General Question XR2 Centre of gravity shift

[Null_pointer]

Hello everyone.
Consider this situation. XR2 is flying in the atmosphere horizontally and the attitude hold autopilot is used. The autopilot uses the trim and other control surfaces to achieve certain attitude because the ship acts as a normal plane. No problem with that.
But when XR2 is flying down reentry the ship is no longer acts as a plane but rather as a brick (because of high aoa). In this case the control surfaces are useless for changing the attitude. The main thing that is used by the XR2 to control the attitude is a centre of gravity shift. This sound reasonable.
I presume this idea was borrowed from small submarines. The submarine batteries are usually put on the small cart which can move back and forth. So to change the submarine attitude you can move this cart. This is very nice solution because you dont need extra fins for the submarine.
So here is my question.
How the center of gravity shift works if there is no moving parts in the XR2. I know the XR2 is a model, you can do anything you want with the model. But still, we say that that thing is realistic, right?
The only explanation I have is there is two tanks of fuel of whatever, and a pump is used change the liquid position to change the center of the gravity.
Could you explain me please how this really work in the XR2 and real plane-like spacecrafts.

 

[dbeachy1]

to Orbiter-Forum!

From the section titled Center of Gravity Shifting on page 29 of the XR Flight Operations Manual:

Note that the pumps that pump fuel forward or aft for COG shifting require a lot of
power, and therefore the APU must be active in order for them to operate.

 

[Fabri91]

I've never tried it, but would this mean that without fuel on board the CG shift is ineffective, since there's nothing to shift around?

 

[Gr_Chris_pilot]

Hello everyone.
Consider this situation. XR2 is flying in the atmosphere horizontally and the attitude hold autopilot is used. The autopilot uses the trim and other control surfaces to achieve certain attitude because the ship acts as a normal plane. No problem with that.
But when XR2 is flying down reentry the ship is no longer acts as a plane but rather as a brick (because of high aoa). In this case the control surfaces are useless for changing the attitude. The main thing that is used by the XR2 to control the attitude is a centre of gravity shift. This sound reasonable.
I presume this idea was borrowed from small submarines. The submarine batteries are usually put on the small cart which can move back and forth. So to change the submarine attitude you can move this cart. This is very nice solution because you dont need extra fins for the submarine.
So here is my question.
How the center of gravity shift works if there is no moving parts in the XR2. I know the XR2 is a model, you can do anything you want with the model. But still, we say that that thing is realistic, right?
The only explanation I have is there is two tanks of fuel of whatever, and a pump is used change the liquid position to change the center of the gravity.
Could you explain me please how this really work in the XR2 and real plane-like spacecrafts.

Many real world aircrafts like airbus a320 using gravity shift to optimize fuel economy and best angle of attack for each flight phase.... This can be done using fuel transfer pumps to transfer fuel to the aft-fore fuel tanks.... The a320 have a specific aft tank near the tail only for this reason..... :thumbup:
Some aircrafts are UNFLYABLE without gravity shift via fuel transfer like SR-71 blackbird and Concorde so yes its realistic and applicable to the xr2

 

[Null_pointer]

I tried to fly XR2 with no fuel at all (only 5 kg of APU fuel) and COG shift works perfectly fine. Is this a model limitation? The COG shift should not work without fuel in tanks.

 

[Cras]

I've never tried it, but would this mean that without fuel on board the CG shift is ineffective, since there's nothing to shift around?

No. I often dump all the main tank fuel before re-entry and COG still works fine. Moving the CoG is essential in Orbiter, if you don't do it, the space craft just will not behave properly during the re-entry phase. Its nice to have a though in the back of your head that something is getting shifted around to make the CoG actually occur, but the XR-2 is known and loved for being awesome, not because it is realistic. It is Single Stage to the Moon after all, unless you modify the config files (part of that awesome I was talking about) to tone down the magic a bit.

 

[agentgonzo]

I tried to fly XR2 with no fuel at all (only 5 kg of APU fuel) and COG shift works perfectly fine. Is this a model limitation? The COG shift should not work without fuel in tanks.

Moving the CoG in the XR2 is just done by 'magic'. We can rationalise it by saying that it's the fuel moving, but all that happens is the code says "my CoG is now here". I would imagine that dbeachy could put in code to limit the moving of the CoG when low on fuel but it's not in there at the moment. It would be a nice feature.

As well as the 'gravity shift' mentioned above, airline pilots also have to 'trim' their planes when they fly (to make sure that it's nicely balanced). Like getting all the passengers spaced out so that they're not all at the front/left etc.

 

[Null_pointer]

Moving the CoG in the XR2 is just done by 'magic'.

I red an article about a Shuttle and I have not found anything about COG shifting (because there is no fuel to pump when reentry). And the Attitude Hold autopilot uses only the RCS and control surfaces to achieve certain attitude. I am not an aircraft engineer but I think that a Shuttle is not a plane, is more a brick. I dont understand how it is possible to sustain a attitude without the COG shifting (it should fall to it nose because it COG is in the center and wings that provide drag force are at the back).

 

[C3PO]

At 40° AoA and hypersonic practically everything flies like a brick.

My guess would be that the STS has a CoG so far aft that it's actually unstable enough that the computers can control it during reentry. Shuttles actually flew pretty well for an unpowered delta-wing, but the rate of descent was high compared to commercial airliners.

The STS used the fly-by-wire to control the attitude, so it didn't have to shift the CoG forward. But if you used the real CoG in Orbiter it would be very hard to control with direct control, especially since Orbiter uses slow airfoil movements to make keyboard control possible.

Programming a CoG shift is easier than the fly-by-wire you would need for a unstable CoG. But since XR-2 are fictional we can just say that the fuel indicator displays usable fuel, and you're not allowed to empty the tank for instability issues. It's a safety feature, OK?

 

[Urwumpe]

The shuttle actually will backflip during reentry, when exceeding 50° AOA, the 40° AOA is near a small island of stability where only minimal control force is needed.

The CoG shifting of the Shuttle by dumping forward fuel simply ensures that this island of stability exists where it is needed.

 

[fcn]

Oh, CoG shifting. Well, I use it everyday. Isn't this the same thing as when taking a bus, if you're standing you lean forward while bus is accelerating, and the other way around? It works fine for me, helps me keep myself balanced with less energy.

As for XR2's CoG shifting, maybe it just uses another mass but the PAO was misinformed or misunderstood, so the public documents say a different thing than the actual thing. Or maybe the company tried to keep it as a tech secret, so they just tried to trick us into thinking that it shifts using fuel but it doesn't. (Maybe I'm taking things a little bit more serious than I should.)