Assistance troubleshooting my spaceplane

[mekasha]

Hello!

I've finally made the jump trying to made an SSTO spaceplane for deploying satellites cheaply. I've had some minor success, but have two main issues I'm hoping to get some ideas on:

• As you'll see from the attached screenshot, I had to put a reaction wheel in the middle of the plane. Without it, I can get up to around 19km, but then the plane tilts forward in a dive that I can't pull up out of until ~15km. With the reaction wheel, I can get to 19km, gain speed, then point up. I make sure to use fuel from the rear tanks first to keep my CoM forward, so I have no idea why I really need the wheel/why I get stuck in a dive I can't pull up from. (edit: Stock Aero, stock everything except Alarm Clock)
  
• I'm using an Aerospike, but it seems to maybe not be the best. Even with the 1.5 fuselage tanks, I barely have enough LF/OX to get to a 72km circularized orbit, with enough to then de-orbit after launching my probe. Is there a better engine? I tried an LV909, but it didn't seem to have enough thrust to actually get me out of orbit.
  

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Edited April 9, 2015 by mekasha

[TheXRuler]

It Sounds like your problem is that you CoM is shifting to far forward and therefore your control sufaces no longer have enough authority to keep your nose up once you reach the upper atmoshpere.

I tend to drain fuel from the front to the back, as control authority gets worse the higher you go. You could try TAC Fuel Balancer (I know it says 0.25 but don't worry about it, I have not found any issues so far and I use it all the time.

The area where the aerospike excells is not a very high ISP but a realtively constant ISP throughout all altitudes. One of the problems might be your ascent profile, it would help quite a bit if you could provide that (or some approximation of it).

[Starhawk]

If you are emptying the rear tank first, your CoM is probably moving so far forward that control surfaces and the cockpit torque aren't enough to overcome the tendency of a nose-heavy craft to dive down. The crux of the problem is shifting CoM.

For the next few weeks until 1.0 drops, this will continue to be an issue. With 1.0 fuel will be used evenly from all tanks in a stage, so this will no longer be a design constraint.

Your craft does not need the weight of the aerospike. For my spaceplanes in the 10-20 tonne range, I get almost all the way to orbit on the turbojet (up to 15-20 tonnes you only need one) and use two 48-7S engines for circularization and orbital maneuvers.

A very efficient profile is possible in stock aero. Flying only on turbojets, climb to over 10 km. Between 10 and 20 km you want to climb fairly slowly and get your speed up close to 700 m/s. Between 20 and 30 km, you climb very slowly, getting your speed to about 1600 m/s by 30 km. Still climbing extemely slowly, between 30 and 36 km keey building speed to around 2100 m/s surface velocity. At around 36 km, depending on number of intakes, your engine will become oxygen deprived. If you throttle back slightly, the engine will keep running and providing thrust.

At this point, you will have essentially orbital velocity even though you are still climbing through the atmosphere. It doesn't take much thrust to maintain velocity and get your apoapsis up to where you want. It's a slow climb, but if you have patience you can make orbit using only 30 m/s or so from the rocket engines. Of course, this will all change with the new aero in 1.0.

I would use a single turbojet, add more lift, and replace the heavy aerospike with small light OMS engines. The balancing part can be very tricky, especially with the cargo bay being on the CoM. I often try to place my tanks lateral to the CoM if I can't place them right on it. Also, keep the two ram intakes to feed air to the single turbojet.

Hope this helps.

Happy landings!

[GoSlash27]

mekasha,

your design looks like it should be fairly aerodynamically unstable, so your pitch problem is likely due to shifting CoM as the jet fuel tanks drain.

Yes, I'd definitely get rid of the aerospike. It's just not worth it in the scale of your spaceplane.

For that matter, a single turbojet is plenty for your needs, and easier to manage.

Scope out my tutorial here for more tips.

http://forum.kerbalspaceprogram.com/threads/102182-So-you-want-to-build-a-space-plane-(-25-stock)

Best,

-Slashy

*EDIT*

A single turbojet is adequate to achieve a 70 km apoapsis at nearly orbital velocity for up to 21 tonnes of spaceplane and a pair of 48-7Ss is more than plenty to circularize. It's just a matter of flying the proper profile, which is basically squeezing everything you can out of the engine.

Edited April 7, 2015 by GoSlash27

[StainX]

I always hate to recommend a mod for "stock people" but if you do a lot of spaceplane stuff, you might take a look at RCS BUILD AID mod.

Also... try to place your fueltanks/engines as close as possible to the center of mass... this helps a lot on having a stable craft. (skylon style)

[mekasha]

Thanks all, I added a few of the big wing sections per below, and was also much less aggressive on climbing. With those few tweaks, I was able to land on Minmus and return (though I did use the fuel from the probe for the Minmus trip). Landing on landing gear without atmosphere is definitely a bit different.

[capi3101]

Glad you were able to work things out. Since you're still relatively new to the realm of spaceplanes in general, I'll point you to '>my general advice for rookies. Keptin's guide is a must for all beginners, DocMoriarty's guide is good for specifics if you're planning on sticking with the pre-1.0 stock aerodynamic model for the time being. And should you decide to switch to a different aerodynamic model, there's always a chronology of my shenanigans in that area.

I would second StainX's suggestion of the RCS Build Aid mod and I would go so far as to call it an "essential" mod; if you can build your plane such that you don't have to worry about the CoM shifting in flight, you'll meet with a great deal more success. That mod gives you the tools you need to do that accurately - it tells you where your CoM will be when the tanks are dry, i.e. in which direction the CoM will shift and how far it will go.

Have you tried landing on Kerbin yet?