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This rocket will not fly, why?


Captain Sierra
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I'm sending an automated refueller/wellhead to the Mun as part of prelims for something.

yfknylk.png

The thing is too unstable to do a grav turn. As this is not the most un-aerodynamic thing I've seen shot into space in 1.0, I want some explanation as to why Squad explicitly said before the update, "If it looks like it should fly, it probably will," but this, which looks like it should fly, is very much not.

If a few radial drills not enclosed in a fairing is enough to make my CD shift that high, then you cannot pass 1.0.x off to me as anything more than a broken mess.

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As Robotengineer said, no fins, no fairings. When you start your grav turn all you've got to keep that thing stable is the three mainsails, that coupled with the huge aerodynamic drag from all those exposed parts and you're not going to be able to manage to wrestle that thing into space.

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Another minor thing to consider: rotate the rocket 90 degrees if you're planning to go east (or west, I guess). It's not a good idea to have bilateral symmetry solely along the axis you're turning into, and the problems that causes only amplify as you add more weight to the equation.

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Everyone beat me to it. Fairings and fins. Right now your payload is producing enough drag to pull the centre of drag ahead of the centre of mass, and that means rocket goes flip.

Fairings reduce the drag of the payload (a the cost of weight, which more than evens out) and fins near the bottom bring the centre of drag downwards, stabilizing your flight like a dart.

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It doesn't have fairings. One does not simply send naked rockets anymore. Also, fins.

The thing flew better like this than it did when it had a fairing on it. Fairings are more harm than good when your payload is basically a 2.5m stack with a few radial bits.

It looks very top heavy to me. Add a few fins at the back of the rocket and try again.

Payload itself is <20 tons, which should be a piece of kake lift for this rocket (this is my delta IV heavy lookalike, which is rated for 32 tons in this configuration).

im not seeing any reaction wheels or control surfaces of any kind o.o it may be a torque issue? try adding some fins if you already have reaction wheels o.o

There's a large reaction wheel on the payload, and those three mainsails have pretty good control authority with their gimbal.

As Robotengineer said, no fins, no fairings. When you start your grav turn all you've got to keep that thing stable is the three mainsails, that coupled with the huge aerodynamic drag from all those exposed parts and you're not going to be able to manage to wrestle that thing into space.

Does the ISRU machine have some huge drag coeff or something? ALso the mainsails give pretty good control until I get 20+ degrees off prograde (by which point I'm already losing control to begin with)

Another minor thing to consider: rotate the rocket 90 degrees if you're planning to go east (or west, I guess). It's not a good idea to have bilateral symmetry solely along the axis you're turning into, and the problems that causes only amplify as you add more weight to the equation.

This I will definitely try. I'll see what fins do but I don't expect much.

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And I shall chime in by NOT saying it's all about the fairings!

I see three fairly long booster stages there. As others have mentioned, KSP (sort of unrealistically) drains fuel from the top of a rocket first, and the bottom last. Thus, as each stage is spent, that stage's center of mass is moving way back toward the engines while the center of drag is staying put.

My recommendation is to set up fuel lines so that fuel drains from the bottom of the stacks first, keeping the center of mass high where you want it.

Also, you can put as many reaction wheels way out on the end of the rocket as you want, but it won't actually help. Reaction wheels do not apply linear force like a lever; they apply torque like a power drill. Thus they will be much more effective if they are clustered near the center of mass, which as I mentioned might be surprisingly low on the rocket.

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This I will definitely try. I'll see what fins do but I don't expect much.

Fins will actually be counterproductive on that rocket design! Since you have bilateral symmetry on those boosters, the airflow to the fins will be unbalanced, resulting in your craft veering hard towards a direction you may well not want it to. Just fair warning. If things start going haywire and it starts pushing hard no matter how you try to control it, lose the fins. Another easy thing to try: lower your attachment point for the boosters. It sounds weird, but it helps offset center-of-mass issues as the rocket drains fuel.

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With the fairing on, an extra reaction wheel, and turning my yaw maneuver into a pitch maneuver, it still loses it when it hits the trans-sonic barrier.

And I shall chime in by NOT saying it's all about the fairings!

I see three fairly long booster stages there. As others have mentioned, KSP (sort of unrealistically) drains fuel from the top of a rocket first, and the bottom last. Thus, as each stage is spent, that stage's center of mass is moving way back toward the engines while the center of drag is staying put.

My recommendation is to set up fuel lines so that fuel drains from the bottom of the stacks first, keeping the center of mass high where you want it.

Also, you can put as many reaction wheels way out on the end of the rocket as you want, but it won't actually help. Reaction wheels do not apply linear force like a lever; they apply torque like a power drill. Thus they will be much more effective if they are clustered near the center of mass, which as I mentioned might be surprisingly low on the rocket.

THe second reaction wheel I just added is actually between the sustainer and the upper stage, so its not far off the CoM.

EDIT: If it makes a difference, I'm using clustered 909s instead of a 2.5m engine on the payload. How gracefully does stock handle that?

Edited by Captain Sierra
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Another thing to consider is slowing your ascent to orbit a bit. Once you hit 250m/s, throttle it back a bit and let your climb be more gradual until about 15km up. Then throttle it up to about 1.75 TWR (ie. around 7m/s acceleration). If you do it right, you'll probably be going around 350 to 450m/s at 15km, and won't actually cross the trans-sonic barrier very hard until you reach about 20km. At which point the atmosphere is so thin that it won't cause your ship to flip out. If your ship still flips out, seriously, lower the boosters a bit. Having the engine bells even like that may look nice, but practically speaking it means your center of mass gets all wonky on the way up.

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Are you doing a proper gravity turn? The new aero makes the old "gravity turn" of pre 1.0 obsolete; trying it will cause your rocket to spin around and disintegrate.

Also, the reason why the rocket doesn't fly even though it "looks like it should" is because rockets in real life obviously have complicated computers to keep the nose pointed directly into the prograde vector with a very tiny margin of error. When control systems are compromised, you get this:

(skip to 1:52)

However, this nearly impossible in KSP due to physics inaccuracies, lack of good enough flight systems, etc. The easiest way around this (and what FAR and HSP players have always done) is adding fins to the bottom of the rocket.

If you still resist from blaming your own design and blaming the game instead, try FAR. The newest version uses voxel aerodynamics so everything your craft experiences will be similar to what it would experience in real life and won't experience anomalies from the rushed new aerodynamics (keep in mind that using FAR also causes rockets to realistically spin out if you pitch over too fast and/or don't have fins at the bottom of your craft.)

Sorry if you couldn't understand any of this. It's pretty late at night and I want to go to sleep already :P

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Another thing to consider is slowing your ascent to orbit a bit. Once you hit 250m/s, throttle it back a bit and let your climb be more gradual until about 15km up. Then throttle it up to about 1.75 TWR (ie. around 7m/s acceleration). If you do it right, you'll probably be going around 350 to 450m/s at 15km, and won't actually cross the trans-sonic barrier very hard until you reach about 20km. At which point the atmosphere is so thin that it won't cause your ship to flip out. If your ship still flips out, seriously, lower the boosters a bit. Having the engine bells even like that may look nice, but practically speaking it means your center of mass gets all wonky on the way up.

So, on a scale of 1 to 10, how detrimental is breaking mach 1 at 6km alt? Thats about where I'm hitting it and soon as I do, I lose all control and the thing flips out.

Is stock aero now just this punishing to less-than-perfectly-cylindrical rockets or am I just struggling because I have over 1000 hours of experience telling me what should and should not work?

- - - Updated - - -

Status update: I shortened the boosters & swapped their engines to skippers. TWR is about 1.2 on launch now so I'm breaking Mach much higher up. Haven't gotten there yet. Last launch broke apart due to stability issue created when boosters were no longer strutted to upper stage.

I'm starting the gravity turn at 2km up and this rocket, with SAS off, does a near-perfect automatic gravity turn once I get it tilting.

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...ALso the mainsails give pretty good control until I get 20+ degrees off prograde...

I recommend turning quickly once your speed reaches 70 m/s, tipping it over more than you might think necessary. BUT when the speed gets over 300, put your heading pointer back inside the prograde marker and keep it there until your speed gets up over 400 m/s, then start pitching again.

Also, I find that three fins on each side booster at right angles to each other, work fine.

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Breaking Mach 1 at 6 km without fairings or fins is going to create problems. If that booster is rated for 32 tons of payload and you're only taking up 20 or so, then your initial TWR is probably too high. Thrust limit or throttle down the engines to not break Mach 1 until 12 or 15 km and see how that works.

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I'd add four decently large fins (maybe the delta wing) to the bottom of that thing centered on the main body. Right at the bottom of the tank. Also, try to stay under 450 m/s or so until you're above 20-25k. Nothing I've sent up has managed to stay on course if it goes much faster than that. Fairings are overrated and in many cases detrimental I've found. They tend to create huge noses on the front end of the rocket that leads to massive drag, though FAR may have something to do with that as well. Most rockets I've sent up fly better without fairings.

Finally, maybe it's just a personal thing but I almost never make rockets with only two boosters. They never seem to fly as well as rockets with four. Maybe try swapping out the two large boosters for four smaller ones to achieve the same TWR and delta v?

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Is stock aero now just this punishing to less-than-perfectly-cylindrical rockets or am I just struggling because I have over 1000 hours of experience telling me what should and should not work?

Most of us have been re-learning everything after 1.0! Keep going, because after the first bit it's actually kinda fun.

And I can now get anything to fly with MOAR FINS! Fins to make it stable, then more fins to make it turn because its now TOO stable :P

XjtFghW.jpg

Yes, thats an actual rocket from my career save. I regret nothing.

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Status update 2: She went to space! I nearly incinerated the sustainer with the sepatrons (that doesn't seem intended . . .) but I got it up there. Lessons learned:

  • Rockets REALLY do not like going faster in lower atmo.
  • Throttle pacing will be required on pretty much all my designs (which pride themselves on a 2.0+ TWR with max payload).
  • Gravity turns are a lot more shallow than I'm used to (I my grav turns used to hit 45 degrees by 15km, now I don't hit that angle until 22km).
  • SAS seems to compensate very poorly for aerodynamic forces on rockets as I did a lot of wrestling. Perhaps locking the fins to no control authority, purely as static stabilization, will be of benefit.

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Status update 2: She went to space! I nearly incinerated the sustainer with the sepatrons (that doesn't seem intended . . .) but I got it up there. Lessons learned:

  • Rockets REALLY do not like going faster in lower atmo.
  • Throttle pacing will be required on pretty much all my designs (which pride themselves on a 2.0+ TWR with max payload).
  • Gravity turns are a lot more shallow than I'm used to (I my grav turns used to hit 45 degrees by 15km, now I don't hit that angle until 22km).
  • SAS seems to compensate very poorly for aerodynamic forces on rockets as I did a lot of wrestling. Perhaps locking the fins to no control authority, purely as static stabilization, will be of benefit.

It really is a learning experience with the new aerodynamics model. Even those of us who cut our teeth on FAR still find interesting new challenges with it. Be persistent, try new things, and you'll get the hang of things as time goes on.

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It really is a learning experience with the new aerodynamics model. Even those of us who cut our teeth on FAR still find interesting new challenges with it. Be persistent, try new things, and you'll get the hang of things as time goes on.

Tell me about it. I've colonized planets . . . and now I gotta relearn how to get to space.

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Craft file and i will fix it for you. I can easily see your rocket is top heavy and you have no controll but those mainsails. You didn't tell when you start your gravity turn. I generally start my gravity turn at 20k and keep my velocity fairly low untill that point comes. If you are trying to turn while those 2 boosters are attached its probably causing uneven drag.

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EDIT: If it makes a difference, I'm using clustered 909s instead of a 2.5m engine on the payload. How gracefully does stock handle that?

You'd be better off with a poodle, at least a quarter of a tonne lighter than the 4 LV-909s and with more thrust and better Isp.

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