Space Plane Help

[SelectHalfling0]

Hey all! I posted a spaceplane question a while ago (when .25 first came out) and have another one for you!

I usually fly my spaceplanes in the upper reaches of the atmosphere horizontally to gain near orbital velocity, and then rapidly pitch up to gain altitude.

When I do this my spaceplanes tend to uncontrollably yaw left or right randomly, before spinning out (fortunately manually controlling the plane brings it out of this "death yaw").

Anyone know how to fix this? I am using an unmodded install, with mostly the new spaceplane parts.

-SelectHalfling0

Edited November 8, 2014 by SelectHalfling0

[LaytheAerospace]

Asymmetrical flameout?

[Wanderfound]

It's either asymmetrical thrust due to oxygen starvation or inadequate yaw stabilisation. For the first, throttle down or turn off some engines. For the second, moar tailfin or some flank-mounted Vernors.

[Geschosskopf]

You might also want to lock the gimbal of the jet engines. I've found leaving it free contributes greatly to spaceplane instability.

[RexKramer]

I think some more information would be helpful to answer your question.

1. How high are you?

2. How many jet engines?

3. A picture of your plane would be helpful.

Lacking the required information, I'm assuming you are using a spaceplane with at least two jet engines. So, you are probably flaming one engine out, which causes the plane to yaw right or left, depending on which engine flamed out first.

There is not really a good fix to eliminate asymmetrical flameout (in stock KSP at least). One solution is to cut your jet engines prior to flameout, and switch to rocket engines. You can delay the flameout by reducing thrust at altitude, but eventually you are going to run out of atmospheric oxygen.

You might also want to reconsider your flight profile. Pitching up at the top of your climb is not really an effective way to reach orbit.

A different approach would be to accelerate horizontally, getting as fast as you can at the highest altitude which does not cause flameout. I typically achieve 2400m/s orbital velocity at 41,000m. At that point, I shut off the jets, and activate the rocket(s). At that point, a small amount of rocket thrust will push the AP above 72,000m (if it isn't already). There is no need to 'pitch up', just continue burning prograde.

At AP, another short burn from the rocket(s) will circularize the orbit.

Also, not sure how KSP handles this, but jet engines typically only work well with the air flowing straight into the intake. 'Pitching up' is going to reduce airflow into the intakes, making your flameout possibly occur at a lower altitude. To see this effect clearly, fly level at about 10,000m altitude. Pitch your aircraft up (or down) so it is pointing away from the prograde marker. Watch your intake air as you do this- you should see the intake air indicator drop off significantly as you point the nose away from prograde. Rockets don't really care which direction they are pointed, but jet engines do.

[Yakky]

In the very upper reaches of the atmosphere, there isn't much air to push against. Suddenly upsetting your attitude via rapid pitch changes might just be more than your control surfaces can recover from. An enormous tail is one option, but an easier fix might be to just add some thrusters and monopropellant (or Vernor engines) to help you maintain stability during that transitional regime. It could be one of those rare cases where you want to have reaction-control (R key) and SAS (T key) enabled at the same time. Probably would only be necessary for a very short period of time until you get completely out of the atmosphere.

BTW I also agree with the poster above. A sudden pitch-up may not be your best move (although to be fair, I've used this approach a few times myself). A very slight pitch-up or no pitch-up, combined with cutting over to rockets to just get your speed up a bit more, should be enough to propel you up out of the atmosphere and might actually be more efficient.

Edited November 8, 2014 by Yakky

[Empiro]

Are you in FAR? In FAR, I generally don't pitch up so much that it says there's a large scale stall or large sideslip. I also turn on both SAS and RCS when switching to rocket engines to give myself greater stability and control.

[SelectHalfling0]

I've got a pic (albeit bad) here: 2 rapiers bring it to near-flameout altitude (20-25 km), and it rides horizontally until it reaches 1400 m/s, and then it pitches up, bringing the rapiers into rocket mode. Once it reaches rocket mode the problem disappears. I would like to note that the COL is slightly in front of the COM. This has not caused major problems for past vehicles though.

Also, the 2 wingtips on the main wings are no longer there. That change was made after I took this image.

No FAR or NEAR. Just stock.

Edited November 8, 2014 by SelectHalfling0

[Hodo]

Yeah it is an assimetrical flame out issue. One of your engines is suffering from air starvation and losing power before the other, this is causing your craft to push to one side and spinning your craft out. The easiest way to fix this is to throttle back as you get higher and keep an eye on your air flow to each engine. The other option is to download either of these to mods...

Mechjeb,

or Kerbal Engineering Redux (KER)

Both provide HIGHLY useful information like air requirements for your engines.

[SelectHalfling0]

Luckily, I already have KER. Thanks!