End over End at Mach 4

[Captain Vlad]

Been fiddling with a Rapier/Nuke engined SSTO tonight and I've having a problem. The plane's a wee bit underpowered IMO, but if I manage her climb angle she can make speeds/altitudes that I shouldn't have any problem turning into a successful orbit. But, when I get to the point where I switch my Rapiers to closed cycle, the plane's nose will start to pitch up and it ends up tumbling end over end. I'm usually toggling the engine around 1,600-1,700 mps at 28,000 meters or so when I hit the button.

My first instinct is 'center of mass issue', but fuel use doesn't seem to be moving the center of lift ahead or in-line with the center of mass, and the center of thrust looks to me like it's lined up fine with the CoM. I've got all kinds of stable spaceplanes, and don't know if I'm missing something or if this is actually some problem I've never encountered before.

Any assistance would be appreciated...it's driving me nuts.

Edited January 8, 2015 by Captain Vlad

[*Aqua*]

The air intakes could be the reason. They have a lot of drag which could lead to a plane that wants to fly backwards. Additionally there are some only on the top side of the craft which places the 'center of drag' a bit above your CoM.

Possible solutions:

1) Close intakes when switching to closed-cycle. This reduces the drag of the intakes. (Could all be done in one action group.)

2) Place the intakes behind the CoM.

3) Distribute intake drag evenly by either placeing the radial intakes on the side of the craft or add additional ones at the bottom.

It could also be that the wings are placed too high. Your craft gets lighter when you burn fuel which leads to an increased effect of the wing weight. Your CoM will rise and not be in line of your CoT anymore.

Possible solutions:

1) Rotate the wings back to a horizontal angle.

or

2) Place the wings a bit lower.

To see how the CoM changes as you burn fuel remove the fuel in the editor and watch the CoM orb. It should move around a bit.

There are some other things I can see to improve your craft:

- Don't angle the gears. It could lead to instable behavior when moving on the ground.

- Your yaw rudder is a bit misplaced to the right (at least it looks like that on the screenshot).

[Captain Vlad]

The air intakes could be the reason. They have a lot of drag which could lead to a plane that wants to fly backwards. Additionally there are some only on the top side of the craft which places the 'center of drag' a bit above your CoM.

I hadn't thought of that. I have an OCD tendency toward symmetrical inlet arrangement that I didn't indulge on this plane...it may've been helping my other designs without me knowing it. I'll fiddle with the intake/action group arrangement and see if it helps...I suspect it's this and not the wings given it's always happening at the precise moment I switch to closed cycle. I have the 'close intake' button mapped to a different key, too, which might be aggravating the problem.

I'll fiddle with this and the wings and see if I can get her into orbit. Thanks.

Thanks for the heads up on the rudder, too, I have a tendency to place it and forget it and not notice it's misaligned until the third or fourth mission. Don't get me started about the landing gear, I have a hell of a time getting them straight (it'll roll down the runway...sort of straight. Getting perfectionist with the wheels I save for last cuz I've been known to fiddle with it over an hour.)

[eddiew]

It's probably irrelevant, but in your 2nd shot, your tailfin looks off-centre... Though I imagine that would give it a habit of yaw-spinning, maybe that momentary grapple with the controls is destabilising it?

Also, are you using FAR (don't see it in your icon bar)? You'll find planes fly more like planes with FAR than they do with stock

On the other hand, pitch-up when you kick in the main rockets isn't unusual, since the top-side is often loaded with tailfins and the CoM isn't vertically aligned with the CoT. I end up loading my spaceplanes with a double reaction wheel to make sure I can keep them in line. Brute force can be quite effective

[*Aqua*]

Don't get me started about the landing gear, I have a hell of a time getting them straight (it'll roll down the runway...sort of straight. Getting perfectionist with the wheels I save for last cuz I've been known to fiddle with it over an hour.)

Just place them at the center of the tanks. They align straight in snap mode. Then use the offset gizmo if you want to move them. Or you use the adjustable landing gear mod. You'll need an updated firespitter dll you make it run on 0.90.

Your plane will never roll down straight the runway. When physics kick the weight of the center body will bend your plane into a 'v shape'. Even with struts you can't prevent that completely it's just less noticible with them. The bending usually is asymmetrical. Not perfectly aligned gears will worsen it as they will want to move in different directions.

[Captain Vlad]

Just place them at the center of the tanks. They align straight in snap mode. Then use the offset gizmo if you want to move them. Or you use the adjustable landing gear mod. You'll need an updated firespitter dll you make it run on 0.90.

Your plane will never roll down straight the runway. When physics kick the weight of the center body will bend your plane into a 'v shape'. Even with struts you can't prevent that completely it's just less noticible with them. The bending usually is asymmetrical. Not perfectly aligned gears will worsen it as they will want to move in different directions.

I'm still getting used to the gizmos, and that sounds like a handy trick. I usually place them 'by hand' and sometimes I'll put them on the outside or inside of things for more or less height off the runway then manually adjust. Thanks.

This plane has some of the more unusual runway behavior I've seen. When I first start to accelerate, it'll turn to the right, but fairly gently and I can turn back to 90 degrees if I do it gently and it'll stay once it gets up to speed. I tested it at 1/3rd throttle and if I keep the speed low the instability gets worse and worse. That's why I wasn't terribly worried about it at the moment, since my usual gear placement trial and error involves flips and explosions. Like I said, tends to be the last thing I 'perfect'.

Also, after a test flight just now, gonna mark this one as answered. I did adjust the wing some, but I feel the intake advice was what really helped; high-speed/altitude stability is worlds better (though I did manage to find its limits...). The help is much appreciated.

On the other hand, pitch-up when you kick in the main rockets isn't unusual, since the top-side is often loaded with tailfins and the CoM isn't vertically aligned with the CoT. I end up loading my spaceplanes with a double reaction wheel to make sure I can keep them in line. Brute force can be quite effective

I built a cargo SSTO awhile back and it works very, very well for me. This design is sort of a shortened version of that plane. But the cargo bird has a reaction wheel on the nose...I think I may add one in the same spot on this plane and see if that helps too...

Didn't in the first place as I wasn't going to lug space station parts into orbit with this one.

- - - Updated - - -

Update: She did make orbit. She didn't have as much fuel left as I'd wanted, but I may be able to change that with some alterations in flight profile. More than enough to meet up with my space station, though.

[eddiew]

Glad that you got there in the end ^^ Another option, since you have rapiers, is more thrust to weight!

I cooked this pair of satellite lifters up a few weeks ago, based on a classic Wanderfound design. Technically they're not very stable, but with 6 gimbling engines at all altitudes, 1.3 TWR, and a double reaction wheel in the cargo bay, there's always enough torque to point the nose wherever the heck you want it. Both models try to nose-up about 30-40k, but just tap W a bit and they're back under control. Later on I'll add vernors to the nose, but I haven't researched those yet.

Also they get to orbit in about 4-5 minutes, so function and fun in one