Help with FAR SSTO Please

[Kurnoc]

So Im fairly new at the game, after making my first successful Duna trip I decided to start fiddling with SSTOs, mainly spaceplanes. Ive read as much on the subject as possible, including most of the Kerbodyne thread and tried to follow all the guidelines that seem fairly consistent through out(well accept the CoM in front of the CoL, Im a pilot IRL and that is just no bueno..) The plane can get to orbit, but it has a tendency to start wobbling all over the place at random. It also has a very odd seesaw motion when it first loads onto the run way.

All I want the aircraft to do is reliably get to a 90x100 orbit and deploy one of the micro sats I use, the sats weigh about a ton and Ive been using the large Rockomax solid booster to get them to orbit, but those cost more then the cost of fuel and to circularize the orbit around the planet the sats have to use their own fuel, which really bites into the mission profiles their able to run, theyve only got about 2.5k Dv on board and my piloting skills are meh x.x

[eddiew]

Obvious starting place; bring up the FAR window, check the stability calculations at the following altitudes and speeds:

0km and mach 0.3

10km and mach 1

20km and mach 2

25km and mach 4

...and see what's red

My gut feel is that you've got a lot of lift at the front and a very small tailplane, and that it's giving you pitch instability - but the analysis window is the way forward!

[Wanderfound]

Another tailed delta...

From the look of it, you quite likely have too much wing up forwards. If you post screenshots of the analyses suggested by eddiew, we can get into more detail.

[Kurnoc]

Sorry Wanderfound, Deltas are pretty, and if I wanted to fly a traditional plane Id just go do it x.x The aircraft has tons of pitch authority, It doesnt really need flaps so the flaps are set to provide 20 to pitch and roll, theres also a set of elevators between the engine nacelles and the main body. Other then a slight bit of sideslip around 10k its pretty much green thru out, but I get this weird circular oscillation the entire flight, if SAS is off it just barrelrolls the whole time and crashes, like I said in my first post I can get it into orbit about every other try, Im sure my meh piloting skills are partly to blame, but Id rather like to get the plane good and then just use it to practice until my skills are up to par.

[eddiew]

If I'm honest, I expected more red

We can see a bit of yaw at the 10/1 checkpoint, which you could fix with a bigger tailfin, or ignore because you're probably going faster than that by then anyway, what with your high TWR. You've got a bit of a pitch issue at 25/4 which could do with some attention since this is your run-up to space; either nudging the wing back, or expanding the tailplane a little would probably show some effect here. If the plane's dynamically stable, then you (and your SAS) don't have to fight it so much, and you should experience less wobble.

If the SAS is causing oscillations, you may find you've got too much authority; sometimes turning down the flap deflection actually helps smooth this out. I find a lot of mine are jumpy below 10km in the thick atmosphere, but then smooth out as the air thins. As another option, an SAS tuner such as Pilot Assistant can be useful - bump up the scalar value by a few multiples for the axis that's giving you grief, and it may well settle out.

Edited April 10, 2015 by eddiew

[Wanderfound]

Get the SAS PID tuning sorted first, but the other possibility if it's still troublesome: body flex. Get some struts in to reinforce nose and tail, and try mounting the horizontal stabilisers to the engines instead of the tailfin. You can offset them back to the same position if desired.

[Van Disaster]

Going to get quite twitchy with CoM & CoL in close proximity - I always read that guideline as "don't move CoM aft of CoL unless you're prepared to manage the sensitivity", I usually put them on top of each other anyway - you might want to fiddle with the control surface deflection for various sets somewhat because generally when you ask for a deflection you'll get it pretty much instantly. You might want to try Pilot Assistant, it's a more tunable SAS replacement.

Try taking some fuel out & rerunning the analysis too, CoM is going to wander around as you burn...

[capi3101]

Can't offer much that these guys haven't already; I would suggest installing a mod called RCS Build Aid, though, especially if you're planning to do a lot of future spaceplane flights. It'll show you how much and in what direction your fuel load is shifting in flight, which is going to be an issue until 1.0 is released. Just as useful as KER, IMHO.

Since your plane is barrel-rolling with SAS off, it could be that the CoM is shifting upwards, above the CoL, as the tanks drain. Just a thought - these guys would tell you if there's any validity to it or not (I'm a FAR rookie, and not a pilot IRL...).

[Van Disaster]

One thing that often happens is CoM gets above and getting close to behind CoL just because you're at high alt & pitched up a lot - combine that with a little shift due to fuel burn & you can get quite badly unstable just in that particular bit of the envelope. Unfortunately because you're at high altitude recovery can be pretty awkward itself. You can check for that sort of thing by pitching the plane up in the SPH & playing with fuel levels.

[Hodo]

Sorry Wanderfound, Deltas are pretty, and if I wanted to fly a traditional plane Id just go do it x.x The aircraft has tons of pitch authority, It doesnt really need flaps so the flaps are set to provide 20 to pitch and roll, theres also a set of elevators between the engine nacelles and the main body. Other then a slight bit of sideslip around 10k its pretty much green thru out, but I get this weird circular oscillation the entire flight, if SAS is off it just barrelrolls the whole time and crashes, like I said in my first post I can get it into orbit about every other try, Im sure my meh piloting skills are partly to blame, but Id rather like to get the plane good and then just use it to practice until my skills are up to par.

Ok looking at your planes information from the graphs there I can tell you some minor things that I noticed.

First your level flight speed at take off is REAL high, 102m/s that is about 200kts, which means your take off speed would be about 210-230kts. You being a pilot know that is FAST.

Next if you look at your last screen shot, of the graph you are looking at 25km ASL, at Mach 4, and your level flight is going to be 15deg AoA. which is quite steep for a delta wing aircraft.

Lets take this aircraft for example, I wish I had taken pictures of the FAR readouts on it.

But it is simple delta wing design with a tail, much like a Mig-21. It has a high take off speed of about 90m/s when full and no flaps, but with flaps it actually reduces down to about 75m/s.

But this craft, actually had a 70m/s take off and a 60m/s landing speed.

Notice the amount of wing in comparison.

And I not only second, but cant suggest more that you get the PID tuner it is the best thing ever besides TAC fuel balancer.

[Van Disaster]

And yet that "high" takeoff speed is around a real-life F-104, let alone whatever we're going to end up with for a real HOTOL spaceplane, which looking at Skylon and what our govt are proposing for spaceport runway length is going to appear fairly ridiculous, I think. We don't have ground effect either... nothing wrong with high takeoff speeds.

F-104 landing speed ~210mph. Shuttle ~220mph.

[Hodo]

And yet that "high" takeoff speed is around a real-life F-104, let alone whatever we're going to end up with for a real HOTOL spaceplane, which looking at Skylon and what our govt are proposing for spaceport runway length is going to appear fairly ridiculous, I think. We don't have ground effect either... nothing wrong with high takeoff speeds.

F-104 landing speed ~210mph. Shuttle ~220mph.

And yet a F-16 takes off at about 148mph.

The F-104 was a manned missile. The space shuttle is a brick with wings.

I admit many of my designs have a large wing surface area but this is because I believe in simplicity in design and ease of control in all aspects of flight. No craft does anyone any good being a handful to fly or land.

[Van Disaster]

Yeah, but as spaceplanes we're basically building missiles - on the way up it's competing with rockets, it's on the way back down again you want it to actually fly. On the way to space the wings are there to minimise gravity losses and the powerplant is airbreathing because we're spending long enough at low altitudes that it's worth doing - the only maneuvering you're likely to do is to turn to align with your orbital insertion plane. Compared to optimising mach 4 high altitude flight - which really would want either drastic sweep or just minimal span - takeoff speed is really not worth optimising, I think. On the way down again wing loading is way less due to lack of propellant load so that will bring landing speeds down, thankfully.

[Guest]

Didnt read the whole thread

Couple of tips:

- remember that with FAR enabled, ALL parts will produce drag and lift dpending on their orientation to 0 AoA.

- Stock ksp nodes joint force are too weak for FAR aero stress, so you need to add lot of struts between tank parts. Or download the joint reinforcement mod by ferram4 too. You can also put struts between the tip of your wings and the tank body and adjust their weight strategically

Edited April 11, 2015 by RevanCorana

[Kurnoc]

Thanks very much to both eddiew and Wanderfound, the SAS thing fixed the wobble pretty much 100%. Still a tiny bit right after take off but thats probably just the tailstrike wheels popping off the ground and it adjusting/catching up to it.

@ Hodo Yes my take off speeds a bit on the high side, but the only time Im worried about that is when I run outta runway. This aircraft gets off the ground before it reaches the hangar. When it comes for landing i actually think I have too much wing, this thing will glide for 30 minutes+ at low altitude if I dont pop the chutes x.x Its also extremely stable at those low speeds/altitudes. Ive actually been thinking of dropping the front strake to try and save some weight and maybe cut some of that glide out...