Frustrating Launch

[Chads]

Hello,

I'm trying to start launching larger payloads, and one of my missions is to build a fairly large station around the Mun.  I thought I'd lift the Cupola, Science Lab, Hitchhiker's Cabin, and the 1.25m RCS fuel in my first launch.  I have a few docs on the science station, some extendable solar panels, a few batteries--typical stuff.

The rocket is fairly tall, but not monstrous.  I think I have it set up as 2 stages plus "boosters" (asparagus style liquid--4 of them, all attached to the main fuel tank).  The main engine is the Mainsail, and the "boosters" are each pulling a Skipper.

Just before the first pair of boosters is ready to be dropped, the rocket snaps back from a 10-25 degree bank to the east, and flips over to the west.  I thought perhaps there was something causing asymmetrical drag, so I flipped it around (so the drag would be on the opposite side from where it started), and I'm getting exactly the same result.

Any thoughts on what might be causing this?  I've tried stripping it down (got rid of docks), lowering/raising the boosters, removed a stage to shorten the rocket, etc.  No idea what's causing this.  

[MinimalMinmus]

The litany against tumbling states "MOAR FINS". Preferrably delta deluxe winglets.

The reason a rockey tumbles is due to it getting aerodynamically unstable. This happens due to speed, or getting too far from your prograde node. But it's nothing fins can't fix.

[Vanamonde]

As your speed increases, drag on the front of the rocket may be increasing but held in check by the control authority of 3 engines. Once you drop two, it may be that you're not longer able to hold attitude with the remaining engine. This happens to me sometimes. What does your rocket look like? Big fairing/cargo up front? 

[The Aziz]

All my launches are supported by aerodynamic overlay turned on (F12 by default, I think?). Then it's simpler to keep the gravity turn as optimal as possible, but keep an eye on light-blue arrows, they shouldn't be too long (but can appear, you're doing a turn anyway), or your rocket will flip; same goes for red arrow, don't let it go too far from the CoT.

[SupperRobin6394]

5 hours ago, MinimalMinmus said:

But it's nothing fins can't fix.

Can it fix a broken heart 

[MinimalMinmus]

2 minutes ago, SupperRobin6394 said:

Can it fix a broken heart 

It cannot, but MOAR BOOSTERS can.

[Aegolius13]

Sounds like you have a goodly amount of drag in your payload.  The Cupoa is pretty blunt, and other little add-ons like radially-attached docking ports will add a little drag of their own.  One little trick I use when launching the Cupola is to put a BACKWARDS 1.25m decoupler and an advanced nose cone in front of it, and release this once I hit orbit.  This does not negate all the drag from the Cupola, but seems to help a bit.

You could also consider putting the payload inside a fairing (as narrow as possible to surround your stuff).  I find myself doing this on most launches despite the weight.  It's a nice insurance policy against both drag and heat, and I think it makes rockets look more attractive / realistic.  

Echo what others said on keeping close to prograde, and putting fins on the back. 

[Zhetaan]

Check your tank drain order.  Usually, all tanks in a given stage drain at the same time; you want them to drain from the bottom to the top.  You can fix this in the VAB; the idea is to keep the mass concentrated at the front and the engines at the back where their few degrees of gimbal have a long lever arm over which to act.  If you start draining fuel mass from the middle, then the drag forces on the front get leverage, too.

I was going to suggest the nose cone trick myself, but @Aegolius13 beat me to it.  If you have a lot of docking ports, try a fairing--the trick to fairings is that they should be as close to the diameter of the booster as possible.  Adding a bit of radius for the docking ports is all right but keep the rocket as streamlined as possible.  Another trick for this is to use 3.75-metre fuel tanks and a 3.75-metre fairing around a 2.5-metre payload stack.  That ought to give you all the room you need, but if you don't have 3.75-metre parts yet, then I suppose that won't happen.

Keeping the rocket streamlined means switching out the 1.25-m monopropellant tank for the 2.5-m tank--even if you have to drain it by ninety percent, avoid abrupt changes to the stack radius because they lead to horrible drag problems.  You could use the stack adapters but you should also avoid gradual changes in stack radius because they still lead to drag problems--they just tend to be necessary and somewhat less horrible drag problems.

Another thing to try is to give up on the asparagus staging for two or more of your boosters.  You trade delta-V and shorter central burn time for longer booster burn time because the Mainsail isn't using your Skippers' fuel.  That gives you more booster-augmented control to get you out of the worst part of the atmosphere.  Maybe that will leave you with a rocket that can still make orbit.  Alternatively, you can deliberately use an inefficient turn:  go up more and turn later.  It wastes fuel, but flipping and crashing wastes more.

As many have suggested, add fins.  Try the winglet control surfaces first and go bigger if you have to.  Some have said to keep to within a few degrees of prograde, and that's good advice except that it won't make an unstable rocket any more stable; it's just a hope that you can keep the forces balanced on a knife's-edge until you get out of the atmosphere.  Since you have to turn, that may not be possible.  One thing for this is to go faster:  as you do, the drag forces on the tail of your rocket almost slot it into a path through the atmosphere unless your front drag forces are truly obscene.

Or you could break this up into two launches and connect your station with the big Clampotron Sr. docking ports.

Edited November 16, 2017 by Zhetaan

[Chads]

Thanks for the info.  A lot of things to take away from all the suggestions!

I do have 4 Deluxe Winglets on the tail of the central rocket.  To the best of my recollection, it looks something like this:

• Hex probe (I was trying to launch it unmanned, and I'd bring crew up later)
• Cupola (I'd swapped this out with an adapter to make it less blunt, to no avail)
• Science Lab -  4 Clampotron Sr. ports radially attached
• Hitchhiker's Room
• Clampotron Sr.
• Fairing (to close the space around the Clampotron)
• 8 support bars for stability
• Decoupler
• Medium Fuel Tank (the Mainsail size ones from here on down)
• Poodle Engine
• Decoupler
• Big Orange fuel tank
• Mainsail

The boosters are the cone fuel tank, a medium sized one, and an engine on the bottom.  4 of them total, radially in pairs.  First pair drops when fuel is spent, then second pair.  However, I never get to the point where the fuel is spent to drop the first pair.

This is happening just at the end of the first portion of the atmosphere, just as I'm starting to see the visual effects of resistance.  It makes sense this is entirely a drag issue.

I was NOT draining tanks from bottom to top--I thought you go the other direction.  This would only really affect the boosters.  I'll switch that around.

I'll turn on the aero readings, and pay attention to that.  I didn't even think of that as something to look at.

I'm thinking, maybe I need to go with a slower launch until I get to the mid-atmosphere.  I am considerably off from Prograde on this launch, so that's certainly something else to watch.

 

Thanks very much for the info!

[Spricigo]

 

7 hours ago, Chads said:

Just before the first pair of boosters is ready to be dropped, the rocket snaps back from a 10-25 degree bank to the east, and flips over to the west.

That is the typical description of the moment the Center of Drag move ahead of the Center of Mass.

Unless you have a perfect symmetrical rocket perfectly aligned with the airflow (which happens to be along surface prograde/retrograde axis), drag forces will cause a torque around the CoM and that torque will increase as you get away from that attitude*. While you CoD is behind the CoM that torque is pushing the rocket back to the direction of airflow, when CoD is ahead of CoM that torque Push the rocket away.

So you need to reduce drag  at the tip of your rocket (or/and increase at the tail) and reduce mass at the tail of your rocket (or/and increase at the tip). Several ideas of how exactly solve your issue was already mentioned (fairings around the payload, fins on bottom, tanks draining from bottom to up), I will just add that you can take most of the fuel from the central stack and put it in the side boosters and adjust the booster position to have a better mass/drag distribution.

16 minutes ago, Chads said:

To the best of my recollection, it looks something like this:

That is a good phrase to go before a screenshot.An image that probably  would give us much better info than the many words that followed it.

*technically that is not always the case, but lets just ignore the edge case by now.

 

BTW: Question about how to play the game, should go in Gameplay Questions subforum.

[Chads]

Thanks, I'd been reducing weight at the front...doh!

 

 

Edited November 16, 2017 by Chads

[Atkara]

My typical launch profile is to start my gravity turn at ~80-90 m/s and make sure that by the time I reach 15000m altitude, the vehicle will have an attitude of 45 degrees. Once there, I let it get as low as 35 degrees and eversince I started meddling with spaceplanes, once the navball switches to orbital mode, I lock to prograde follow, till apoapsis hits the expected orbital insertion altitude and the vehicle leaves the atmosphere. I also play with the throttle to keep TWR around 1.5 -this can be monitored on the navball's right side readout.

With this, I've gotten all sorts of payloads into orbit, in fairings of pretty much any size. I also use fins to be on the safe side, although in 90% of occasions, I haven't seen them really kick in.

Edited November 16, 2017 by Atkara

[Nicias]

5 hours ago, Aegolius13 said:

Sounds like you have a goodly amount of drag in your payload.  The Cupoa is pretty blunt, and other little add-ons like radially-attached docking ports will add a little drag of their own.  One little trick I use when launching the Cupola is to put a BACKWARDS 1.25m decoupler and an advanced nose cone in front of it, and release this once I hit orbit.  This does not negate all the drag from the Cupola, but seems to help a bit.

You could also consider putting the payload inside a fairing (as narrow as possible to surround your stuff).  I find myself doing this on most launches despite the weight.  It's a nice insurance policy against both drag and heat, and I think it makes rockets look more attractive / realistic.  

Echo what others said on keeping close to prograde, and putting fins on the back. 

I like to do both of these. Going up from the cupola, 1.25m decoupler backwards, then 2.5m fairing (upside down) then big nosecone. This big fairing goes down to cover all of the draggy bits.

[Slam_Jones]

24 minutes ago, Nicias said:

I like to do both of these. Going up from the cupola, 1.25m decoupler backwards, then 2.5m fairing (upside down) then big nosecone. This big fairing goes down to cover all of the draggy bits.

I'm curious as to why you put the fairing on upside-down?

[Pthigrivi]

Do you know what your TWR is off the pad? It may be that the Mainsail is more engine than you need and you're just going so fast that drag is getting away from you.

[DAL59]

Press f1 to take a screenshot.

[A_name]

You can also use a small fuel tank at the top of the stage and lock it to act as ballast, it’ll keep your COM forward.

[Aegolius13]

8 hours ago, Chads said:

Thanks, I'd been reducing weight at the front...doh!

Not an uncommon issue... all the way since Robert Goddard himself. 

https://en.wikipedia.org/wiki/Pendulum_rocket_fallacy

[Spricigo]

4 hours ago, Slam_Jones said:

I'm curious as to why you put the fairing on upside-down?

That way he can discard the fairing base after use. Not like 175kg will make that much of a difference but why not do some mass reduction? 

[Vanamonde]

This question about playing the game has been moved to Gameplay Questions. 

[Chads]

I was able to get it into space finally by using fairings over the draggy parts (which pretty much included the entire top stage, as well as the bulk of the science lab), and by monitoring the TWR, maintaining it around 1.5 until I got pretty close to the edge of the atmosphere.

Thanks to all for the assist!