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Bobe

Building a shuttle - CoM/CoL weirdness

Question

Something I've never really accomplished is building a reliable shuttle, so I set out to do just that. I've already done some test flights, but it's far from stable. The issues stem from the orbiter's aerodynamics which I can't figure out.

1T8VpF7.jpg

The vehicle is completely stock except for the docking assembly in the forward bay, but here is the craft file with the assembly removed (it brings the CoM just in front of the aft docking port): https://www.dropbox.com/s/xyv532298gm371q/Shuttle%20Orbiter.craft?dl=0.

I can't figure out why the centre of mass is so far back and why the centre of lift is upside down. I've double checked to make sure the wings are the right way up. The centre of thrust shown here is with the main engines disabled. In-orbit stability is another issue considering the two OMS engines aren't perfectly aligned unless I tilt them up to an awkward angle.

Also, sometimes the CoL will act up and just appear as a dot in the middle of the tail without a vector:

aG690Q3.png

 

Edited by Bobe

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13 minutes ago, Bobe said:

I can't figure out why the centre of mass is so far back

Because those vector engines on the back are really, really heavy. 

13 minutes ago, Bobe said:

and why the centre of lift is upside down.

Because you're in the VAB instead of the SPH. The VAB assumes that you're going to be flying a rocket, so the CoL calculations assume "airflow"  from the top straight down. The SPH assumes you're flying a plane, so it has "airflow" from the front to the back, with a 5 degree angle of attack. Take just the orbiter part and load it in the SPH. 

One thing with shuttle replicas that you need to be careful of is being mislead by the CoM/CoL relationship. The CoL doesn't tell the whole story of all the aero forces acting on the shuttle. There is also drag, which (because the CoM is so far back) has a long lever arm to flip the nose around and make you fly backwards. 

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Full tank of fuel at the back with no corresponding forward tank will affect CoM that way. Try tweaking out the fuel and see where it ends up.

And SPH++, much better for making the orbiter section before bringing it to the VAB to add the external tank and boosters.

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Ok, making the orbiter in the SPH makes sense. It didn't actually occur to me that the forces were calculated differently.

I suppose what I meant about the CoM was not that it didn't make sense, because obviously the fuel tank and engines are heavy, but rather I didn't understand how I was meant to balance it out considering basically everything is in place. I guess this is where the reality of KSP physics comes to kick you in the butt, but so many people seem to achieve similarly realistic-looking orbiters without any trouble.

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@Bobe You don't have a payload in it yet, which can greatly affect CoM. It's also important to note that CoL is close to irrelevant for the ascent (it flies up like a conventional rocket), CoL only becomes important during reentry and landing, when that rear tank is likely to be near empty.

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1 minute ago, Red Iron Crown said:

You don't have a payload in it yet, which can greatly affect CoM.

Obviously, but I'm trying to design it in reverse order of mission profile. The objective here is to ensure it can glide down to a stable landing (with the fuel tanks empty).

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1 minute ago, Bobe said:

Obviously, but I'm trying to design it in reverse order of mission profile. The objective here is to ensure it can glide down to a stable landing (with the fuel tanks empty).

This is the correct approach, IMO.  With that being the case, like RIC said, empty the fuel tanks because it will be reentering empty on an actual mission. 

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Just now, FullMetalMachinist said:

With that being the case, like RIC said, empty the fuel tanks because it will be reentering empty on an actual mission. 

Naturally. I think in the screenshot and in the craft file I left the tanks full, but if I recall correctly them being empty still didn't bring the CoM in front of the main landing gear.

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59 minutes ago, FullMetalMachinist said:

Because those vector engines on the back are really, really heavy. 

Also the Mk3 cockpit is really, really light.

 

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1 hour ago, Bobe said:

I suppose what I meant about the CoM was not that it didn't make sense, because obviously the fuel tank and engines are heavy, but rather I didn't understand how I was meant to balance it out considering basically everything is in place.

What works for me is to deviate slightly from the real life NASA orbiter and move the wings, and especially the control surfaces, waaaaaay back. 

The problem you're likely running into is that (like I mentioned before) the drag forces that are acting on the forward part have a huge leverage advantage because the CoM is so far back. At the same time, the elevons on the back of the wings are close to the CoM, so they have very little leverage, and aren't strong enough to keep things pointed the right way 'round. 

So, to help out you can move the wings back as far as you can without destroying the "replica" feel, and make sure that you have some body flaps, lots of people forget about those. 

Also, this:

2 hours ago, Bobe said:

but so many people seem to achieve similarly realistic-looking orbiters without any trouble.

Is not really true. It may look like they're not having much trouble, but IMO designing and flying a successful shuttle replica is one of the hardest things to do in vanilla KSP. 

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L9RqgNO.png

Semi-success (that's KSC in the distance). The ride up was a little awkward, MechJeb did its best to try and keep the thing rotated 180 degrees. There's still so much lateral movement off the pad that the ascent guidance has to compensate for and it ends up as this large sideways curve. The tricky part is balancing the thrust of the SRBs with the throttling of the main engines during max Q and hitting acceleration limits (bumped up to 30 m/s^2); if the main engines throttle down too much the boosters will overpower the stack and slowly put it in a death spin.

In any case, orbital flight was fairly good, I just need to tweak the OMS engines slightly, my maneuvers spun out just a few delta v from completion. Re-entry was not so great. I got lazy with my pitch up maneuver and underestimated how much drag it ignores with a prograde attitude. The transition into atmospheric flight was very chaotic, it had a hard time staying level and eventually went into an uncontrollable spin. Once I had slowed down enough and the air was thick enough for the control surfaces I managed to gain enough control to take it in for a rather soft water landing. At least I know it handles well on final approach, and I managed to deploy a large, useless tank of fuel to orbit.

 

39 minutes ago, FullMetalMachinist said:

The problem you're likely running into is that (like I mentioned before) the drag forces that are acting on the forward part have a huge leverage advantage because the CoM is so far back. At the same time, the elevons on the back of the wings are close to the CoM, so they have very little leverage, and aren't strong enough to keep things pointed the right way 'round.

That definitely seemed apparent on re-entry. I'll try moving the wings back, but they can only go so far. As mentioned above, the control surfaces seemed to handle well enough when not going so fast, but I had to experiment with which elevons were assigned to which axis (typically outer roll, inner pitch).

 

39 minutes ago, FullMetalMachinist said:

and make sure that you have some body flaps, lots of people forget about those. 

Doesn't the engine mount sort of act as a body flap? What sort of part would be a suitable to put there?

Edited by Bobe

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7 minutes ago, Bobe said:

Doesn't the engine mount sort of act as a body flap? What sort of part would be a suitable to put there?

Not so much. I mean, yeah it contributes a small amount of body lift, but that's about it. I would put a couple of the BigS control surfaces down on the bottom edge. Set them to pitch only, and maybe put them in an action group to toggle extend/retract down to be like flaps. 

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CoL shouldnt even be in the VAB, it should automagically be CoP...

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