Newbie help with rocket

[jonpfl]

All,

I am trying to build a rocket that will get me to the Mun, let me land, do some science and get back.

I am curious why this rocket flips around 20k when I try to turn it to 45 degrees?  Why do some rockets flip and some don't?  I am trying to keep the weight lower in the ship which I assume would keep it from flipping.

What else?

Thx
jonpfl

[HebaruSan]

Don't turn 45 degrees abruptly; that's old, out of date advice. Do it gradually, starting around 1km and reaching 45 degrees by about 10km.

As for that specific rocket, is it flipping before or after you drop that first stage? In general the answer to problems like this is to move the center of mass forward or the center of lift+drag back, but how to do that would vary depending on when you have the problem.

[jonpfl]

54 minutes ago, HebaruSan said:

Don't turn 45 degrees abruptly; that's old, out of date advice. Do it gradually, starting around 1km and reaching 45 degrees by about 10km.

As for that specific rocket, is it flipping before or after you drop that first stage? In general the answer to problems like this is to move the center of mass forward or the center of lift+drag back, but how to do that would vary depending on when you have the problem.

When I get to 20k apos I try to turn to 45 degrees and it just goes crazy (so still the first stage).

1) When you say "center of mass forward", can you expand on that?  I thought I was supposed to keep center of mass as low on the rocket as possible.

2) How do I figure out the center of lift + drag back?

Thx

jonpfl

[harrisjosh2711]

think of a rocket like a dart, you want a heavy tip. It sounds to me that you have an unstable rocket on your hands.

[jonpfl]

31 minutes ago, harrisjosh2711 said:

think of a rocket like a dart, you want a heavy tip. It sounds to me that you have an unstable rocket on your hands.

Yeah, I need to remember that, I think I have the fuel flowing the wrong direction!

Thx

[HvP]

@jonpfl I think you can get rid of those small fins near the top part of your rocket. They are going to catch the wind acting like a sail if you turn too hard and drag the front of your rocket around. Maybe move the small fins to where the larger ones are near the Swivel engine, and move the larger fins to your bottom stage.

There's also added drag when you connect stacks of different diameter parts without a size adapter. It will be smoother if you add a Rockomax Brand Adapter to narrow the gap between the size 2 parts at the bottom and your size 1 parts above it. Just remember to use the decoupler that fits the smaller size at the top of the adapter so it will separate from your Swivel engine.

Also, I'd guess that the mainsail engine on your bottom stage is WAY too overpowered for such a small rocket. When you ram into the air going extremely fast in the lower atmosphere it increases the drag at the front of your rocket even more than normal. If you want to keep the larger diameter on bottom then a Skipper would probably work just as well and be more efficient.

And don't forget solar panels and an antenna if you're going to the Mun.

(I'm going to suggest this thread is moved to the Game Questions forum, you'll get more help there.)

Edited June 12, 2017 by HvP

[StrandedonEarth]

Model Rocketry 101: for stability you want the center of mass ahead of the center of pressure (think of where a side-view silhouette cut-out would balance). Too far ahead and it won't turn at all. So, fins as far back as you can, just big enough to keep it stable.

Edited June 12, 2017 by StrandedonEarth

[HebaruSan]

3 hours ago, jonpfl said:

When I get to 20k apos I try to turn to 45 degrees and it just goes crazy (so still the first stage).

Right; don't do that. Turn 5 degrees when you get to maybe 50-100 m/s, then keep turning very gradually so you're at about 45 degrees by 10 km altitude.

3 hours ago, jonpfl said:

1) When you say "center of mass forward", can you expand on that?  I thought I was supposed to keep center of mass as low on the rocket as possible.

Imagine your rocket floating motionless in the air, and suddenly a sustained gust of wind comes along. The center of mass acts like an anchor, around which the rest of the ship is blown by the wind. The fins and lower density parts act like a windsock and force the ship to rotate so they're pointing in the direction the wind is blowing.

          ^ center of mass              ^ center of lift+drag
                           -----> wind ----->

That's similar to what happens during a launch, except the "wind" is just normal air that you're flying through on your way to space. It blows opposite to the direction you're moving, so you want to make sure that your rocket can operate properly if its windsock parts are pushed towards the back.

3 hours ago, jonpfl said:

2) How do I figure out the center of lift + drag back?

One easy way is to add fins to the very bottom of your rocket, bigger than fins you've used elsewhere, to ensure that your rocket tends to point nose-first into the wind.

33 minutes ago, HvP said:

@jonpfl I think you can get rid of those small fins near the top part of your rocket. They are going to catch the wind acting like a sail if you turn too hard and drag the front of your rocket around. Maybe move the small fins to where the larger ones are near the Swivel engine, and move the larger fins to your bottom stage.

Good eye, I didn't notice those. I agree with the suggestion to scrap them. Upper stages often don't need fins at all anyway since you're out of the atmosphere by the time they're used.

[Chel]

Most of the time the problem can be averted with SAS, but sometimes, as I have found out, SAS can be used against you. It can cause uncontrollable rolling, but as soon as you turn it off, the craft stops rolling. Anyway, getting off topic. I would also say that if you have the electric capacity, you could stack some reactions wheels on somewhere, although you would probably need an RTG or solar panels to stop you from running out of power from the reaction wheels.

[qzgy]

On 6/11/2017 at 11:42 PM, The_Cat_In_Space said:

Most of the time the problem can be averted with SAS, but sometimes, as I have found out, SAS can be used against you. It can cause uncontrollable rolling, but as soon as you turn it off, the craft stops rolling. Anyway, getting off topic. I would also say that if you have the electric capacity, you could stack some reactions wheels on somewhere, although you would probably need an RTG or solar panels to stop you from running out of power from the reaction wheels.

Actually, if you have a well designed rocket, reaction wheel spam shouldn't be necessary. Just stick draggy stuff on the back.

Looking at the design, I have to concur with what most people here said - don't turn too abruptly and stick the fins lower down onthe rocket.

Edited June 25, 2017 by qzgy

[Jas0n]

The old go straight up and then turn to 45 degrees is from before the aerodynamics were changed. Now it's better to turn gradually from lower.