Flyable booster flipping out

[Jarin]

So I'm trying my hand at designing an Aurora-style reusable flying booster, but I seem to be failing horribly at aerodynamics. No matter how far back I shove the CoL, the thing still insists on flipping around and hurtling butt-first to its doom. 

It shouldn't be a fuel balance problem. That shot is with worst-case balance; all tanks empty except the rear tank completely full. Flight tests have it leap up off the runway just fine, then start flipping out not long after takeoff. No part or aero mods involved so I haven't a clue what the problem might be. Craft file here.

 

[Gaarst]

The CoL shown in the VAB is only the resultant of lift forces: it doesn't account for drag.

So even if your CoL is way at the back with the CoM in front, the whole part of the craft that is in front generates massive amonts of drag and moves your CoP (centre of pressure) forwards. This is why your booster is unstable.

[adsii1970]

21 minutes ago, Gaarst said:

The CoL shown in the VAB is only the resultant of lift forces: it doesn't account for drag.

So even if your CoL is way at the back with the CoM in front, the whole part of the craft that is in front generates massive amonts of drag and moves your CoP (centre of pressure) forwards. This is why your booster is unstable.

Using the picture above as an example, how would one move the CoP forward? Would wing reconfiguration work? Is it strictly a weight problem?

[Gaarst]

9 minutes ago, adsii1970 said:

Using the picture above as an example, how would one move the CoP forward? Would wing reconfiguration work? Is it strictly a weight problem?

You want to bring the CoP backwards.

You do it pretty much like the CoL, just keep in mind that wings only move CoL, but all parts have an influence on the CoP (though you can't see the CoP). Seeing this design, I'd suggest bringing the wings forward, closer to the centre, then moving the CoM as well to keep it balanced.
Adding wings at the back would shift the CoP too, but you will have trouble balancing your CoL after that.
CoP is the sum of aerodynamic forces (lift + drag), so as long as the CoL and CoDrag are not close to each other (ie: CoP moving around the craft depending on AoA) your plane will not be stable as at one point, the CoM will be too far from it, either in front of or behind it. So just moving the CoM in this case will not help.

What I understand about aerodynamics is that CoL matters when flying levelled (small AoA) as everything is inline with no insane amounts of drag coming from the fuselage (CoP is then essentially at the same place as CoL); while CoP matters when reentring or flying at a high AoA: the whole length of fuselage is exposed to airflow, generating tons of drag which will make your plane/booster flip if too far in the front.

Edited May 20, 2016 by Gaarst

[adsii1970]

@Gaarst: Maybe @SQUAD could add an indicator for CoP. Seeing your explanation makes it seem like a common sense thing to add.

 

[Gaarst]

Just now, adsii1970 said:

@Gaarst: Maybe @SQUAD could add an indicator for CoP. Seeing your explanation makes it seem like a common sense thing to add.

It is. Having a CoP display would solve a lot of similar issues, especially when replicating space shuttles (insanely heavy Vectors at the back -> CoM at the back -> wings at the back -> draggy front -> flip when flying).

[Snark]

Moving to Gameplay Questions.

The fundamental problem is that your CoM is way in the back.  It needs to be in the front.

With a CoM way in the back like that, it means two things:

First, you've got that huge draggy fuselage sticking out in front that's going to want to trail behind the CoM.

Second, your CoM is so close to your control surfaces that they have almost no control authority.  Their lever arms are so short that they simply can't exert any significant amount of torque.

Move the CoM way far forward.

[Jarin]

Any recommendations on how to do that? I've rebuilt this thing several different ways, but fundamentally, it's still a bunch of fuel tank with a big engine at the back, and I don't see how to change that. 

[Jarin]

I would also note, "CoM too far back, don't know how to bring it forward" nicely encapsulates 90% of my trouble with spaceplanes in general, so I'd really appreciate any tips people can offer.

[bewing]

The other way to fix Snark's "lever arm" problem is to forget about putting the control surfaces at the back end. Leave the CoM back there, and put great big canards on the front end.

[Jarin]

Okay, I can make it fly if I lock the forward fuel tank for ballast, but that still seems like a shoddy workaround. Also, no-go on the giant canard concept. Helped with getting off the runway faster but not much else.

 

[Snark]

Locking the forward tanks is the easiest solution.  Another is to get rid of the big-heavy-engine-on-the-back (or, at least, replace with something much smaller) and add side-mounted engines much farther forward.

[AeroGav]

5 hours ago, Snark said:

Locking the forward tanks is the easiest solution.  Another is to get rid of the big-heavy-engine-on-the-back (or, at least, replace with something much smaller) and add side-mounted engines much farther forward.

Yet another victim of the way rocket engines drain fuel unevenly.

I always assumed this was an artificial way of making the game more difficult, or to give us a reason to spend tech points on unlocking larger tanks.  Apparently though this was actually done to help   us so that we didn't need to worry about lower stage engines leeching off the upper stages of our rockets.   How nice of them !

So, to make a rocket engine not spoil the flying qualities of a space plane, you will need to observe the following rules , assuming no mods

1. The rocket fuel tanks need to be on the centre of gravity, so there is no fore-aft shift as it burns off.

2.  Each rocket engine can be fed by only one tank, or it will drain the front first and screw up your cg. So this tank needs to be the largest possible.

3.   Jet fuel tanks drain evenly so you can put some in front of and some behind the rocket tanks, and  not get any shifts.

4.   Rule 2) implies rocket engines a bit behind CG, which means you need something sufficiently heavy ahead of cg to balance your ship.  It can't be fuel or cargo as the plane will become uncontrollable when empty.  Maybe your jet engines can be mounted on the sides of the fuselage towards the front - looks ugly but hey, if it works.   If your space plane has a large mass in non-removable payload - science bay, hab module, passenger cabins, ISRU gear - you're in luck, they can go at the front to balance the engines.

5) Rules 1) and 2) imply a lot of parallel fuselage sections increasing drag/frontal area,  and requires splitting the cargo bay into a forward/aft compartment, which must be loaded evenly, since the prime real estate at your CG will all be taken up by rocket fuel tanks.

Locking tanks isn't user friendly imho, or you're a much better pilot than me.    Rocket engines go through their fuel FAST,  the CG changes happen too fast for your to trim them out,  and when the rockets are burning, everything is going to be happening so fast you'll never get the time to micromanage fuel transfers and tank locking.

I get the impression most folks just work around that by locking the nose angle with SAS while in the rocket boost phase freeing up the player to manage fuel, and hoping the SAS doesn't run out of pitch authority to overcome the CG shifts that occur.   

Alternatively, install these lightweight mods

1. GPOSpeedFuelPump - can configure tanks to automatically balance in assembly building.  Tanks can be given a priority level too, for example you can configure wing tanks to pump their contents into fuselage tanks as these empty.  Real airplanes do this, i don't see it as cheating.

2. Tanklock, enables you to set action groups to lock fuel tanks. Again, real airplanes do this , it's not cheating.

3. Modular Fuel Tanks - you can swap the contents of tanks, eg. turn LF/O tanks into pure LF or Ox stores and not waste any tank space.  I have some house rules, eg. not putting xenon in something not designed for it (it's a pressurized gas), and not putting oxidizer in wing tanks (assuming it's cryogenic, wing tanks are going to boil off much faster than cylindricals)

 

Edited May 21, 2016 by AeroGav

[Pecan]

9 hours ago, AeroGav said:

...1. GPOSpeedFuelPump - can configure tanks to automatically balance in assembly building.  Tanks can be given a priority level too, for example you can configure wing tanks to pump their contents into fuselage tanks as these empty.  Real airplanes do this, i don't see it as cheating.

+1.  Great mod :-)

[Jarin]

20 hours ago, Snark said:

Locking the forward tanks is the easiest solution.  Another is to get rid of the big-heavy-engine-on-the-back (or, at least, replace with something much smaller) and add side-mounted engines much farther forward.

 

14 hours ago, AeroGav said:

[snipped, lots of good spaceplane info]

This is great stuff that I'll keep in mind for spaceplane designs, but it's only partially related to what I'm doing here. If you check that link in my question, the design idea is a strap-on booster that can make a stageless lift to orbit (the unattached booster "plane" has some 4200+ d/V) with two or more boosters that can fly back on their own once empty. That means no good way to replace the big heavy engine, and no jet engines involved at all. It also means that how it drains the fuel is less important because it's going to be ideally hitting the ground mostly or completely empty, and needs to be able to handle balanced gliding with no fuel onboard.

This is the most stable configuration I've managed to come up with so far. I moved the cargo bay with system components all the way forward and overloaded it a bit. Doubling up on the reaction wheels helps a bit too, but it still had a horrible lateral-instability problem (thus the extra vertical stabilizers on the wings). It'll sideslip like nobody's business still if I'm not careful, but basic turns seem better now. The final nosecone is kept full and locked, which costs me about 250 d/V, but it's there for emergencies, and that sacrifice makes it flyable.

Also important: the rear huge landing gear was originally attached to the bottom of the wings. That turned out to be a BAD IDEA, as the wings would inevitably snap on landing. Gear is now attached to the fuselage, then repositioned out into place with the move tool.

Edit: though it occurs to me... I could get some extra boost with a set of whiplashes running on fuel stored in the wings. Hmm....

Edited May 22, 2016 by Jarin

[Snark]

25 minutes ago, Jarin said:

the design idea is a strap-on booster that can make a stageless lift to orbit (the unattached booster "plane" has some 4200+ d/V) with two or more boosters that can fly back on their own once empty. That means no good way to replace the big heavy engine

Why not?  As originally suggested:  downsize the heck out of that engine, and add more engines amidships.

For example:

• drop the Mammoth
• in its place, stick a Mk3-to-2.5m adapter, with an inverted 2.5m nosecone to close it off.
• Up near the middle of the plane, attach four FL-T800 tanks on the left and right sides, above/below the wing plane.
• Four Vectors on the FL-T800s.

There you go.  More fuel storage, much farther-forward CoM, plenty of engine power.  If you don't like the idea of Vectors, maybe use aerospikes, and put a Mainsail on the back instead of the nosecone.

[AeroGav]

On 20/05/2016 at 9:29 PM, Jarin said:

I would also note, "CoM too far back, don't know how to bring it forward" nicely encapsulates 90% of my trouble with spaceplanes in general, so I'd really appreciate any tips people can offer.

I don't know if that was your intention but you just made a replica of the British Aerospace HOTOL.   Unfortunately the HOTOL became unstable as it's fuel burned off so most successful designs look more like a Skylon.

Here's something I built in an hour,  the needs of CG and the stupid way rocket fuel is drained create a rather fat ship. 

 

A single mk3 rocket fuel fuselage long is our only rocket fuel tank, it sits right in the middle of the ship with the CG marker dead centre.   I left the tapering adapter on the rear of the fuselage empty, but maybe it should have been filled.   There's three light, efficient Terrier motors on the back.   Two thuds attach radially to the massive fuel tank near the front, counterbalancing the weight of the Terriers.   Also  near the front are 4 whiplash engines, which draw fuel from the wings.  Yeah biplane.  Sorry but none of the stock wings are big enough for efficient high altitude flight on something this size.

I brought far too much jet fuel and the maiden flight was clumsy, lots of porpoising.  But it did at least get to orbit.