How do you transition from jet to rocket in a spaceplane without spinning?

[Dunbaratu]

With any sort of spaceplane design, you come to a point during the flight where the air thins out to the point where you need to turn off the air-breathing engines and switch to a rocket engine.

I have never been able to do this without either spinning uncontrollably at the moment of transition, or leaving the engines off long enough that a large amount of the vessel's momentum is lost to drag during the transition period.

If I wait for the engines to flameout on their own in a twin engine plane, they generally don't flameout at exactly the same time as each other, leading to uncontrollable spin from the few seconds of uneven thrust when one engine was on and the other was not.

If I hit 'x' to kill the engines first, they take too long to spool down to zero and I lose too much momentum waiting for that.

If I try to kill the engines just before the flameout, by watching the air intake's rightclick menu and as it starts getting dangerously close to zero using a pre-assigned action group that I assigned to the "toggle" action on the engines when hitting ALT-1, then only one of the two engines turns off when I do it! Even though the two engines are built with symmetry and are the "same" engine as far as the action group tab is concerned, only the left engine actually toggles off when I use ALT-1. Thus, uncontrolled spin results.

It's one of the infuriating things about spaceplanes and it's the reason I never use them despite having over 1000 hours in the game. I think if this was solvable they'd be usable. And clearly it has to be solvable because other people use them. I just don't understand how they're making the two jet engines cutoff at the same time to avoid the spin.

Edited February 1, 2014 by Steven Mading

[Kasuha]

All my spaceplanes up to now (not that many of them) are capable of getting orbital speeds in upper atmosphere. So I fly at ~40 km, raise my apoapsis to ~150 km, coast to apoapsis and circularize there. It gives plenty of time to do any switching.

I also usually use designs with odd number of jet engines (3 or 1) and switch off all but one engine in higher atmosphere. That solves any problem with unbalanced thrust.

To effectively switch engines, use action groups. One key to activate your rocket engine, one or more keys to activate/deactivate your jet engines. Deactivating a jet engine makes it to spool down immediately too, getting rid of a lot of problems.

[Dunbaratu]

All my spaceplanes up to now (not that many of them) are capable of getting orbital speeds in upper atmosphere. So I fly at ~40 km, raise my apoapsis to ~150 km, coast to apoapsis and circularize there. It gives plenty of time to do any switching.

How on earth do you get them to have air above about 20km? I've tried additional intakes but that is bugged because they don't spread to the engines correctly and only one of the engines gets all the additional air when you do that.

I also usually use designs with odd number of jet engines (3 or 1) and switch off all but one engine in higher atmosphere. That solves any problem with unbalanced thrust.

Where do you fit the rocket that is needed? At some point you do need one, to lift periapsis after you get to your apopasis. I was trying to put the rocket in as the middle center piece.

To effectively switch engines, use action groups. One key to activate your rocket engine, one or more keys to activate/deactivate your jet engines. Deactivating a jet engine makes it to spool down immediately too, getting rid of a lot of problems.

But the action groups aren't toggling both the two engines in a symmetry group like they should. They're only toggling one of them. This is the solution I was trying but that problem made it not work as a solution.

[Rhomphaia]

But the action groups aren't toggling both the two engines in a symmetry group like they should. They're only toggling one of them. This is the solution I was trying but that problem made it not work as a solution.

It is a known issue that if you assign an action group to a symmetry grouped part, then later remove and replace the part the action group only acts for one. Setting up action groups should be the final stage of the build.

[uui8457]

But the action groups aren't toggling both the two engines in a symmetry group like they should. They're only toggling one of them. This is the solution I was trying but that problem made it not work as a solution.

In my action group for transisting I turn off the Jet engines, close all the intakes and start my rocket engine(s)

When closing all the intakes at the same time, the engines cut off immediately and doesn't make my plane spin, no matter what.

Are you sure you have both of the engines in your action group? If you for example attached them one-by-one without symmetry and then clicking on just one of them for the action group, only one will of course acctually turn off.

Hope you get it sorted out!

[Dunbaratu]

Hmm. I just did an experiment and it turns out I only get the "one engine toggle" problem when I build the craft in this order:

1 - Turn off bi-symmetry.

2 - Attach fuel tank to left side of plane (getting only a tank on the left side because symmetry is off)

3 - Attach air intake to fuel tank (getting only a tank on the left side because symmetry is off)

4 - Attach jet engine to fuel tank (getting only a tank on the left side because symmetry is off)

5 - click action group tab.

6 - click jet engine.

7 - assign "toggle engine" to action group 1.

8 - Turn on bi-symmetry.

9 - Detach original fuel tank from left side and reattach it now that symmetry is turned on - causing it to clone the entire tree of parts from there on down and make a copy of it on the right side.

That may seem like a weird way to do it but it's sometimes necessary because it won't let you attach parts, thinking there's phantom collisions with other parts, unless you build the entire structure one-sided and then clone it later.

If on the other hand I build it in THIS order, it works:

1 - Turn on bi-symmetry.

2 - Attach fuel tank to left side of plane (getting a matching one on the right side too because of symmetry)

3 - Attach air intake to fuel tank (getting a matching one on the right side too because of symmetry)

4 - Attach jet engine to fuel tank (getting a matching one on the right side too because of symmetry)

5 - click action group tab.

6 - click jet engine.

7 - assign "toggle engine" to action group 1.

But unfortunately it's not always possible to build it that way because of the "refuse to let you attach parts symmetrically that you CAN attach one at a time" bug.

Edited January 31, 2014 by Steven Mading

[Dunbaratu]

It is a known issue that if you assign an action group to a symmetry grouped part, then later remove and replace the part the action group only acts for one. Setting up action groups should be the final stage of the build.

That sounds like the problem I'm having. I've just never encountered it before. The strange thing is that even when I tried re-assigning the action group after symmetrizing it still has the problem. I have to delete the jet engines entirely and re-attach new ones (thus clearing off the previous action groups for it) before I can get proper symmetry from its action group. If I just go into action group 1, remove the action, and then re-assign it, with the engines still there, they still lack symmetry in the action groups. Only removing the engine any putting a new one one in its place seems to make it "forget" the buggy data and allow the action groups to have symmetry.

Once the game has gotten it in its head that the part's action groups should not be symmetrical, that mistaken notion stays true for that part from then on, even if you try to reassign its action groups after finishing the build. You have to throw that part away and go get a new part from the parts bin to break free from the problem.

[Kasuha]

How on earth do you get them to have air above about 20km? I've tried additional intakes but that is bugged because they don't spread to the engines correctly and only one of the engines gets all the additional air when you do that.

My last "plane" used 4 intakes and one engine. The trick is going very fast. Trying to get above 25 km if you're below ~1500 m/s is pointless. And the other trick is decreasing the throttle. As you already go at near orbital speed, you don't need lift or much thrust. And when the engine is at low throttle it can go with the little air it gets.

Some time ago I had a ship with six intakes and one engine and it was still able to "pulse" at 60 km. 0.23 resource optimizations made these things really easier, although they introduced other problems.

Where do you fit the rocket that is needed? At some point you do need one, to lift periapsis after you get to your apopasis. I was trying to put the rocket in as the middle center piece.

Rocket engines don't flameout one by one. So I mount them in pairs, symmetrically.

But the action groups aren't toggling both the two engines in a symmetry group like they should. They're only toggling one of them. This is the solution I was trying but that problem made it not work as a solution.

Already discussed as I can see - you need to either set it up as the last thing, or re-apply the setting after adjustments. Just open the action group, remove the action and put it back. That'll put both engines in again.

Edited January 31, 2014 by Kasuha

[capi3101]

Once I've opened up my intakes and my air starts getting down into the questionable zone (around 0.05-0.07 per engine), I start monitoring thrust levels from the engines. When I start noticing uneven thrust, I throttle back a little bit, and I keep this up until I'm down to about 1/3 thrust or I note that I've stopped accelerating. Only then do I light the rockets. I still keep the jets going at that point and continue to throttle back - at that point, the important thing is that you continue accelerating.

Throttling back avoids flame out.

[Dunbaratu]

Working around the symmetry bug seems to have made this a lot easier. It's working now and I got it into orbit. I still have no idea how to make intakes go to the correct engines so I can't make high altitude multi-intake setups work, but at least I can get both engines to shut off at once now.

I still have the annoying problem that planes flip over on their backs in utterly unrealistic ways but that's a later problem and a seperate topic. (Yes, I know I'm supposed to put the lift a bit behind the center of mass. I build balsa gliders as a hobby. That doesn't seem to be the cause of the flipping.)

Edited January 31, 2014 by Steven Mading

[Kasuha]

I think this picture tells the story. Notice the altitude, pitch, speed, and throttle level. The ship is still accelerating.

It's very light ship, but it can be done with bigger crafts, too.

Funny thing is, I found it most comfortable to do the piloting from map view. Because when you get to your accelerating altitude, you want to stop ascending or descending and it's easiest done in map view by keeping the apoapsis (later periapsis) in your close vicinity by adjusting the pitch.

[Kasuha]

I still have no idea how to make intakes go to the correct engines so I can't make high altitude multi-intake setups work

You can't. Intake air is distributed like electricity. The only way to ensure it is going to the correct engine is to have only one engine active.

By the way, the same problems are here with ion engines. If you don't have sufficient electricity supply, ion engines will cease to function one by one, regardless of their placing or symmetry.

[Rhomphaia]

Working around the symmetry bug seems to have made this a lot easier. It's working now and I got it into orbit. I still have no idea how to make intakes go to the correct engines so I can't make high altitude multi-intake setups work, but at least I can get both engines to shut off at once now.

Intake air is provided to the engines in order of placement, the engine placed last will be starved first. Even with symmetry the game counts one engine as being placed before the other.

If you have an odd number of air breathing engines you can work around the issue by placing the central engine last and taking your throttle cues from it.

With an even number of engines, you either need to cut the throttle before this becomes an issue or provide enough control authority to counteract the imbalance. (reaction wheels or a HUGE rudder)

[Dunbaratu]

Is there any way to force a jet engine to change the normal fuel depletion rules such that it tries to take from the backmost fuel tank first? I think my flipping over problem is happening because the front fuel tanks drain first and this moves the center of mass back as the fuel is used. I tried a yellow fuel hose going from the back tank to the front tank to try to replenish from the back to the font but that makes no difference. It always removes fuel from the front first. With a single engine design I can use the mouse to counter this and transfer fuel during flight with the engine on, but with two engines I cause asymmetry by trying to do that since I can only move one side at a time.

[Anton P. Nym]

Right click on the tank you want to reserve, then click the green triangles next to the fuel (and oxidizer, if there) bars; that shuts down fuel flow out of that tank. Now that Tweekables are in play, you can do that in the VAB/SPH and have it persist into flight.

When you need the fuel/oxidizer again, right click on the tank and re-enable flow by clicking on the red circle-bars next to the fuel bars.

-- Steve

[Kasuha]

Is there any way to force a jet engine to change the normal fuel depletion rules such that it tries to take from the backmost fuel tank first?

There is a lot of options if you understand how fuel distribution works. But what exactly is needed to do depends on your plane design.

[Dunbaratu]

Right click on the tank you want to reserve, then click the green triangles next to the fuel (and oxidizer, if there) bars; that shuts down fuel flow out of that tank. Now that Tweekables are in play, you can do that in the VAB/SPH and have it persist into flight.

When you need the fuel/oxidizer again, right click on the tank and re-enable flow by clicking on the red circle-bars next to the fuel bars.

-- Steve

That does it. Thanks. I'm not very familiar with many of the new 0.23 tweakables.

[vetrox]

Just want to quickly but in here.

A great tool that mechjeb has is a "prevent Flameout" tool. Although i dont use mechjeb much any more I found this to make piloting space planes alot easier as you dont have to worry about your throttle.

It also has an intake control option too that opens intakes when you need them.

Sometimes these little tools can make the tedious job of getting an ssto into space that little less troublesome. I find it takes a little bit of the guesswork out...in fact i mightr go redownload mechjeb just for this!

Another troublesome thing I find with space planes is sometimes at higher altitudes (and also sometimes at lower altitudes but i find this rare) that if you yaw whilst in flight one intake will suddenly pull your plane left or right and get a bit more air whilst causing another intake to have less thuss causing a flameout and spin.

Edited January 31, 2014 by vetrox

[BubbaWilkins]

Editing symmetrical parts after they have been assigned to action groups breaks the action group assignment and only applies to the root part.

1) All intakes contribute to the air pool and all engines draw from the pool. If you have an issue where only one engine is running, than you only have enough air to run one engine.

2) Intake air is a function of velocity as much as anything. you don't just climb to 20k meters and then run flat out. You level out between 10-15 and let your velocity increase as you slowly climb higher. I have many craft with only 1:1 intake/engine setups which can get above 20k without flaming out, but they are usually travelling 1500m/s before I get that high.

3) CoM and CoL are both dynamic. what works at takeoff and low altitude may not work all that well at high speed and high altitude. Even mass lost to just fuel burn can throw a craft out of balance.

4) Remember that control surfaces lose their control authority in the higher/thinner atmosphere. Make sure you have reaction wheels or RCS.

[Dunbaratu]

Editing symmetrical parts after they have been assigned to action groups breaks the action group assignment and only applies to the root part.

1) All intakes contribute to the air pool and all engines draw from the pool. If you have an issue where only one engine is running, than you only have enough air to run one engine.

This is very broken logic because unlike the other parts that use this resource distribution algorithm, jet engines can run partially effectively on partial air. This algorithm only makes sense when a part's resource requirements follow an all or nothing boolean logic, not when they scale across a gradient. When they scale across a gradient this algorithm guarantees uneven distribution any time the resource supply is between 100% and 0% of what is needed. I've found that adding air intakes LOWERS the effective altitude at which I can use jets because of this algorithm. I have to shut them down the moment the air intakes drops to 95% of the needed amount, instead of leaving them on up to about 5-10%.

I hope the devs don't classify this behavior as "correct". I hope they understand it's a problem.