How to do this with Action Groups...Now with pictures!

[Darkeldar]

I've been flying my spaceplane with Sabre S engines, and I trying to come up with how to make action groups to do what I need to do.

Before takeoff I shut down the Oxidizer flow on the LFO tank, but action groups does not give me this option, and I have to manually turn the flow back on in flight.

When I reach 20km altitude, I want to switch the engines from air-breathing to rocket mode, will this process take more than one group? Do I have to shut down the engines during conversion?

I assume I just create another group for the return to air-breathing?

At least I found out why I was having engine problems at takeoff.

Edited October 15, 2013 by Darkeldar

[Sirine]

Normally people accomplish this by using "fuel line", else, Alt-Click on both fuel tanks to manually transfer fuel does the same thing.

or, fuel line in both direction, in + out, this make sure that all fuel flow are free.

And for the Action Groups...they can't be done.

[Darkeldar]

I already have a separate fuel line from the LFO tank, so getting oxidizer to the engine isn't a problem, but turning it on when I change engine mode is what I'm trying to accomplish.

What do you mean action groups can't be done? Are you saying that I can't have the engines in two AGs, Stage and a Custom?

[RabidMonkey]

Add a separate, ejectable LF-only tank with a fuel line running to the LFO tank. When the SABRE is in air-breathing mode, it will pull LF from the LFO tank which will be replenished by the LF-only tank. Then, when you switch to rocket mode, you should still have a full LFO tank for burning, and you can jettison the LF-only tank as unnecessary weight.

[solidgeoff]

I'm scratching my head over why you're shutting down the oxidizer flow at the beginning. If you're running your sabres on airbreathing mode, they shouldn't draw from the oxidizer. I usually put a LF only tank somewhere on the ship. The previously mentioned drop tanks always work well on a decoupler.

To answer your action group question directly, you can absolutely switch from air breathing to rocket mode at 20km. Use the "Toggle engine mode" option, and make sure to group them all under the same action group. Afterwards, hitting the same button will toggle them back to air breathing.

Sometimes, I have a spaceplane that has both turbofans and sabres. In that case, I will put "toggle engine" for the turbofans (turning them on or off) in the same action group as the "toggle engine mode" for the sabres. in this scenario, my turbofans turn off and the sabres switch to rocket mode.

Finally, I often will put engines on an action group that shuts them off in case of some emergency, like a flameout. I might put them all on "toggle engine" on action group 9 or something.

Hope that helps. In sum, you only need one action group to change from air breathing to rocket mode.

[Hodo]

TAC Fuel Balancer will solve all your problems.

Otherwise it can't be done in the stock game.

[Darkeldar]

I'm using B9 currently.

So air-breathing mode will not draw oxidizer from a LFO tank, and no I don't want "drop tanks", though it sounds like I could drop 1 pair of fuel lines as the engines would draw liquid fuel from all the contiguous LF tanks.

So all I need to do is to put the mode change into a custom AG, and the spaceplane will handle the change-over. If this works, I'm going to rebuild the Spike with a larger LFO tank.

And I had a chance to try it at lunch, so I took Spike 3 up to 19k, did the mode change AG and transitioned back before I ran out of oxidizer. I stopped the flight there as I wanted to verify that I only needed one set of fuel lines.

Thanks!

Edited October 15, 2013 by Darkeldar

[solidgeoff]

Feel free to post a picture for further critique, Darkeldar.

[Darkeldar]

Well here we go....Spike 3, my kerbalized SR-71 or A-12

I did get rid of one pair of fuel lines as all the liquid fuel tanks are contiguous.

Edited October 15, 2013 by Darkeldar
added pictures to thread

[solidgeoff]

Looks good to me. You shouldn't have any issues with the action groups. does the craft have any way to restore electric power? Also, depending on your goals, all those liquid fuel tanks may be excessive, if you're just trying to get to space and back. Some of the B9 parts have good "empty" fuselage parts.

[Darkeldar]

You can look at 3.5 here http://forum.kerbalspaceprogram.com/threads/53011-Spike-3-5 where I shorten her takeoff run by adding a pylon to the nose gear. Got her up to 1189 m/s, and comfortably over 22K with adding the radial air intakes.

But yes she's a big fuel tank, and when I build Spike 4 she'll be carrying more oxidizer. So far (and it may be that I haven't been in rocket mode that long) I haven't had any electrical issues. I currently don't have any plans for making her SSTO but I'll see about removing some fuel as I know she has endurance, as I've only used less than a tenth of her fuel for my test flights.