Balancing Large Spaceplane with LARGE CARGO BAY!

[ajburges]

Holy cow! What are you trying to lift?

I'm becoming more convinced we need a better way to quantify drag in KSP. My recently retired heavy lifter could get payloads just shy of an orange tank to orbit (without stealing fuel from the payload). It only needed 8 turbojets, 1 Mainsail, and about 95 t of fuel to do so. My new design has 8 RAPIERs, 4 turbojets, 1 Skipper, and about 134t fuel. The new design has a smoother (and maybe faster ascent), better OMS efficiency, greater RCS thrust, but I don't know if payload will be able to hit orange tank. The craft is still in testing. Both had larger cargo bays. (In my mind cargo bays are to deliver modules, not fuel.)

My problem with heavy lifters is I see some noticeable slow downs. Even with sub 150 part missions and the larger wings. Controlling orbital inclination is a real core when the game slows down.

Edited July 30, 2015 by ajburges

[Nich]

I am going to have to go with drag is king for SSTOs. The shock cones are way better then the ram intakes and I really like the precooler. 1 shock cone and 1 precooler to feed 2 engines. I am torn on lift but generally more is better. I used 10 lift on a 10.9t SSTO and got the same results as 3 lift on an 8.6t (same craft different wings)

[ajburges]

I am going to have to go with drag is king for SSTOs. The shock cones are way better then the ram intakes and I really like the precooler. 1 shock cone and 1 precooler to feed 2 engines. I am torn on lift but generally more is better. I used 10 lift on a 10.9t SSTO and got the same results as 3 lift on an 8.6t (same craft different wings)

I really don't get the precooler hype. Shock cones give me enough air as long as I balance placement and it's been ages since my intakes were the critical thermal part. My large planes can't handle any more parts as is.

More lift is just more parasitic drag. My terminal velocity at 10° is subsonic. It's supersonic at 0°. The worst wing loading is at takeoff and initial climb. You only need enough to manage that well. V² lift will work fine once you go supersonic and most space planes are lightest at landing.

Edited July 30, 2015 by ajburges

[selfish_meme]

Way too many engines, if the Stock Payload Fraction challenge taught us anything it's that too many engines is bad. Nich is right that drag is king, my testing shows also that the pre-coolers are not worth it. You can get a 36.8t orange tank cargo to orbit quite easily with 4 Rapiers and a Whiplash, see my craft here . Use shock cone intakes (with small nose cones behind the rapiers), use less engines even if it takes longer to ascend, with Rapiers you need to hit 400m/s as fast as possible off the runway, then slowly ascend until you reach 10km where you should now exceed 1000m/s (reach it too soon and you risk overheating) and you should be pushing past 1200m/s by 20km. Never pitch above 20 degrees and usually keep it around 10 degrees until you leave atmosphere.

- - - Updated - - -

I really don't get the precooler hype. Shock cones give me enough air as long as I balance placement and it's been ages since my intakes were the critical thermal part. My large planes can't handle any more parts as is.

More lift is just more parasitic drag. My terminal velocity at 10° is subsonic. It's supersonic at 0°. The worst wing loading is at takeoff and initial climb. You only need enough to manage that well. V² lift will work fine once you go supersonic and most space planes are lightest at landing.

He's using only 1 intake and 2 pre-coolers for 2 engines, so the pre-coolers add enough air to feed a second engine off the one intake. I would still use 2 intakes and ditch the pre-coolers entirely, not sure without testing which would be more efficient.

Edited July 30, 2015 by selfish_meme

[Vegvisir]

I like these threads, they're interesting. Spaceplane enthusiasts, unite! I have a long way to go I think....KSP can be hard :/ but very fun

[Wolf Baginski]

One thing I have found is that shifting some fuel forward while in flight can help

Right-click on tank-1

Alt-Right-click on tank-2

The info windows will sprout buttons to deal with fuel transfer.

This can also remedy a cross-feed error, if you still have fuel left when in orbit.

It's not a cure-all, and it's easy to get nose-heavy. One thing you can do to check is empty a tank while in the SPH and see what that does to the balance.

Air-breathing engines will drain fuel from all tanks. Rockets drain fuel and oxidiser from the furthest away tank first. If you're using one of the fuel-switcher mods you could put fuel and oxidiser in different tanks. Maybe you have room for a small oxidiser tank as ballast, maybe even an oxidiser-tank nosecone. But what happens on re-entry? It would be no help on trim if you've used all your oxidiser.

On a practical note, I use Action Groups to toggle intakes and multi-mode engines on the same key. Though things can get awkward if you used a Stop command on only part of the set. Maybe set all the engine on automatic switching, and let them switch when intake-air stops.

[ajburges]

The beauty of space planes is that being noise heavy only cuts your control authority. Move some fuel back for more pitch control. Being tail heavy OTOH is a recipie for a RUD.

I often disable fuel flow of my forward tank so that my craft keeps balanced for the first few seconds of orbital burn. Makes my craft much more manageable as I move fuel from the wings to the body.