Problem with a shuttle configuration : Center of Lift, Center of Mass

[XB-70A]

Hi everybody,

 

I am actually snatch my hairs with a 6-seats capable shuttle design I am working on. It is clearly inspirated from the now well knew Dream Chaser, but it gives me so much problems that I surnamed it Doom Chaser.

I worked on two different variants :

The A is about endurance with about 3 km/s of dV available :

 

And the B much more for aerodynamic performances test craft :

 

In both case they are my first attempts to place a medium orbiter with enough lifting surface at the top of a launcher. All the others concept were placed in a booster/fuel tank-riding position and their lifting surface was just sufficient to ensure them to safely glide back to home.

Here the situation is totally new to me, the CoL is far away from the CoM and CoT more important. The result is a lifting force too important to be countered by the vectored thrust, the whole shuttle flipping all the time once the dynamic speed becames sufficient to gives the orbiter wings and body enough lifting force :

42 m/s relative speed is still to low to give the orbiter enough lift.

50 m/s and the problem starts to appear.

Logically the autopilot tries to counter it by max gimballing the engines and it gives some results. But as a simple computer it will not keep on gimballing the engines and will wait for new problematics information to come.

About two seconds only after the lifting force is so important that event he thrust can't counters it anymore.

 

 

I've tried a booster riding configuration too to bring the CoL closer to the CoM but the result is absolutely the same. Of course I could also place it in a fairing to erase that problem definitely but it will ask for a larger and costful launcher.

 

Is anyone of you with more knowledges got ideas ?

(I should have studied more seriously my manuals about aerodynamics some years ago...)

[AeroGav]

I don't know if you're making a replica of the whole dreamchaser stack or just the orbiter.

If you're only bothered about the orbiter looking right,  might it be easier to radially attach four smaller boosters around the orbiter, so the lift from the orbiter's wings is near the bottom rather than the top?

Otherwise you need to put giant wings on the very bottom of the booster to try and balance the lift coming from the spaceplane.

[bewing]

Or launch straight up. It's only if you try to do a gravity turn that you will see this problem.

[XB-70A]

 

Just now, bewing said:

Or launch straight up. It's only if you try to do a gravity turn that you will see this problem.

Alas I'm against that profile of launch. They are inefficient (the whole already launcher got some difficulties to lift that 26 tons craft) and, I don't know it's just like it was killing the fun of climbing. But I understand it can sometimes be the sole solution to send something in orbit. Like those huge stations some made.

 

3 hours ago, AeroGav said:

Otherwise you need to put giant wings on the very bottom of the booster to try and balance the lift coming from the spaceplane.

I actually found the same solution just as after I created that topic and tried it. I was against the fact to place wings on the launcher but remembered a bug where the wings even offsetted into a tank were giving lift. The result is more than positive :

 

It's like cheating and did not resolve the main problem of how to counter that lifting force. I mean like they have to counter in reality. I've seen two different configuration on artist impression.

The most well knew at the top of a launcher :

 

And one inside a fairing :

 

Damn... I just discover by the way that the Atlas got a configuration with boosters...

[erik_t]

The answer in two parts:

 

The first answer is that the entire stack is of course a lifting surface. You can flip out of control quite easily with a large axisymmetric fairing. The difference is that this configuration is giving you lift even at zero angle of attack. I wouldn't be surprised if the real thing mounted the payload at a small negative angle of incidence to ensure that there is zero lift along the entire stack when pointed exactly along the velocity vector.

 

The second answer is that NASA/ESA/Roskosmos/SpaceX/etc have waaaaaaaay better SAS and control algorithms than we do

 

Were I attempting to emulate this launch vehicle, I would try to very slightly adjust the angle of incidence of the lifting body, and I would have no shame in hanging big honking fins on the tail of the booster.

[AeroGav]

5 hours ago, AeroGav said:

 

If you're only bothered about the orbiter looking right,  might it be easier to radially attach four smaller boosters around the orbiter, so the lift from the orbiter's wings is near the bottom rather than the top?