Science mode launch help

[daniel1234]

Can someone please tell me where my launch config is going wrong? I'm in science mode and only have the first two techs unlocked, so my items are limited. This setup flies fine until I eject the solid fuel stage (runs out around 4000m) - at that stage it starts spinning out of control. I've also tried it with two solid fuel stages and a decoupler, as soon as the first is ejected control is lost. At this stage I'm just aiming to get into space.

Launch config (bottom to top):

Rt-10 solid fuel booster with two basic fins

Decoupler

LV-T30 Liquid fuel engine

FL-T100 fuel tank

Cockpit

Chute

SAS is enabled from launch.

Any help?

[FancyMouse]

This one doesn't look powerful enough to reach space anyway - you probably just want to redesign one.

[g.koster]

you can stack more fuel tanks on top of one other for one engine

[Harry Rhodan]

you can stack more fuel tanks on top of one other for one engine

This. Right now your CoM is riding right on top of your CoT. That is extremely unstable.

€: And the amount of thrust is also too much. Your second stage has a TWR of around 7.5 while you should aim for something like 2 at max.

Edited May 10, 2015 by Harry Rhodan

[Jananton]

Hmm, the reason you spin out of control after dumping the solid fuel booster is that you're probably going too fast in the lower atmosphere.

In the VAB right click on the booster and set the thrust to 50% for a start and try again. At 4000 meters up you shouldn't exceed 150 to 200 m/s otherwise the density of the atmosphere and the resulting drag will pull you in a spin for sure.

Also, although some things have changed, don't you have a LT-V200 fuel tank yet with two techs unlocked, that would be more suitable here and make sure you have a pilot in the seat so you have SAS.

Greets,

Jan

[DeMatt]

As g.koster says, you can stack additional fuel tanks on top of each other, and they'll supply fuel like one big fuel tank. Additionally, two fins is not enough for full directional stability - you need at least three, using symmetry to place them as a ring. The reason you flip out is because it's a positive-feedback situation - once you start turning, without fin drag to offset the capsule drag, the flip force just gets stronger as you turn more.

So I'd redesign it as follows (top to bottom):

• Parachute
  
• Capsule (with monopropellant removed, right-click on it in the VAB and then drag the bar to zero)
  
• Decoupler
  
• 3x FL-T100 fuel tanks
  
• 3x Basic Fins (in a ring on the bottom FL-T100)
  
• LV-T30 (with throttle limited to 50%)
  
• Decoupler
  
• RT-10 SRB (with thrust limited to 70%)
  
• 3x Basic Fins (in a ring on the bottom of the RT-10)

If you wanted to do a strictly SRB flight, I think you'll need three RT-10 stages, and have fins on at least two of them. And of course actual orbit will need a bigger rocket yet.

[daniel1234]

Thanks for all the help! Over the last few hours I've worked out a lot of what you guys have been saying from experimentation - I spun from a mixture of too much Delta-V and not enough fins, plus that thing wasn't powerful enough anyway. I've noticed that SaS has little effect, I think because that setup has rigid fins (they don't move when I change pitch/yaw/roll) and the engine has no vectoring. Am I right in saying this or am I missing something? One more question, can someone explain why this is the case:

Right now your CoM is riding right on top of your CoT. That is extremely unstable.

I'm guessing my CoT should be nearer the bottom of the rocket, and the CoM near the middle? Again, why?

[ghpstage]

Thanks for all the help! Over the last few hours I've worked out a lot of what you guys have been saying from experimentation - I spun from a mixture of too much Delta-V and not enough fins, plus that thing wasn't powerful enough anyway.

Its not too much dV, you can never have too much dV!

The problem is with the acceleration. The thrust to weight ratio is too high, right click on the SRB during design and turn it down..

I've noticed that SaS has little effect, I think because that setup has rigid fins (they don't move when I change pitch/yaw/roll) and the engine has no vectoring.

SAS only stabilises using the equipment it has available, if your craft cannot produce enough torque to overcome the torque generated by aerodynamic forces then it won't do squat. More control authority can help prevent unruly rockets form flipping, especially if SAS counters the rotations early, but ultimately theres only so much that it is practical to add and it wouldn't solve the underlying problem. Edited May 10, 2015 by ghpstage