asparagus staging redux

[Starchaser]

Ok, I've asked this question before, and I was told it was because I had an unbalanced rocket. I didn't really believe that side torque of less than 0.1 kNm would be an issue, and I managed to launch that payload, but in a very brute force manner.

But, now, part 2 of the issue. Here's a rocket with no side loading. 

https://imgur.com/a/dbpvM

It absolutely won't launch. 

https://imgur.com/a/Dzm0C 

Here is the whole rocket. Now, I was able to change the delivery to a 2 stage rockomax mainsail/skipper and it went into orbit fine. But that's not the point. From everything I understand there is nothing wrong with the first rocket, and making the change cost me an extra 13,000 ish. I play in career mode and funds are important as well. 

I am really getting angry and frustrated over this.

Edit:

If anyone wants to try to fly it, here it is.

https://drive.google.com/file/d/0B99G8SQzn7guNk5UZFlEMkhEdUE/view?usp=sharing

I'm using all the near future mods, but I think all of that is stock, except for the mechjeb and KER required parts

Edited October 29, 2017 by Starchaser
added the ship

[renhanxue]

Missing part "orbital-engine-0625", so I can't load it.

That being said, what does "absolutely won't launch" mean? Does it perhaps flip over when you reach a speed somewhere in the neighborhood of 300 m/s? In that case, that's not because it's unbalanced, that's because it's aerodynamically unstable. The aerodynamic center is in front of (or perhaps more accurately in the case of a rocket, above) of the center of mass. Try sticking four fins at the bottom of the rocket, but I suspect in this case it may not be enough because of all the incredibly draggy stuff you've got hanging off it. The lander should be covered with a fairing, and you don't need pretty much any of those struts at all. If you need to stabilize the asparagus boosters just use autostruts. External fuel lines are downright exotic in the post-1.2 era as well (they're essentially never needed, just enable fuel crossfeed on the decouplers and the external tanks will be drained first automatically - only disadvantage is that KER still doesn't calculate dV correctly).

[Starchaser]

Thanks for the response.  I will attempt to make the changes you suggest. The image was a reload of one of the intermediate attempts to launch this, as I said, I got it into orbit with a single stack eventually. One of the 'bring it into the assembly building to try something else' was to add fins. You're right, it didn't help. Are you sure about the autostruts? in another question I asked, regarding stacking multiple probes, the answers there said to avoid autostruts totally.

[Spricigo]

There are a few instances where you should avoid autostrut.

1.Inside fairings.

2.Too long connections.

3.Heaviest or Root may change (for the respective AS option)

AS:Grandparent from boosters to core attachment tend to be safe. At least I'm yet to see AS:Gp being an issue.

Edited October 29, 2017 by Spricigo

[linuxgurugamer]

Cover all that stuff with a fairing

[Snark]

On 10/29/2017 at 5:18 AM, Starchaser said:

But, now, part 2 of the issue. Here's a rocket with no side loading. 

https://imgur.com/a/dbpvM

It absolutely won't launch.

You don't explain what you mean by "won't launch".  Looks to me like you've got enough thrust to get off the pad, so that's not it.

My guess is that it's horribly aerodynamically unstable and flips in flight, amirite?  Here are the things that look "off" to me about the rocket:

1. You've given it very little ability to steer.  There are no aerodynamic control surfaces, and over 80% of your takeoff thrust is coming from Reliants, which don't gimbal and can't help with steering.  The only steering you have comes from the lone Swivel.  Given the other problems the rocket has, I'd be doubtful that the lone Swivel would provide enough control authority.
2. You have no fins on the back.  Nothing to help stabilize its direction.
3. You've got a horrible draggy contraption on the front end.  That big ol' fairing with the 1.25m base is sitting on top of the Mk1 pod with its 0.625m top.  You need to smooth out your front end so that it's an uninterrupted 1.25m stack, that comes to a point on the front.
4. I can't tell where the CoM is for your ship, but I'd guess it's pretty low.  This will contribute to the instability.

Things you can do to make it better:

• Add fins.  This is a no-brainer, it's really easy to do and will help you a lot.  I'd suggest the AV-R8 winglet, since they're steerable and will give lots of control authority.  Mount them as low down on the stack as you can get.  Put an AV-R8 way down low on the bottom of each radial booster, then put a quartet of Basic Fins around the bottom of the central stack.
• Lower the radial boosters.  Mount them as low down on the central core as you can.  This will allow you to put those fins even lower (relative to the CoM), giving them more control authority.
• Make sure you've chosen staging order correctly.  As you launch and turn eastwards, make sure that you're jettisoning the boosters on the top and bottom (i.e. east and west) sides of the rocket first, and the boosters on the left and right sides (i.e. north and sount) second.  Why?  Because pitch instability is generally more of a danger than yaw instability, and you'll want to hang on to those fins longer.
• Set fuel flow priority.  For stability, you want your CoM to be as far forward as possible.  You can help with that by setting fuel flow priority so that the bottom-most tanks drain first.  Since you're using stacked tanks, doing this will cause your CoM to rise rapidly as you drain fuel, which will help your stability.  It's important to do this because if you don't, then by default each stage will drain all its tanks simultaneously, which doesn't help your stability much.
• Fix that draggy monstrosity at the front end.  How best to do that depends on what you've got hidden inside that fairing.

[Starchaser]

9 hours ago, Snark said:

My guess is that it's horribly aerodynamically unstable and flips in flight, amirite?  Here are the things that look "off" to me about the rocket:

1. You've given it very little ability to steer.  There are no aerodynamic control surfaces, and over 80% of your takeoff thrust is coming from Reliants, which don't gimbal and can't help with steering.  The only steering you have comes from the lone Swivel.  Given the other problems the rocket has, I'd be doubtful that the lone Swivel would provide enough control authority.
2. You have no fins on the back.  Nothing to help stabilize its direction.
3. You've got a horrible draggy contraption on the front end.  That big ol' fairing with the 1.25m base is sitting on top of the Mk1 pod with its 0.625m top.  You need to smooth out your front end so that it's an uninterrupted 1.25m stack, that comes to a point on the front.
4. I can't tell where the CoM is for your ship, but I'd guess it's pretty low.  This will contribute to the instability.

Things you can do to make it better:

• Add fins.  This is a no-brainer, it's really easy to do and will help you a lot.  I'd suggest the AV-R8 winglet, since they're steerable and will give lots of control authority.  Mount them as low down on the stack as you can get.  Put an AV-R8 way down low on the bottom of each radial booster, then put a quartet of Basic Fins around the bottom of the central stack.
• Lower the radial boosters.  Mount them as low down on the central core as you can.  This will allow you to put those fins even lower (relative to the CoM), giving them more control authority.
• Make sure you've chosen staging order correctly.  As you launch and turn eastwards, make sure that you're jettisoning the boosters on the top and bottom (i.e. east and west) sides of the rocket first, and the boosters on the left and right sides (i.e. north and sount) second.  Why?  Because pitch instability is generally more of a danger than yaw instability, and you'll want to hang on to those fins longer.
• Set fuel flow priority.  For stability, you want your CoM to be as far forward as possible.  You can help with that by setting fuel flow priority so that the bottom-most tanks drain first.  Since you're using stacked tanks, doing this will cause your CoM to rise rapidly as you drain fuel, which will help your stability.  It's important to do this because if you don't, then by default each stage will drain all its tanks simultaneously, which doesn't help your stability much.
• Fix that draggy monstrosity at the front end.  How best to do that depends on what you've got hidden inside that fairing.

In short order, point 1. I gave it more steering than that, includiing, on a launch that came after the attempt pictured, 4 fins. There are also 2 reaction wheels, a 1.25m wheel right under the command pod, and the probe hidden in the fairing has a small reaction wheel. 

Point 2, I did attempt it with fins, and I always use the AV-R8's when I use fins. It still flipped about. However you suggest putting them on the boosters, and that I didn't do.

Point 3.The fairing is that size because of the orbital survey scanner on the top probe. If I try to keep the fairing to 1.25m it clips through or won't let me place it. I wound up launching it with 2.5m parts, although all I did was remove everything under the first tank under the lander, put a rockomax adapter on it, then launched 2 stage rockomax tanks/engines and fins.  

Point A, I did this. but only on the center stack. Next time I try asparagus, I'll try on the boosters too.

Point B. This is confusing to me, as other people have asked me why my boosters are so low, when I've had them low.

Point C. This is a must do. I wasn't aware it was possible. I mean, I saw the fuel priority buttons, but had no idea what they were for or how they worked,

Point D. The only way I fixed that permanently was when I learned about interstage fairings. I've launched a few multi-probe rockets since then, and to make them smooth I've had to use rockomax tanks. I was trying to avoid it for cost purposes, but less launch stress is helpful too.

Thanks for your thoughts and insights. I will definately incorporate the new info in my future launches

It actually leads me to a different question. Someone above suggested enabling crossfeed on the decouplers and getting rid of the fuel pipes. It seems I could use fuel priority to make the tanks drain in pairs....but without a connection to the boosters aside the one you want to drain, will that work?

[Snark]

13 minutes ago, Starchaser said:

I did attempt it with fins, and I always use the AV-R8's when I use fins. It still flipped about

Fins won't help if they're too close to the CoM.  It would help to have a screenshot of the ship with CoM displayed.

14 minutes ago, Starchaser said:

This is confusing to me, as other people have asked me why my boosters are so low, when I've had them low.

Well, here's the problem:  I strongly suspect that your CoM is way too low on your rocket.  This means that, 1. it'll flip easily, and 2. adding fins on the back won't help a lot because they're not far enough behind the CoM to have a decent lever arm to work with.

The reason for mounting the boosters really really low is not because "low boosters = more stable" ... but because it gives you a really low attachment point for the fins.  Which gives the fins more of a lever arm to work with, so they're more effective.

Heck, if you wanted to, you could even stick a Structural Fuselage at the bottom of each radial booster (i.e. beneath the lowest fuel tank, on top of the engine).  It's super lightweight, and will give you a much lower attachment point for your fins while (relatively speaking) moving your CoM upwards.  If you did this, I bet you could just leave your boosters where they are now-- just put the fins way lower down.  It'll make them much more effective.

Another reason to mount your boosters low down is because they drain before your center stack does, so putting them low down means that your CoM will rise fairly rapidly as they burn, which will help stability.

17 minutes ago, Starchaser said:

There are also 2 reaction wheels, a 1.25m wheel right under the command pod, and the probe hidden in the fairing has a small reaction wheel. 

Pretty much irrelevant.  During launch, aerodynamic forces are so much huger than everything else that reaction wheels might as well not exist.  There's no way you're going to be able to stabilize yourself with reaction wheels; it needs to be aerodynamically stable.

20 minutes ago, Starchaser said:

Someone above suggested enabling crossfeed on the decouplers and getting rid of the fuel pipes. It seems I could use fuel priority to make the tanks drain in pairs....but without a connection to the boosters aside the one you want to drain, will that work?

Absolutely it works, I use it all the time. 

1. Attach your boosters using 4x radial symmetry, same as you're doing now.
2. Turn on crossfeed for the radial decouplers.
3. Use the staging UI to split the decouplers into two stages of two each, instead of all four of them being lumped together in one stage, the way they are when you first place them.

There, that's it.  You're done.  It works.  The pair that stages away first will drain first.  When they go empty, you stage them away, and the next pair will drain.  When they go empty, stage them away and your center stage will drain.

There's just one catch to be aware of:  you need to watch when the fuel tanks run dry and stage then.  And by "watch when the fuel tanks run dry", I mean actually watch the tanks themselves via their right-click menu.  Why do I make such a point of this?  Because it's different from when you use fuel ducts.  Fuel-ducts are one-way, whereas a decoupler with enabled crossfeed allows fuel to flow in both directions.  This means, for example, that the engines whose tanks run dry first don't show up as "empty" in the staging display, because those engines can "see" (and still have access to) all the fuel in the other stages.

For example, if you're using fuel ducts, then when a pair of boosters runs dry... it dies.  You can see it die, because it stops spewing flame.  Also you can see its fuel bar in the staging display approach zero as it gets close to running dry.

With crossfeed-enabled decouplers instead,  it doesn't work that way.  All your engines will show the same percentage on their fuel bars in the staging display.  And when a pair of boosters runs dry... it will keep firing because now it's draining fuel from the next stage.

So, you have to watch the tank content so you'll know when it's time to stage.  No big deal, just something to be aware of.

In summary:  An asparagus ship with fuel ducts is more of a pain to put together in the VAB.  An asparagus ship with crossfeed-enabled decouplers is (slightly) more of a pain to fly... but only slightly.  And of course it doesn't have to pay the weight/drag of adding fuel ducts.  Overall, these days I prefer to do the crossfeed thing, myself, where possible.

[Spricigo]

@Starchaser, may you please post a image of your tech tree, so we know what parts are available to you. A little description of the intended purpose of the craft and a description of what is inside the fairing may be usefull but just for the lifter design not really a necessity.

I still have a day of work between me and KSP but I think I can come up with something that works and even cut the cost a little bit.

[Spricigo]

well...looking for what you have in this craft,  I think You can use this one.  (I wondering why you weren't using FL-T800 tanks) Anyways, tested it with a dummy payload  with a similar shape of your 2 topmost stages, and 6,7t. As you can see it reachead a nice orbit with 200m/s in the core lifter and 1300m/s in the transfer stage.

I hope you like the launch profile: T0: press spacebar, T43s:press pacebar, T1min33s:press spacebar.

Yes, that is it. You don't need to steer or use SAS (in fact you should let it disable until its time to circularize), fire all engines and the rocket will fly itself to space, you just need to drop the spent boosters.

Album https://imgur.com/a/j4A0g will appear when post is submitted

 

 

Edited November 1, 2017 by Spricigo

[Starchaser]

I had to step away from KSP for a few days. I'll drop a screenshot of my tech tree when I get home. I'm at work right now. I didn't see I had notices until I relaunched Chrome to post my docking image