Jods list of aerodynamics issues

[Jod]

OK, so I was building an SSTO freighter for about a week now and here is what I picked up during this lengthy and painful process.

Flying an SSTO to orbit consists of 4 flight stages(assuming a plane with wings + lots of air intakes and jet engines for the initial ascent), during which you will mess up in many horrific yet very distinct ways:

1: Take-off.

2: Up to 10 000 m

3: 10 000 - 30 000 m

4: Up to space

1: During take-off there are 2 things to look out for:

-Lift center balance. It shouldn't be too far ahead of the mass center or the craft will nose up, then dive and crash. Too far back and you simply won't take off.

-Landing gear placement. It needs to be very sturdy, including what it is based on, some things just won't hold it. Should usually be placed so that the wings would have at least 3 grades angle pointing upwards, 5 is best. Also there's the problem of anything behind the rear landing gear clipping the terrain and exploding. So place them as far back as possible.

Possible issues:

=Can't compensate for the nose up momentum, goes into a loop and crashes: +CoL too far ahead of CoM

=Can't take off at all(could be any of the following): +CoL too far back of CoM +Wings are at less than 3 degree angle +Not enough lift

=Rear end clips terrain: +Landing gear placed incorrectly And possibly +Too much nose up effort, ease up on the controls

Preferable:configure the craft to take off of the runway, not halfway to the ocean. it is a good idea to do so and will sometimes make landing that contraption much easier.

2: The smoothest part of the flight where if you could take off less things can go wrong with good piloting. Then again:

-Control surface placement. Control surfaces will apply torque. Torque depends on distance from the CoM. Place control surfaces as far away from CoM as possible for best effect(along the craft, not to the side, unless you want to do barrel rolls like crazy). Don't overdo the amount of control surfaces or the craft will twitch badly.

Possible issues:

=Veering off to the left/right without an ability to compensate: +Tail fins! lots of them. possibly delta wings as tail fins. and don't forget control surfaces

=Twitching wildly, comes in two flavors: +Too much control surfaces, the smallest push results in a huge torque +If the whole structure is shaking you need more struts

=Falling into a roll/loop without an ability to stabilize:

Either: +You are a horrible pilot

Or +CoL too far from CoM, which if engines are active and aligned with CoM produces a nasty loop after engine thrust is changed.

Also: +Too many high drag parts in the front, move them behind CoM

If you took off without going into a loop the only other possible explanation would be +Fuel tanks too far away from CoM and drained unevenly

And sometimes: +Placed high lift control surfaces on the wingtips like a fool. Never place anything that far sideways.

=Rolling when trying to turn: +Only one tail fin with a control surface high up will do that to you, place at least 2, close to CoM vertically and as far away from it on the horizontal plane as possible.

Preferable: balance the CoL and the fuel tanks with CoM so that no nose up/down moment would be produced and an ASAS could handle this part of the flight without too much control.

3: Worst part of the flight, with increasing speed producing huge amounts of lift and drag, air density dropping and jet engines about to stop working.

-MOAR air intakes, you need as many as you can get. ram air intakes work well, radial for some reason didn't do so well for me.

-MOAR control surfaces. If it does not lead you into at least a small twitch at lower altitudes, it is not good enough.

-Finer CoL-CoM balancing, you will really need all the stability you can get

-It is important to understand that for most configurations at these speeds the nose-up/nose-down you are experiencing if trying to maintain heading close to prograde is not due to torque from balancing CoL, but probably from drag which is applied rather mysteriously to the craft.

-Balancing Center of Drag, there is no indicator as to where it is, but it is sufficient to place high drag parts behind the CoM

-Higher altitude allows for more velocity, reaching 1500 m/s on jet engines alone is possible at heights above 20 000 provided enough air intakes to keep engines active.

Possible issues:

=Nose-up/nose-down without an ability to compensate after switching engines/adjusting jet engine thrust/changing heading angle: +Not enough control surfaces, you need more at these altitudes Or +Misaligned engines, you will need them in line with CoM to get to space anyway so keep that in mind Or +Weird drag moment, you need to balance the high drag parts i.e. air intakes

=Jet engines don't have enough air: +Air intakes. you need them.

=Not enough speed gain, getting to 30 000m at 500-800m/s speeds: Either: +Not enough engine thrust Or +Too much lift. Yes, with too much lift you will actually lose horizontal speed in favor of vertical component. Can be circumvented by decreasing engine thrust or maintaining a very small angle to prograde provided your Drag center is behind CoM.

Preferable: at least 1200 m/s before switching off jet engines, it will make reaching orbit easier. and a decent vertical speed as room for error in the likely case of losing control while switching engines

4 Finally, with the worst behind all that's left is to gain height and speed to reach orbit.

-Air drag becomes minimal at about 40 000, at this point RCS becomes a valid control mechanism. Any use of RCS below 40 000 is a waste, if you can't control your craft without it below that point - go back to the drawing board, invest in control surfaces, do something.

-Consider rocket engine thrust to weight ratio as well as efficiency. Aerospikes are efficient but heavy and will usually not lift you up fast enough, so other engines as an additional boost may be needed. Mainsail is a beast and can push most freighters, even the Skipper can manage this with a little worse power to weight ratio but still lighter than 4 aerospikes, for smaller craft consider Rockomax 24-77 as boosters, as highly inefficient they may be, the gain in altitude and saved time makes up for it.

Possible issues:

=CoM not aligned with CoT, a nasty surprise to get when you got all that way up to space: +Just... don't forget this during construction

=RCS balancing: +Adjust for empty fuel tanks during construction, better yet - position fuel tanks close to the CoM.

Preferable: Efficient engines for gaining speed when above 50 000m when there is enough room for slow burn and good booster engines to make that room and push past the nasty 30 000-40 000 barrier.

Bonus: Landing

-Again, balancing Drag. Put the air intakes in the back and you'll be fine. Don't forget to open them during reentry and you'll be safe.

Upd: added info on Drag balancing, that's the last part I needed for a perfectly functional craft.

Upd: added some info on control surface balancing

Upd: added miscellanious info

Hope this wall of text will be helpful to somebody.

All these horrible things happened to me and keep happening to me on a regular basis, none of the characters and events in this post are fiction and should resemble real issues anyone can come across.

Most of this post was written during KSP 0.20, but contains general physics that will probably be useful for years to come unless the space kraken invade Kerbin and all our base will belong to them.

No kerbals were harmed during the production of this craft. All test models were probe-controlled and the final product has zero casualty rate up to date on its extensive duty as a Kerbin-LKO SSTO freighter.

Edited September 2, 2013 by Jod

[Nodrak]

I have found that Centre of Thrust placement is far more important then Centre of Lift Placement. The lift position seems like it really only matters with takeoff. Many of my heavy designs use the end of the runway to get their Angle of attack needed for liftoff. The main reason for this appears to be at way that wings cause lift, which doesn't seem to work like it should. Also, as CoM will change from fuel burn, it makes it impossible to balance over the course of a flight without variable geometry wings.

Until wing lift works a bit more like you would expect, space planes are really just horizontal rockets that glide...

[Jod]

as CoM will change from fuel burn, it makes it impossible to balance over the course of a flight without variable geometry wings.

True, I have problems with that during reentry, but there are workarounds for that. Placing most fuel tanks near the CoM helps. As well as pumping fuel to forward tanks if you have heavy engines in the back.

Also, you haven't mentioned the Drag force. If the craft has a lot of air intakes, it is possible to bind them to an action key to close/open when additional stability is needed.

Wing lift indeed is completely inaccurate in KSP. Don't even get me started on this one, I had an exam on aerodynamics recently.

[AddMeGamers]

Very nice stuff here

[kebabsoup]

Awesome thread! Thanks for sharing your invaluable experience and knowledge! I shiver at the thought of all the Jebediahs, Bobs and Bills who died in the honing of your piloting and engineering skills. But in the end it was all worth it!

I haven't designed the "perfect" SSTO and was stuck for quite a while in SPH (Space Plane Hell) trying to get a stable flying object. But redemption came to me suddenly in the form of some keyboard shortcuts! I realized you can use alt key (mod key) + QWEASD to finely trim your pitch/yaw/roll. I guess it kind of turns your keyboard into a joystick.

With this technique you don't need a "perfect" spacecraft to reach orbit. At take-off, this allows you to gently bring up your nose and avoid the frenetic headbanging syndrome. And during ascent you can easily keep the desired heading without straining your fingers and your brains too much. No mod required! All stock parts lol!

Thx to Scrogdog, I learned about trimming in his thread here >> http://forum.kerbalspaceprogram.com/showthread.php/32621-Learn-to-fly-in-KSP-with-the-stock-Aeris-3A!

[Dispatcher]

Thanks for the helpful information! I've used some of the principle you mention, and am now aware of more.