Need some help with a transporter spaceplane.

[capi3101]

Evening, all.

So, a few weeks before 0.25 came out I discovered DocMoriarty's KSP Space Plane Construction and Operation Guide and starting putting its principles into practice. Overnight I went from struggling with spaceplane design (a dozen or more flights into the drink or off to the side of the runway) to consistently successful designs.

So, I finally decided to go ahead and use the principles discussed in the guide to lift a payload. I had a probe design already - the Moneymaker 7 - which I was launching conventionally to fulfill FinePrint contracts; the probe is 12.21 tonnes. I've learned that 25% is a good low-end payload fraction for a transporter spaceplane, so I set about designing one using DocMoriarty's principles for a plane that weighed 48.84 tonnes (i.e. four times the mass of the payload. The result was the Moneybroker 7:

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Craft File (150 kB)

This is the "Mk-II" because I managed to corrupt the original craft file and had to redesign the plane from scratch.

I've tried to follow the design guidelines as best I can. 48 tonnes, so four RAPIERs. There are a total of twenty Delta Wing/Wing Connector As plus a Cargo Bay - looking at that just now the design apparently is a little light on lift (the numbers say 22). 6 ailerons, 6 elevons, 6 elevators, 5 rudders and a pair of canards. Five Reaction Stabilizers in the cargo bay. Fuel requirements in the side tanks are as close as I can get them. The plane was reasonably balanced (.06 meters difference upwards) prior to the addition of the forward tri-couplers and the last six Ram Intakes - with those additions the CoM moves forward .1 meter. From the screenshots you can see the final design is actually slightly less than 48 tonnes, so it manages a better than 25% payload fraction.

The problem I've got - and this has been consistent for a couple of flights now - is that the plane doesn't get much above 27k and 1500 m/s before it switches over. Further, once switchover occurs, the RAPIERs have a tendency to overheat to the point of exploding.

The Guide says this design should work, so at this point I'm open for suggestions. Anybody have any ideas why I'm getting such lousy performance from the plane? For reference, I'm flying stock with aerodynamics. Any help would be appreciated.

Edited November 15, 2014 by capi3101

[DeMatt]

Hmmm... my thoughts after a test flight:

1. The download page for that guide specifically says that it hasn't been updated for 0.25.
   
2. Smaller probe if you can. Where are you sending those two Goo canisters, Eeloo? And why does it need RCS?
   
3. Those five Reaction Wheels in the bow cargo bay probably aren't doing you any favors.
   
4. I'd say you need a bit more rocket fuel. Easy enough to tweak it into the existing tanks.
   
5. Speaking of fuel tanks, switching from the X200-32 to an X200-16 plus an X200-8 would save you 0.5 tons of dry mass per side. This by itself might be enough to allow you to ignore point 4.
   
6. Don't let the RAPIERs automatically switch. Reduce your throttle and fly more on the jets. It looks like the upper-starboard engine is the one that spools down first, so right-click it and watch its thrust output.
   
7. The RAPIER overheat problem is easily enough avoided by not throttling them to maximum. Just one interval on the throttle gauge is enough.
   
8. Not personally a fan of the RTGs, but if you don't want to futz around with solar panels, I suppose they're all right.
   
9. Might consider switching from the small Elevon 4's to the larger Elevon 3's.
   
10. You can delete at least the aft centerline BZ-52. And switching the decoupler from the 1.25m version to the 0.625m version will save you more mass.
    

[endl]

by the design of the plane i guess your playing with stock aero, you have to spam air intakes to get your plane to gain enough speed/altitude before the engines switch over. if your overheating throttle down. you might want to consider cutting some parts and save some more weight but 4 rapiers should be enough to get you into orbit at your mass

[RizzoTheRat]

I'm also fairly new to space planes so possibly take my advice with a pinch of salt...

I'm not entirely convinced by Rapiers, they're very thirsty, if you mounted your engines side by side instead of over and under you could use 2 turbojets and 2 rapiers. The turbojets will give you more power and use less fuel, and 2 rapiers in rocket mode will be plenty once you get high enough to have to shut the jets off, my tanker aircraft is 56 tonnes on the ground and 4 turbojets and 2 rapiers do the job nicely.

[Martin0]

So you have 2 problems:

1- rapiers switch over at 27km and 1500 m/s

2- rapiers overheat and explode.

How to fix 1: not much of a problem, pitch up less so you don't climb so fast.

How to fix 2: throttle down so that the rapiers don't overheat.

[Wyre311]

http://forum.kerbalspaceprogram.com/threads/96406-The-Arcadia-series-and-the-Zeus-Ships?highlight=Arcadia

Please give PM me with an email to send you my personal ship designs. I believe they are everything you need

[capi3101]

Getting pretty late here this evening and I need to go to bed, but before I depart I wanted to thank y'all for your advice - which was pretty much universally "don't climb so damn fast and kick the throttle down a bit". Did that, also tweaked the design with an extra pair of wing connectors and two additional AV-R8s for rudders, and bumped up the fuel to assume a slow kickover speed. Made it into orbit with fuel for the deorbit. I do want to ask a few more questions of y'all but I figured I'd at least mention a qualified victory before I headed out for the night.