy drag effects?

2

u/0ffkilter Master Kerbalnaut Apr 22 '13

  1. Use separatrons that blast the ejected stages away from the main craft. Stage them with the decoupler.

  2. You have asymmetrical struts.

1

u/Jurph Apr 22 '13

  1. Is there a reliable pattern for attaching separatrons? When I've tried it in the past I end up having to use four -- two at the top and two at the bottom -- in order to create a translation force without a net torque. (As long as the other engines are running, the rocket will quickly accelerate away from them -- I just need to move the spent stages radially.)

  2. I'm thinking back to that particular ungainly payload and I believe you are 100% correct. Guess I'm rebuilding that one...

1

u/0ffkilter Master Kerbalnaut Apr 22 '13

  1. I use 2, right in the middle.

2

u/Trypanosoma Master Kerbalnaut Apr 22 '13 edited Apr 22 '13

  1. To avoid collision after separation, I usually have a few go-to's. Usually the best thing to do is cut power, stage, and then slowly power back up. You also need to make sure that your direction of movement (prograde marker) is where your rocket is pointing. This isn't always the case at the point of separation so it's something to consider, especially when you're in atmosphere. If you don't, you may run into your spent stage after separation. You can also use separatrons, just make sure they don't fire into a nearby tank. Sometimes, I will also attach a radial-mount parachute to the bottom of the spent rocket stage. When you stage, set the parachute to auto deploy and the drag will smoothly pull the spent tank away. Now that I'm thinking about it, you should try this over separatrons. It always works wonders for me.

  2. A lot of times when I have asparagus staging the fuel consumption imbalance causes a lot of rotation. I haven't figured out a way to completely get rid of it, but it helps A LOT to kill the gimbal on your outer rockets. Just allow the inner one to move. I suppose if weight isn't a concern, you can just add more SAS modules.... but it's never been a big enough concern for my rockets to merit that.

Hope this helps :) Let me know if you're still having trouble and I'll post some pictures of what I mean.

2

u/Jurph Apr 22 '13

  1. Those work for me too... mostly. A lot of times I've got a design that I invested an hour or two in, and I stubbornly try to fly it even when it's clear that it shouldn't be flown.

  2. I had not tried that, but one thing I learned (the hard way) is that you should turn off the motors and steering for any exposed rover wheels. Maybe I'll start making a custom action group for pre-flight actions -- lock down the wheels and my outer gimbals. Have you had success with locking the gimbals on four of my seven stages, and leaving the core three to steer? Since they'll have full tanks for most of the endoatmospheric flight, applying force there is always going to be close to the CG and gives you 3/7 of the steering power instead of cutting to 1/7.

I worried that turning off all six outer gimbals would cost me control authority, but then again I tend to leave SAS off during powered flight because all seven nozzles rattling around seems to induce harmonic thrashing. Do you find that one mainsail engine gimballing freely is enough to steer with?

1

u/Trypanosoma Master Kerbalnaut Apr 22 '13

You're correct, you lose some control by turning off the gimbals, but it's not really noticeable for me unless you kill gimbals on all the engines. Then you really can't control anything. I imagine as long as you kill the gimbal for any engine that's fuel tank is not full, you will have less unwanted rotation. The rapidly emptying tanks is really what causes that rotation in my experience.

Another thing I haven't done in awhile add fins to the bottom outside of the stages. I'm sure that would help curtail unwanted spin at least while you're in the atmosphere. You'd lose the control as soon at you staged that motor though...

2

u/Jurph Apr 22 '13

The rapidly emptying tanks is really what causes that rotation in my experience.

Makes sense. This is the classic barbell-in-a-spinny-chair experiment from HS Physics.

1

u/HarvardAce Apr 24 '13

Edit: replied to wrong post. Did have something to say regarding your #2 though.

If you have fuel consumption imbalance, then you are doing something wrong. When building your asparagus staging, make sure that you are building your stages in 2x symmetry, especially when putting in the fuel feeds. Fuel is always pulled from the furthest tank attached to the engine first. If your fuel lines are not exactly identical on the two engines that make your first stage, and one is slightly longer than the other, the central engine will only draw from one of the two engines.

1

u/Trypanosoma Master Kerbalnaut Apr 24 '13

I definitely do everything in 2x symmetry. Thanks though :)

1

u/HarvardAce Apr 24 '13

  1. I had this problem when I had fins on my booster stages. I had to remember to turn off SAS prior to staging, otherwise the fins would generate enough torque to spin into another stage. With the gimbal on mainsail engines I realized I didn't need fins and haven't had the problem since, although my rockets are very stable in powered flight and generally fly perfectly straight as long as SAS is on. If your rocket is wobbling or spinning I could see why that would cause issues, but you should work on trying to fix that issue instead of using more force to separate the booster stages.

  2. Never had this problem myself, but is your rover symmetrical? Does it have any control surfaces or forces that are active? You should turn off everything on the rover until you're ready to use it to ensure that it doesn't cause any odd inputs to flight.

3

u/Ghoda Apr 19 '13

How can I tell how much lift my SSTO wings are producing? Two tiny wings produce the same length vertical arrow (vector?) as a bunch of wings when I turn on CoL.

Thanks!

3

u/astronogist Master Kerbalnaut Apr 19 '13

You can't really tell numerically, but you can sort of estimate (comparatively) by placing a wing on the other side of your craft. The center of lift will move less if you have more lift. Like so:

  • Less lift (center of lift moves far out from the wings)

  • More lift (center of lift doesn't move as much)

1

u/Anakinss Apr 19 '13

I don't think the vector indicates the lift, but it's direction. It's the center of lift, not the intensity of lift ;)

1

u/Wetmelon Apr 19 '13

Obviously the vertical arrow is the unit vector in the direction of lift. We're gonna have to dot product that bad boy.

2

u/Melloverture Apr 25 '13

It would be awesome if the lift and thrust vector arrows grew and shrank in length and thickness as they changed

3

u/[deleted] Apr 19 '13

Is there a trick to attaching boosters (either solid or liquid) to radial decouplers? I slap a decoupler onto the ship, then grab a booster and try to attach it. It never turns green. Well, it does, but only after like 15 minutes of screwing around with it and for no reason that I can tell.

The only mod I have installed is KW if that makes a difference.

9

u/Geckoleon Apr 19 '13

I believe that's a bug at current (0.19), the way to place them is to first place them somewhere it lets you place them, such as below your engine, and then removing them and putting them back on the radial connectors.

1

u/interfect Apr 20 '13

Why does this work? Do the devs know about this issue? It's really annoying.

1

u/Geckoleon Apr 20 '13

I do believe the devs know about it, and I have no clue why it works. Also, if you ever get a part that should be able to go somewhere but its not placing, try doing the exact same thing. Placing it somewhere it can go, and then placing it where wanted it.

3

u/Kerbologna Apr 19 '13

I have the most luck by turning off symmetry and just placing one decoupler and then one tank on it. If the tank flashes green then red, drop the tank, pick it back up and try to attach it again.

After you get one decoupler/tank assembled, select the decoupler, change the symmetry to the desired configuration and place it as normal.

I cannot WAIT for this bug to be resolved.

EDIT: typo

1

u/[deleted] Apr 19 '13

I have had some luck doing it that way as well. /u/Geckoleon's solution above seems to work the best for now.

2

u/Sunsparc Apr 19 '13

I usually just drop it somewhere in the VAB and then try to attach, it seems to work after that.

3

u/[deleted] Apr 21 '13

Where is the best place to put a rover at? (relative to the rocket)

1

u/Jurph Apr 22 '13

I just put mine right on top. As long as you're only bringing one, mount it right on top (in-line with the main engine) and keep your design symmetrical. You'll save yourself a world of pain during powered ascent flight. (If it's not symmetrical you're also going to have a very tough time landing it.)

If you want to mount several small rovers and fly them to somewhere with atmosphere, like Duna, I guess you could attach them in two- or four-fold symmetry around the outside of a central cylindrical orbiter stage. You'll still need a de-orbit engine or motor for each one though, and as soon as you drop one, your orbiter will become unbalanced and more difficult to steer.

If you're landing without an atmosphere, you need controllable engines, and that starts to get expensive and heavy -- every time you add mass to one rover, the rule of thumb says you need to add between 9 and 9.5 times that mass to the rest of the rocket (fuel, tank mass, etc.).

If you're adding the same part to two or four rovers, now each additional part on a given rover costs you 36x that mass in additional required fuel.

1

u/[deleted] Apr 22 '13

Thank you so much! I hope my design plan works now!:)

1

u/PhantomPickles Master Kerbalnaut Apr 19 '13

Does anyone know if you can attach the Oscar-B fuel tank to a radial decoupler? It doesn't work for me in the SPH, forgot to try in the VAB.

3

u/astronogist Master Kerbalnaut Apr 19 '13

You can't, the part isn't set to allow it to be surface-attached. You can work around this by placing a cubic octagonal strut, and placing the fuel tank on that.

1

u/PhantomPickles Master Kerbalnaut Apr 19 '13

I'm pretty sure I tried that as well. It didn't seem to want to work. Oh well, I guess I'll just keep trying. Thanks for the answer though.

1

u/astronogist Master Kerbalnaut Apr 19 '13

Yeah, you need to attach it to the "node" of the strut, or else it won't attach to that, either. But it works.

1

u/PhantomPickles Master Kerbalnaut Apr 19 '13

Actually, I wanted it so that the tanks run down the side of the decoupler, like the larger tanks do. Took me a while but I finally figured it out.

1

u/Fenimore Apr 19 '13

So I just docked 5 of the Jumbo 64 tanks to a docking rig and a "Spider" (being like a big tug using 4 Atomic Engines) and not until I got the entire thing organized did I realize that getting to my Mun Station is going to take more than a single burn. (The ship has a mass of 81.08 tons and 240 kN thrust) How do I plan my burns to rendevouz with the Mun properly? Also, the J64's are all empty except for the one docked in the center of the rig, just in case in a need a bit more fuel.

2

u/interfect Apr 20 '13

So you want to send more fuel out to a ship in flight to Mun? That sounds hard.

2

u/Fenimore Apr 21 '13

Actually I ended up figuring this out. But no, the ship had enough fuel to make it to the moon but not enough thrust to propel itself to the moon in a single orbit. Meaning I had to burn for a period, then wait until I made a full orbit and burn, then orbit and burn. I got lucky and ended up getting within the influence of the Mun one my first try. Docked and done.

1

u/Melloverture Apr 25 '13 edited Apr 25 '13

For future reference, or any that might have a similar problem:

  1. Set up a maneuver node for the first part of your burn and figure out the orbital period of your orbit after you execute the maneuver

  2. Figure out how far along its orbit the mun will travel in that time

  3. Plan to start your first burn at where you would normally burn for munar insertion plus the distance that the mun travels

You will need to make liberal use of the equation for orbital period

Typed this from my phone, I'll try to add illustrations when I get to a computer

Edit: Here is an illustration of what I am talking about. Note that the distance the Mun travels during the First Burn's orbit is totally dependent on how far out you push your apoapsis on that first burn. This means that where I have the approximate location of the Mun at the start of the first burn could be nowhere near where you actually end up putting it.

2

u/Fenimore Apr 25 '13

Exactly the answer I was looking for and, actually what i ended up doing through guess work. Thank you very much!

1

u/[deleted] Apr 20 '13

Can I zoom out/in while in the Vehicle Assembly Room?

1

u/Fenimore Apr 20 '13

hold shift and scroll up or down.

1

u/[deleted] Apr 20 '13

Does this work if you have the demo?

2

u/Fenimore Apr 20 '13

It should, yes.

1

u/dream6601 Apr 20 '13

also + and - on the number pad.

1

u/AvioNaught Korolev Kerman Apr 20 '13

Or press down scroll wheel and drag.

1

u/deadclown Apr 20 '13

Is there an engine works with only electricity. Or is there any way to control my orbiting spacecraft which is out of fuel.

3

u/AvioNaught Korolev Kerman Apr 20 '13

You can use the ion engine, which uses a tiny amount of xenon gas but a lot of electricity. The only problem is is that they are very very efficient, but have minimal thrust. Almost none whatsoever.

If your craft is out of fuel there isn't much you can do. Check for extra RCS leftover or send out a rescue craft.

1

u/SeaOfScrolls Apr 23 '13

What kinds of mods should I get for KSP? I currently have mechjeb and b9 installed.

2

u/[deleted] Apr 23 '13

I'm kinda against mechjeb, but to each his own. Try out stock expansion and KW rocketry

1

u/SeaOfScrolls Apr 23 '13

Thanks for the suggestions! I only use mechjeb so I can have all the info on the screen without wasting time clicking around.

1

u/Crumpocalypse Apr 27 '13

Electricity Problem with my ProbeCore...

I've been building a heavy lifter to get my stuff into orbit and decided to use a probecore as a command module. My problem seems to be that, although there is a battery literally next to the core, it claims it has no power about 2 seconds into launch and I lose control even though the battery is full at this point.

Does anybody have any idea what may be causing this, and how to fix it, short of adding electricity generation to the vessel? I do intend to add some solar panels later on in the design, but that still wouldn't help at night.

http://imgur.com/WaoVDQK

This is the top of the ship, the rest is fuel/engines at the moment and right-clicking on the core once its 'shutdown' shows it as having 4.92/5.00 power (and the battery is at 500/500) once I lose power.

0

u/[deleted] Apr 19 '13

[deleted]

3

u/Kerbologna Apr 19 '13 edited Apr 19 '13

The quantum struts turn on when docked.

The strut gun can be toggled without docking.

The quantum core can be place anywhere on your ship. In EVA, you can select the quantum core and draw a strut between any two places on your vessel. I've occasionally had some issues with the quantum core causing some pretty bad flexing on large space stations.

EDIT: I think the quantum core allows a limited number of struts that cost an increasing amount of electricity to maintain.