Design help needed.

[catloverjerrygarcia]

Kerbin VIa launch vehicle. by AdmFranzvonHippie, on Flickr

That's the design I've worked up with a forum friend of mine.  I just started trying to fly it for a 75 km orbit but it becomes unstable in flight.  And if anyone wants to scream at me for cheating because I use MchJeb go away, I suck at flying rockets and can't play the game without it.

[Geonovast]

Two biggest problems I'm seeing:

The rocket is short and fat.  The biggest issue here is that while your engines gimbal (At least, I think - I'm pretty sure those are swivels), they're pretty close to the Center Of Mass, so their authority isn't that great.

Which leads to the second problem - the blunt, draggy nose.  I would ditch the stayputnik.  Get a second service bay, and stick the HECS in there along with the MJ module, and if you turn the solar panels sideways, you can probably get them in there too without them clipping out.  I do it all the time.  Then stick a nose cone on the top.   You could probably also get rid of that giant reaction wheel since the HECS will have one.

Slide the boosters down a bit, and maybe get some small fins on the bottom to help keep the drag as far back as you can.

[IkranMakto]

[Geonovast]

[catloverjerrygarcia]

@Geonovast  I'm trying to do what you suggest but can't get everything to fit in the second bay...

[Jestersage]

5 minutes ago, AdmFranzvonHippie said:

@Geonovast  I'm trying to do what you suggest but can't get everything to fit in the second bay...

First, what are you actually trying to do? He switch to use a HECS or OCTO probe.

If you do insist on using Stayputnik for recreation, please put it in a fairing if you can.

 

[Geonovast]

You may need to use the Offset Tool (green) with Angle Snap off (Red) to get things to line up so they look right.

[catloverjerrygarcia]

Okay, this is what I came up with, hope it works.  MechJeb is in the nosecone.

Kerbin VIa by AdmFranzvonHippie, on Flickr

Edited August 28, 2019 by AdmFranzvonHippie

[Geonovast]

3 minutes ago, AdmFranzvonHippie said:

MechJeb is in the nosecone.

That might look fine, but it actually won't be hidden from aerodynamic forces unless it's inside the service bay.  Just clipping parts inside another part doesn't actually protect them.

Might be alright though with the other changes though.

[catloverjerrygarcia]

*sigh*  I can't do what you suggest I do, there just isn't enough room in the bay.

Edited August 28, 2019 by AdmFranzvonHippie

[catloverjerrygarcia]

So this is what I did.

20190828125747_1 by AdmFranzvonHippie, on Flickr

20190828125853_1 by AdmFranzvonHippie, on Flickr

[Geonovast]

Oh!  You had a fairing this whole time?  Forget the service bays entirely then, stick the whole thing in the fairing!

You won't need the nosecone now either.  Just taper the fairing in to make a point.

[catloverjerrygarcia]

So even utilizing these suggestions and interpreting them into my own design it still gets squirrely...

[Geonovast]

7 minutes ago, AdmFranzvonHippie said:

So even utilizing these suggestions and interpreting them into my own design it still gets squirrely...

Would you mind removing the MJ module, and then sharing the craft?

[catloverjerrygarcia]

Had a visitor stop by, will make the changes now utilizing a faring.

[catloverjerrygarcia]

Okay, here's the updated design utilizing just a faring.

20190828133344_1 by AdmFranzvonHippie, on Flickr

There was even enough space inside the faring for me to add a goo experiment.

[Geonovast]

Ideally you want the fairing as slim as possible.  Looks like you've got a good amount of extra width there.  If that still doesn't work, try slimming the fairing down (should make it lighter too)

 

Now, I have no experience with MJ, but would it still need SAS?  The Stayputnik has no SAS ability.  You might want to try with one of the other probe cores.  The Stayputnik is really only good, at all, until you've unlocked something else.

[catloverjerrygarcia]

I do have the other cores, and the faring wouldn't go as slim as possible for me, but it might with one of the other cores, back to the drawing board.

Latest modification, the batteries I was using were interfering with the faring build, so I swapped them for the Z-200.

20190828135747_1 by AdmFranzvonHippie, on Flickr

Edited August 28, 2019 by AdmFranzvonHippie

[Kerbinchaser]

Why not make the center core-booster longer and match it with the booster length? Your two boosters are much longer than your center stage, try matching the height of all of them. 

[catloverjerrygarcia]

2 minutes ago, Lo Var Lachland said:

Why not make the center core-booster longer and match it with the booster length? Your two boosters are much longer than your center stage, try matching the height of all of them. 

I started out that way and it was grossly overpowered.

[Kerbinchaser]

1 minute ago, AdmFranzvonHippie said:

I started out that way and it was grossly overpowered.

you can edit how much fuel each booster carries. 

[catloverjerrygarcia]

Okay well I've done some flight tests and it still gets squirrely as it's working on the gravity turn...

More detailed information, the wobble starts around 8000 meters altitude.

Edited August 28, 2019 by AdmFranzvonHippie

[Geonovast]

At this point I think a video would help.

I flew the first design I posted just beautifully to orbit, no problems at all.

[catloverjerrygarcia]

12 minutes ago, Geonovast said:

At this point I think a video would help.

I flew the first design I posted just beautifully to orbit, no problems at all.

Uploading video now, estimate of around a half hour to finish.

[catloverjerrygarcia]

I have a sneaking suspicion I know what the issue is but we'll see what the analysis here says.

20 minutes to go on the video.