Drone SSTOs - Balancing the CoL and CoM with no cockpit

[WanderingKid]

So, having build plenty of SSTOs in the past, I decided for career to try building out a re-usable satellite for fun and profit. After half a dozen crashes in ironman mode, and having screwed around in my playground a while, I finally realize why I just can't get the blinking planes to balance.

The probe core on one end weighs in at .1t. The TurboJet at 1.2t. Run out the fuel and you've got a tail sitter and a plane that loves to flip on landing approaches. The cockpits, on the other hand, weight in ~1.2t, thus balancing the craft. Mk2's are twice the weight so you've got balance for your double Turbojet.

So, without significant mods (such as the one that lets you create weights), has anyone built a stable reusable SSTO drone plane? Other than fuel or cockpits, the only thing I can find that's roughly even weight is another engine. I'm trying to do this in under 30 parts due to upgrades. Right now, I'm thinking I can stick a FL-R25 on the front of it to balance off the wings, since I don't use monoprop and there's part limit concerns for things like fuel transfers and a 'last tank' up front.

Has anyone else done this with ease, and if so, can I ask what trick(s) did you use?

[Vectura]

For stuff like that I usually put the wings right at the very back, so that they are almost attached to the engine.

[WanderingKid]

You end up with a lawn dart that way at launch, though, due to the fuel sitting up front, no?

[GoSlash27]

You don't have to balance the weight, but rather the leverage. The mass times distance of the engine should be counteracted by a lighter mass at a farther distance for perfect balance.

Having said that, remember that the balance doesn't necessarily have to be "perfect" so long as it remains within limits at all times.

I'll post an example shortly.

Best,

-Slashy

- - - Updated - - -

This is a tanker I built back in .24.

[BigFatStupidHead]

I have often built such ships where the ship is centered around the engine. Fuel tanks get surface attached to that, and wings arranged afterwards. Sort of a flying wing arrangement.

[WanderingKid]

@Slashy: While I understand that you can fulcrum a little on one end to balance a lot on the other, the fuel still throws the CoM around a lot for me in comparison. With enough parts, I could use fuselages (or equivalent) to stick a balance point WAY out, but the 30 part constriction is really making me fight for efficiency.

@BigFatStupidHead: Hrm... I'm having some good luck with this FL-R25, but I'll swing back to that if I can't seem to get this to play nicely. I know where you're going with that though. I just tried a dual Engine on the wing arrangement to flip that idea over. It was functional, but asymmetric burnout was causing me serious issues. There's just no good way to monitor when you're about to hit that particular problem, and a third centerline engine to allow for symmetric burnout just puts the problem back into play.

[GoSlash27]

An example using .90

As you can see, the CoM hardly moves at all with changes in fuel or payload mass.

Think of it like a balance beam.

Best,

-Slashy

[WanderingKid]

Slashy, how are you handling Fuel Flow rules where it pulls from the front tank first in that configuration? Or do you make sure you calibrate to having JUST enough fuel?

[GoSlash27]

Slashy, how are you handling Fuel Flow rules where it pulls from the front tank first in that configuration? Or do you make sure you calibrate to having JUST enough fuel?

I calibrate them to have just enough fuel to do the job and return to KSC under power, but remember that keeping the fuel tanks in the CoM means that your CoM doesn't really go anywhere as the fuel drains because it lacks leverage in comparison to the static leverage of the airframe and engine.

I'll post a worst-case with the forward tank dry.

Best,

-Slashy

- - - Updated - - -

This is with no payload and the forward tank empty. IRL this situation would never occur, since the plane will have depleted most of it's fuel on the way up when it still has a full load of O2 and payload. Nevertheless, the CoM remains centered.

Best,

-Slashy

[Pecan]

So, having build plenty of SSTOs ... Has anyone else done this with ease, and if so, can I ask what trick(s) did you use?

Tail-sitters, VTVL. 100t in 59 parts, all sorts of SSTOs.

IF you must do it the hard way, the best way to balance things is to put the heavy, static, mass of the engines in-line with the CoM - ie; out on the wings, just like real planes do.

[BigFatStupidHead]

There is a trick to dealing with asymmetric flame out. When you add an engine (do not use symmetry), add all intakes for that engine. Then add the second engine, and all intakes for that engine. So, you might go: turbojet, ram scoop, ram scoop, turbojet, ram scoop, ram scoop. This is a handy mod that does just that for you.

[numerobis]

I try to put the fuel tanks level with the center of mass -- out on the wings. That way, as the fuel drains, the center of mass doesn't move.

[WanderingKid]

There is a trick to dealing with asymmetric flame out. When you add an engine (do not use symmetry), add all intakes for that engine.

You still get Asymmetric flame outs that way. The first engine to die flames out, and the air goes to the second one to feed it, which now has enough air. That technique keeps them alive longer, not symmetrically flame out. The difference is at high altitudes it usually, but not always, takes them both out. Try it with Jet engines ~20km, you'll see what I mean.

[numerobis]

But by the time you get the flameout, you've lost at least 80% of your thrust, likely more, so it doesn't hurt as much.

[WanderingKid]

I probably needed more than one circular intake/turbojet, but I was getting some serious flat spin problems at one point with dual engines on the wing and a 6.5t craft. I fear it's situational, as your torque and distance from the CoM can really throw you sideways if you place them WAY out on the wing.

[GoSlash27]

I probably needed more than one circular intake/turbojet, but I was getting some serious flat spin problems at one point with dual engines on the wing and a 6.5t craft. I fear it's situational, as your torque and distance from the CoM can really throw you sideways if you place them WAY out on the wing.

You *definitely* need more than one circular intake per engine, and you also definitely want your engines as far inboard as you can get them.

To minimize part count you're going to want to stick with one engine anyway, so you may as well stick with the in-line arrangement. You could hang tanks off-center, but it'll take a higher part count to make that happen.

Best,

-Slashy

[WanderingKid]

Finally got myself sorted out on this using the Monoprop tank up front as a balancer.

30 parts, including an antenna, KER module, Battery, 3 Solar Panels, and Thermostat.

[GoSlash27]

Finally got myself sorted out on this using the Monoprop tank up front as a balancer.

http://i.imgur.com/yDzPGNc.png

30 parts, including an antenna, KER module, Battery, 3 Solar Panels, and Thermostat.

Congrats!

Once you get the first one sorted out, the rest come easier.

Having said that... don't think "mass" for balance, think "leverage". Not only is mass "mass", it's also drag.

Best,

-Slashy

[WanderingKid]

Fair enough. I've done 30t cargo lifters as SSTO's before (haven't quite gotten the Orange Tank figured out yet, close though), but this little light thing with minimal parts kept throwing me for a loop. I just didn't have anything to spare to get the leverage.

[BigFatStupidHead]

Finally got myself sorted out on this using the Monoprop tank up front as a balancer.

http://i.imgur.com/yDzPGNc.png

30 parts, including an antenna, KER module, Battery, 3 Solar Panels, and Thermostat.

Nice work!