Need help taking off my heavy cargo spaceplane

[tsotha]

Okay, so after posting this last night I went back and did some hand calculations (I use exclusively stock so no mods to do it for me.) Comparing the total wing lift with the total mass tells me that I have less than half the wing I need to lift this much.

The successful "normal" looking large craft I've seen usually have dozens of control surfaces stacked inside a cover that hides them (along with dozens of intakes also hidden away), so if you're looking at other peoples' designs sometimes it can be misleading.

This kind of plane is really difficult to actually get working within any kind of acceptable performance parameters. Fortunately in the next release Squad is planning not only to increase the lift of the aerodynamic parts, but also the lift will increase with speed as in real life. So this kind of thing will be much easier to build.

Edited January 15, 2015 by tsotha

[Fearless Son]

You have way too many wing parts on that thing!. The reason you can't lift off the runway is because your rear wheels are too far back and the plane lacks the leverage to overcome that. Unless you want to risk tail-scraping, there's not a lot you can do about it with such a large plane, besides putting the gears on struts as I recommended in my previous post.

Yeah, that is something I tried to compensate for in this iteration of the design. I did as much as I could to move the center of both lift and mass as far backward as possible so that I could put the rear wheels as far back as possible while still not being very far behind the center of mass. But I feel like there is a point beyond which it just becomes too much weight for the lift model to really account for it. Unfortunately as the weight of the design kept having to go up, I kept having to build out the wings further and further to get enough lift. I did at least manage to get the effective center of lift back behind the center of mass by carefully trimming the angle of the lift surfaces of the front canards and the tail assembly.

You want the "center of drag" (not displayed ingame) to be behind the center of mass - it'll make the plane want to point forwards into the air stream, so your approach is correct .

Yeah, I noticed this handles much better than previous versions where my many intakes were all toward the front.

The problem here is almost certainly the way fuel drains; check your payload is also locked if it's fuel tanks.

Quite probable. I make sure to shut off fuel flow from my cargo tank, as well as the frontal fuel tank (that is meant to be manually shunted back into the rear tanks after reaching orbit for the return trip.) Note how most of my fuel tanks are either in the rear fuselage of the plane, or spread out across the rear of the wings. This was to help shift the center of mass backward. I also unloaded several tanks of their excess fuel to carefully manage the balance and keep it where I need it to be.

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I really, really suggest that you experiment with a much smaller plane. The physics and design principles are the same, but with much fewer parts and possibly a better TWR, the experiments will consume far less of your time. I started out like you did -- every flight took like 20 minutes until things became interesting. Every new idea, every different approach, every slight variation in ascent profile: I first had to sit through a 20-minute ascent before I could see what happens.

Be smart, start small. Designing huge vessels is a whole lot easier if you have some idea of what to expect.

Oh believe me, I do, and have in fact made more than one spaceplane which has successfully achieved orbit and returned. My standard go-to is a little single-seater, single-turbojet craft with pair of delta wings, two shock cone intakes, and some stacks of mini-tanks with nothing but oxidizer on top of some of the lightest rocket engines available. Handles beautifully, achieves orbit easily. I use it for "survey Kerbin" contracts since its orbital capacity means I can get it to wherever I need it to be on the planet in a minimum of time, minimum of fuel, and it has some parachutes so I can land it even in difficult terrain if I have to (though it has proven itself capable of runway landings.)

I will post a picture sometime. Lovely little plane.

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The successful "normal" looking large craft I've seen usually have dozens of control surfaces stacked inside a cover that hides them (along with dozens of intakes also hidden away), so if you're looking at other peoples' designs sometimes it can be misleading.

Ah, that would explain it. I think I have seen that trick before. I have considered doing similar by cramming intakes into a cargo bay, provided they do not overlap with one another. That is kind-of airhogging, but I could pretend the cargo bay with the multiple intakes was just some bigger, super-intake without feeling like I was "cheating" on volume. Putting a bunch of control surfaces in there does feel a bit too much of a cheat though.

Hopefully new parts in the next update will make such things unnecessary.

[Anquietas314]

I did as much as I could to move the center of both lift and mass as far backward as possible so that I could put the rear wheels as far back as possible while still not being very far behind the center of mass. But I feel like there is a point beyond which it just becomes too much weight for the lift model to really account for it.

The reason that doesn't work comes down to some reasonably complicated physics related to levers. In simple terms, if you put all the mass at the back of the plane, pretty much on top of the rear wheels, then your rear control surfaces are so close to the fulcrum (pivot - the wheels) that they have pretty much no effect while you're on the runway. If you have large control surfaces at the nose, you can compensate for that though. Ideally you want your rear control surfaces (which are usually the most important in a plane) quite far away from center of mass to maximize their torque.

Having the center of mass so far back also has serious problems when you drop off your payload - you can pretty much guarantee your center of lift will be miles in front of the center of mass afterwards, especially if it's a heavy payload. For planes designed to lift heavy cargo, you want the center of mass to be roughly lined up with the middle of the cargo bay so it doesn't shift 10 meters when you drop off that 40 tonne tank .

Quite probable. I make sure to shut off fuel flow from my cargo tank, as well as the frontal fuel tank (that is meant to be manually shunted back into the rear tanks after reaching orbit for the return trip.)

Yeah, from experience this is a really bad idea unless you intentionally bring way more fuel than you actually need - when it runs out you completely lose control over the center of mass's location and naturally it ends in a fiery ball of death.

It might be useful if you upload a screenshot showing the center of mass, lift and thrust, but realistically I think you'll need to redesign it anyway :/

[Fearless Son]

The reason that doesn't work comes down to some reasonably complicated physics related to levers. In simple terms, if you put all the mass at the back of the plane, pretty much on top of the rear wheels, then your rear control surfaces are so close to the fulcrum (pivot - the wheels) that they have pretty much no effect while you're on the runway. If you have large control surfaces at the nose, you can compensate for that though. Ideally you want your rear control surfaces (which are usually the most important in a plane) quite far away from center of mass to maximize their torque.

Huh. My understanding is that as a rule of thumb is that for a good takeoff, you want the rear wheels to be just slightly behind the center of mass. But if the elevators are too far rear of the center of mass, then you end up with tail dragging. Is this just a case of one or the other?

Now I am starting to think about WWII-era types of designs, with the wide wings toward the front and a long, light fuselage trailing behind it that rides on a small wheel at the rear. If the supersonic physics were more realistic though that would be a problem for a spaceplane.

[Anquietas314]

Huh. My understanding is that as a rule of thumb is that for a good takeoff, you want the rear wheels to be just slightly behind the center of mass. But if the elevators are too far rear of the center of mass, then you end up with tail dragging. Is this just a case of one or the other?

You do want the wheels just behind the center of mass, but what you don't want is the center of mass at the back of the plane at the same time unless you have enough control surfaces at the front to overcome the weight of it. You want the control surfaces far away from the center of mass (and the wheels) because that'll make them most effective. The technical stuff if you're interested. Wiki's explanation is somewhat simplistic though - things get a little more complicated if there are parts of the lever that don't line up neatly with the other parts (tail wings and such).

It's a balancing act between having enough control, and not tailscraping . Incidentally this is why we need bigger landing gears - they'll give you more room to pull up before the tail hits the ground.

Those WW2 planes are usually sitting so that their wings will naturally lift the plane off the ground when they're going fast enough. You can technically do that in KSP, but it doesn't really work so well for large planes.

Edited January 15, 2015 by armagheddonsgw

[tsotha]

Incidentally this is why we need bigger landing gears - they'll give you more room to pull up before the tail hits the ground.

Yes, the lack of choice in landing gear is a big disappointment for me, particularly with the amount of effort they put into plane parts recently.

[Snarfster]

For bigger landing gears I use that tweak scale mod. Now my planes have a size appropriate main gear. Not great, but it'll tie me over until the next update.

[Anquietas314]

For bigger landing gears I use that tweak scale mod. Now my planes have a size appropriate main gear. Not great, but it'll tie me over until the next update.

Well, there is also Adjustable Landing Gear, which is pretty much the gear equivalent of procedural parts / wings. It isn't officially updated with 0.90, but reportedly it works fine as long as you have the latest version of firespitter.dll (apparently not the one shipped with the latest firespitter?). I seem to remember OP didn't want to use mods, though I can't seem to find any discussion at all about them . Perhaps I'm thinking of someone else lol.