I need help with a spaceplane design.

[Jakewhistler]

Is this the right place to ask for help on a spaceplane design? I'd like to post the craft files of both versions. Each fails spectacularly in its own way. And never very far from the end of the runway.

[Geonovast]

1 minute ago, Jakewhistler said:

Is this the right place to ask for help on a spaceplane design?

I think it'll get better attention in Gameplay Questions, so I've moved it there. 

Welcome to the forum, @Jakewhistler!

[AHHans]

3 hours ago, Jakewhistler said:

Is this the right place to ask for help on a spaceplane design? I'd like to post the craft files of both versions. Each fails spectacularly in its own way. And never very far from the end of the runway.

Hi @Jakewhistler, welcome to the Forums.

Yes, the Gameplay Questions subforum is a good place to ask for help with spaceplane design. An posting a craft file is indeed a good way to get said help. Please upload the "*.craft" file to a file hosting service (like Dropbox, Google drive, or whatever) and post the link to the file here.

There is also the FAQ, and tutorials (e.g. this one) that may help you.

[Jakewhistler]

41 minutes ago, AHHans said:

Hi @Jakewhistler, welcome to the Forums.

Yes, the Gameplay Questions subforum is a good place to ask for help with spaceplane design. An posting a craft file is indeed a good way to get said help. Please upload the "*.craft" file to a file hosting service (like Dropbox, Google drive, or whatever) and post the link to the file here.

There is also the FAQ, and tutorials (e.g. this one) that may help you.

Thanks! I did look over the spaceplane tutorial you linked, I actually had it open in a tab already.  So I'm trying to make a runway-to-orbit ship that can carry space station assemblies. Eventually I could retool it for mining and exploration. First I need to get it off the runway for an appreciable amount of time without it exploding or just losing its wings. I'm using four rapier engines, to save on oxidizer in atmosphere and just overall weight efficiency. (Four engines for both space ant atmosphere instead of rockets and jets, that sort of thing.) I've tried using Matt Lowne's craft files but the only one that opens for me is a space station. Nevertheless I've been inspired by his designs. The first iteration I made "piecemeal" wings that work better now that I've autostrutted them. That's this one: https://drive.google.com/open?id=1RuABmApe5UK0IkFZEfMH5rZOgW85GRiD

The second design has space shuttle Big-S Delta Wings. First just one set and then another set mounted on top of the engines, for Moar Lift! This got slightly better results I think, even more slightly better when I added a pair of Vector rockets but the improvement wasn't worth the extra danger when everything went pear shaped. https://drive.google.com/open?id=1GELHayXG8mvaqJ-pCUaDLlJbUqI10vX7

I'd appreciate any help. I know the first one doesn't have any RCS yet but I figured I'll get it off the runway and into higher altitudes first, and add other finishing touches. 

[AHHans]

O.K. I had a look at the first one, the "Rapier Sketch.craft". The first and most important issue I noticed was that you seem to use FAR! FAR changes the aerodynamics quite a lot compared to the stock aerodynamics so that makes a big difference. And I don't use FAR, so there is little that I can help you with that. (And if you use other mods that might be relevant, then please mention them.)

Having said that, I did give it a try with stock aerodynamics. The design is aerodynamically quite stable, with a rather large distance between CoM and CoL. This means that it needs quite a bit of pitch-up trim to fly straight, but in stock that is not a problem, you have enough control authority for that. It does mean that with SAS in "stability assist" the nose drops quite a bit before SAS applies enough trim to keep it stable. (Again, not a problem, but something one needs to take into account.)

You also use quite a few struts, but no autostruts. The relevant difference here is that regular struts generate drag, while autostruts don't. To enable autostruts you need to activate "advanced tweakables" in the settings menu. Once you did that you'll have more options in the PAWs (the "right click windows") of the parts, one of them being the autostruts. Autostruts have their own set of problems - in particular when they flip from one part to another e.g. because fuel consumption changes which part is the heaviest - but autostruts to grandparent are usually safe. For a test I removed all the obvious external struts without activating any autostruts, and the craft still flew fine without falling apart or anything. But that may be different with FAR.

You are using the RAPIERs in air-breathing mode but you bring enough oxidizer to burn all liquid fuel in rocket mode. That means that you bring way too much oxidizer and too little Lf. Accordingly I ran out of fuel when trying to get into orbit like that. I then replace one FL-T800 tank per side with two Mk1 LF fuselages and with that I managed to get it into orbit with some dV and a few hundred units of excess oxidizer to spare.

Once in orbit I noticed that the direction of thrust of the RAPIERs does not go through the CoM. In the atmosphere that is not much of an issue because the aerodynamic forces of the control surfaces give you enough authority to counteract the resulting torque. In space you only have the torque from the reaction wheels to keep you straight which in this case means that you can run the engines only at very low thrust levels without the craft flipping out of control.

Some more tips:

I find it very useful for my spaceplane to put toggling the engines on or off and switching them between air-breathing and closed cycle (plus toggling the air intakes) onto action groups. That way I can easily switch them all at once.

Using dedicated airplane parts like the FAT-455 wings or tail-fin on spaceplanes is risky: the have have low thermal resistance and a prone to overheating and exploding, especially during reentry.

To get the most out of the RAPIERs you want to gain as much speed as reasonably possible with them being in air-breathing mode. That means not getting too high too soon, you'll want to get to at least 1000 m/s in air-breathing mode. My flight profile for your craft (with stock aerodynamics!) looks like that: after take-off pitch about 20 deg up until I got to ca. 6000m altitude (at sub-sonic speeds); then slowly lower the nose to the horizon to gain speed (this will loose altitude but I kept above 5000m); once I got to 440 m/s I raised the nose to about 10 deg pitch up, making sure that I don't drop below 400 m/s(!); the increasing speed and the curvature of Kerbin means that the pitch (relative to the surface) will slowly increase, that's fine and what I want; once the thrust of the RAPIERs drops below 170 kN I switch to closed-cycle mode (because that craft has excess oxidizer, on my craft I usually wait till near flame-out before switching); when the time to apoapsis gets above 1 min I switch SAS to prograde hold; when the apoapsis gets to 80 km I throttle down, coast, and circularize.

 

[Jakewhistler]

I'm sorry, I forgot to mention that I use FAR. It came recommended in the Basic Aircraft Design tutorial. What made sense to me was that KSP adds the drag from each component and not just those on the outside. The example given is a probe inside a fairing has more drag than just the probe on the rocket. This made sense to me. Does it not do this? (That guide was posted seven years ago but edited just before KSP 1.9.1 came out, but after 1.9, so I don't know what that means.) Other than that there's Mod Manager and Docking Port Alignment Indicator. (Sometimes I really wish this machine could play Orbiter. So much instrumentation, but no spacecraft construction!) I didn't initially auto strut the wings because I didn't want to poke the Kraken. I'm running on a laptop and KSP doesn't run at its best. I've read that excessive autostrutting could cause problems, and those wings have more parts than I'm comfortable with, that's why I went with the S-wings on the second version. I don't remember if I saved that one auto-strutted or not. 

I'll try the fuel tank replacement, I hadn't thought of that, or measured my fuel, really. The airplane vertical tail made me nervous though, I'll replace it. So far all my r"e-entries" have been from the atmosphere anyway so it hasn't come up.

Edit: I tried your flight profile, and I think I just need to practice with it: I keep ending up rolled a little to one side or the other and attempts to correct it lead to more problems. But on one of my flights the Structural Wing Type B on both wingtips fell off and it flew just fine until it didn't. They move the CoL forward though so I'm going to leave them. And it doesn't seem to want to climb that fast. I've also added batteries and the RCS, so that's covered for now.  (I didn't notice that rapiers don't have an alternator! I just sort of take it for granted. I need to find a solution to that...)

  But thanks for your help! I'll keep practicing, and hopefully I'll get it well above 1Km. 

   -Jake

Edited April 29, 2020 by Jakewhistler
Update

[Bar27262]

A good spaceplane design is one that works aerodynamically and has a good enough liftoff capability. My guess is you're trying to make a kickbooty spaceplane that is capable of orbital manuevers, flying out and flying back into KSC. So, let me go over some key essential things.

1). A good design is with Delta Wings and a lot of other wings/fins to have the air flow through. Also airbrakes for aerobraking purposes. Adjustable air intake is also good too because that lets you adjust for air flow. The essential part is you need to avoid any "extra" parts that might slow down your craft. Go for what is necessary and will help your plane aerodynamically.

2). Engine location is important. When I first started out making spaceplanes I always used the MK2 bicoupler with heavy engines. This was bad because it make the spaceplane center of mass go towards the back rather than the central part which is where the center of mass should be. I recommend you place these on the Delta Wing as close to the fuselage as possible. Connect one fuel-line to it. Struct it properly so the fuel-tanks with the engines don't go flying off and bouncing around. :)

3). Crewed or AI, the AI drone core reduces weight significantly as it is just a small drone core compared to the cockpits. The drawback is that the AI drone core will need something to generate power. You can have a small cargo-bay to stash the radioactive isotope generators and a few batteries. If you are not that far ahead, you can opt to place a few batteries and a solar panel instead. 

4). Lights are somewhat important if you plan on doing night-time flying whether it's on Kerbin or anywhere else. I recommend four. (two on the delta-wing top side to illuminate the runway as real planes have, two on the bottom of the fuselage for illumination)... The lights drain power so you'll need some batteries for the cargobay.

5). Landing gear is also important, there should be two on the wing with one forward and one in the back to deter tailstrike. Size depends on how big of a craft you're gonna be building.

All in all, building a successful spaceplane is easy but it requires trials and usually these trials are by fire  (if not worse... LoL!) and at the end of it you'll come out with a successful spaceplane. The biggest joy for me on this game is when I launch a space shuttle (Mine is a AI-drone named the "Cargolifter MK1" designed sort of like the space shuttle.) and I get it into orbit, open the little cargobay doors and I release the satellite into "the wild" (so to speak.).. 

Of course you can also make a cargo spaceplane that can take off from the runway. Heck, even a spaceplane boat designed to "surf" the waves of Laythe and look for new stuff. The possibilities are as endless as your imagination goes! Good luck my friend! 

 

[Lt_Duckweed]

5 hours ago, Jakewhistler said:

What made sense to me was that KSP adds the drag from each component and not just those on the outside. The example given is a probe inside a fairing has more drag than just the probe on the rocket. This made sense to me. Does it not do this? (That guide was posted seven years ago but edited just before KSP 1.9.1 came out, but after 1.9, so I don't know what that means.)

That section of the guide hasn't been true for nearly 5 years now, but has never been updated.

 

Things in a fairing or cargo bay do not produce any drag.  Things not in a bay create drag based on the shape of the part, but if node attached to other parts, the surface area that is attached will no longer be counted.

[Linkageless]

Here's a small tip, RCS thrusters inside a closed cargo bay don't work, but when it's open work just as well as if they were outside.  Hence you can have clean lines and RCS once in space, if you wish.

[Jakewhistler]

19 hours ago, Lt_Duckweed said:

That section of the guide hasn't been true for nearly 5 years now, but has never been updated.

Things in a fairing or cargo bay do not produce any drag.  Things not in a bay create drag based on the shape of the part, but if node attached to other parts, the surface area that is attached will no longer be counted.

Thanks! I'll have another look at FAR and see how much I want to keep it or not.

15 hours ago, Linkageless said:

Here's a small tip, RCS thrusters inside a closed cargo bay don't work, but when it's open work just as well as if they were outside.  Hence you can have clean lines and RCS once in space, if you wish.

I don't mind the lines of RCS thrusters. I just hadn't added them yet; I wanted to test the overall shape and handling of the plane first. Shape doesn't matter so much in space. Kind of like the shuttle Enterprise's test flights. (But not nearly as successful )