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Help me troubleshoot spaceplane that levels off at 10k


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Though fuel drains backwards, which would mean it would be easier to pitch up, if anything.

I cannot personally test the craft, as it has a kerbal engineer module on the nose, but from what I can see, it looks fairly good in design. I used to have this same problem when building spacecraft in .16, when trying for the K-Prize, and I determined that the cause was that I was not going fast enough. What speeds are you achieving at 10km?

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My best guess is that your CL is too far back in relation to your CG, and as the jet fuel is being burnt away, the CG is shifting forward.

Remounting your existing wings a bit forward may correct this, or you may need to put some more non-lifting mass at the back end.

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One way to increase altitude is to reduce weight. In your image you're carrying four jet fuel tanks, you can safely loose at least two and still have plenty jet fuel. You might even be able to get to orbit with just one.

Once properly balanced (can't judge that from your current image) I see only one reason why this would have difficulties reaching orbit: it could do with a few more intakes. You don't need twenty or thirty intakes per engines as some people do, more will make things a bit easier but two to four will suffice.

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initially looking at it, it looks like it shoulf fly fine, even with FAR on.

Make sure the fuel is draining backwards, if not it may cause your nose to become to heavy preventing lift (you may not have enough nose lift to bring it up, courtesy of FAR), so make sure any fuel lines are draining properly and in the correct order so that the weight is more towards the back (making the front lighter for easier pitch).

Also, take into account that FAR likes to change aerodynamics, you may need slightly more lift to have influence in the thin atmoshere. If you can, attach one or two more control surfaces to the wings or another Canard to the front, to provide more lift of course.

Generally speaking though, its probably fuel, if all is draining well, consider making slightly longer wings so you get more lift, or adding a few control surfaces (the small stock KSP flaps would work) to the wings (make them flush with the surface, when you pitch up they will activate and give more lift).

my .02, hope it works out

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Okay, a few points:

A) FAR is just fine for newbies, provided they're ready to learn some actual aerodynamics rather than the utterly nutty setup of stock KSP. In fact, my wife requested FAR so she would not learn bad habits, as she put it. But hey, if you like monstrous planes with eight wings clipped into each other, fifteen air intakes per engine and three times the amount of engines it ought to need to even fly, then stock KSP planes are for you.

B) If the aerodynamic center (or aerocenter - it's not the center of lift) is in front of the center of mass, the plane will straight up flip out and kill everyone involved. So that's not what's happening here, and is anyway counter-indicated by the fact that as you go supersonic, your aerocenter shifts backwards.

C) Fuel drain can and will shift the CoM as well - but for me that only becomes a problem when going to rockets for the atmosphere transition burn. Usually, that happens well above where the atmosphere is a major factor, typically 35-40 km up. So probably not that. And as mentioned before, it looks very different.

D) As your supersonic velocity increases, depending on the structure of your plane, your control authority may decrease selectively - particularly I always have trouble getting enough pitch-up. Bigger control surfaces farther from the center of mass and aerocenter are typically called for. Also, ensure that control surfaces are specialized - otherwise they may work against each other, giving you less, not more, control authority. My guess would be that that is the problem you are encountering. To test it, you can fly your aircraft at lower speed and see how high up you can maintain pitch-up, or you can try lengthening the fuselage, adding bigger canards, and making sure they're the only surface controlling pitch - that should see some effect too. B9 delivers some nice big stabilators that should serve you.

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I would personally say that 600 m/s is on the edge of too slow for an attempted SSTO flight. I am generally in the region of 1000 or more at that height, which by velocity alone should push you to a higher altitude. Once you are above 13-15km, you should be able to use rocket engines to sufficiently nudge your way out of the atmosphere. Keep in mind that you don't need to blast out of the atmosphere, as enough speed will allow you to drift out on relatively low fuel usage.

This is generally the process I use for both my FAR and stock game files.

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How long is it taking you to reach 10k, what is your climb angle (how far above the horizon line is the velocity vector) and how far above your velocity vector is your nose while you are climbing?

Shifting fuel shouldn't be an issue unless you're taking an inordinately long time to get to 10k, you shouldn't have spent enough fuel getting there for things to have shifted much.

600 m/s is fine for 10k. Actually, if anything it's too fast, you're fighting a heck of a lot of drag doing 600 at 10000 unless FAR changes things more than I thought.

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600 m/s is fine for 10k. Actually, if anything it's too fast, you're fighting a heck of a lot of drag doing 600 at 10000 unless FAR changes things more than I thought.

Well, FAR has you flying in air, not chicken soup, so 600m/s at 10km is fine, in fact that's usually what my ascent profile tends to hit, or close to it (I usually do my initial climb-out to 12km, then level off and pour on the power. Typically I hit mach 5 (1500-1600 m/s) between 25 and 35 km, whereupon I pitch up for atmosphere transition and engage rockets). But as I mentioned above, I doubt this is a profile problem, or a stability problem. This is a problem of control authority. Incidentally, one I've had myself, and solved by the approach given in my post above.

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Bigger wings and more control surfaces might help. Maybe it's got enough lift to work in the thicker parts of the atmosphere but fails higher up when the air gets thinner.

I'd definitely swap the large control surfaces for pairs of small ones. Apart from looks there's no reason to use the large ones. The lift/weight ratio of the small ones is much better. This won't really change the look of your ship, and will instantly give you more lift which may well solve your problem.

Edited by Moar Boosters
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