CargoSSTO Flat Lining at Low Altitudes

[WDude4k]

Hola Kerbonauts!

I've recently built a C-17 (plane) and wanted to try and turn it into an SSTO. I've made some progress, like adding rocket engines, but the thing can't pitch up past 12km.

I shall keep testing and messing with control surfaces. Ideally I don't want to add much more to this other than control surfaces due to it's bulk, but I'm open to suggestions.

Let me know if anyone anywhere has ideas I'd appreciate it.

Thanks!

Some Pictures:

^The Forces (as of right when posting this. Chances are they will be moved slightly within a few hours of the post)

^Take off! Great control on all sides. I could even do a loop if I so pleased.

^Here is where I've actually lost the ability to affect the climb. This guy is leveling out and I can't stop it. Call in the crash cart 'cause he's flat lining!

^Clear! *zap*

Edit:

I have made several adjustments to.... everything. CoT and CoM are lined up, but I still have problems getting past about 35km.

Edited August 4, 2014 by WDude4k

[LordFjord]

Can't see the pics.

Reasons for the plane being unable to pitch up are usually:

- pitch control surface locations (far away from center of mass for most effect)

- in general low amount of lift compared to the weight

- center of mass too far ahead of center of lift

[KerbMav]

- center of mass too far ahead of center of lift

Activate unlimited fuel in the Alt+F12 cheats menu and try again. (Turn it off again afterwards and never touch it again, ya hear me?! )

If the plane works better, you have a problem with the center of mass moving as the plane consumes its fuel.

In KSP the atmospheres are smaller than in reality, so air pressure drops quite fast as you climb up. Thinner air means less controlability from wings and control surfaces. Although I think the center of mass is your problem here.

[Wanderfound]

As others have said: your pics aren't showing up. Chuck 'em on Dropbox/Photobucket/Imgur etc.

I recently had similar issues adapting my old planes for the new nerfed-jets FAR. You might find some things of use in this thread: http://forum.kerbalspaceprogram.com/threads/88200-Power-or-aero-%28issues-with-old-planes-under-new-FAR%29

[MarvinKitFox]

Your centre of mass / centre of lift is quite acceptable.

Total wing area should be adequate.

Total engine power is *ample*

However, your centre of THRUST is well above the COM.

And your centre of drag is even further above thet.

At low altitudes you have enough air "grip" to offset this. At 12k, you dont.

Further...

unless you have a huge number of clipped-into-invisibility ramscoops on there, you have WAAAAAY to little air intake for that monster.

To keep 8 turbojets fed and happy, on a SSTO vehicle, your want about..... 48 ramscoops facing the wind.

I count 4 of the feeble circular air intakes, and thats it!

To fixx..

Mount a cubic octagonal on yout plane's floorpan, pointing out.

Add a forward-facing ramscoop to it.

Rotate the cubic into your fuselage, to preserve appearance.

Repeat 40 times.

The addition of mass and drag below centreline will improve the balance of your plane, and the air from the scoops will allow your engines to keep producing good thrust to 26km, and still acceleration on lower thrust at up to 38km.

Engine flame-outs on such widely dispersed engines might be... embarrasing.

Consider manually snuffing your outer engines, and reverting to a single turbojet hidden centreline in the cargo hold, for the high altitude push.

[Wanderfound]

Further...

unless you have a huge number of clipped-into-invisibility ramscoops on there, you have WAAAAAY to little air intake for that monster.

To keep 8 turbojets fed and happy, on a SSTO vehicle, your want about..... 48 ramscoops facing the wind.

I count 4 of the feeble circular air intakes, and thats it!

Not so much. My big SSTO spaceplane (two turbojets, six RAPIERs) manages fine with 8 ram intakes, 8 nacelles and four radial intakes. And the radials are probably unnecessary overkill.

So long as your engines don't choke below 30,000m (at low climb rate and angle of attack; if you're climbing steeply, they'll choke much earlier), you're all good.

[MarvinKitFox]

Not so much. My big SSTO spaceplane (two turbojets, six RAPIERs) manages fine with 8 ram intakes, 8 nacelles and four radial intakes. And the radials are probably unnecessary overkill.

So long as your engines don't choke below 30,000m (at low climb rate and angle of attack; if you're climbing steeply, they'll choke much earlier), you're all good.

2 turbojets, 6 rapiers.

As the rapiers have a way to switch over to lox-burning, they don't factor in the air-starvation game. Especially if you have them on automatic.

So you have 4 ram intakes AND 4 nacelles AND 2 radial intakes, feeding each turbojet.

Yet you have the gall to complain about my advice of supplying 6 ram intakes?!?!?!?!?

Thus guy is running EIGHT turbojets from 4 circular intakes!

Have you even bothered to take a look at his plane?

Edited July 30, 2014 by MarvinKitFox

[geb]

Corrected image embedding for those too lazy to view source and grab the URLs:

As MarvinKitFox said, you need to line up the thrust vector so that it's pointing through the centre of mass. That will mean that when you get up into thin air and start losing aerodynamic control, your ship is still balanced on its engines. Unbalanced thrust from too high up will make the ship turn downwards.

[Jebadiah Kerban]

ur turbojet fuel tanks are too big so u need to replace them or u remove 1/2 of the fuel and all of the oxidizer or change tanks that should help cos ur engines are too weak and ur plane is too heavy and also never use the glider like wings its the worst wing to use its only for gliding rather use something else. U can send the craft to meh and il fix it rlly i can FIX or idk but when ur at 0.1 IA (intake Air) Turn the engines off and close the intakes cos intakes produce drag and drag is bad but also add more engines and also lower ur thrust at 40,000 M or Ft and ur thrust should be at 2/3 and at 60,000 ft trust must be 3/3 or full PM me for anything else also bring up ur gear that produces drag

[Wanderfound]

2 turbojets, 6 rapiers.

As the rapiers have a way to switch over to lox-burning, they don't factor in the air-starvation game. Especially if you have them on automatic.

So you have 4 ram intakes AND 4 nacelles AND 2 radial intakes, feeding each turbojet.

Yet you have the gall to complain about my advice of supplying 6 ram intakes?!?!?!?!?

Thus guy is running EIGHT turbojets from 4 circular intakes!

Have you even bothered to take a look at his plane?

Eight turbos on four circulars are certainly underfed, but I was speaking in general, not specific; I hadn't seen the images. Regardless, forty eight ramscoops is extremely excessive and unrealistic unless you're dealing with a 200 ton vertical SSTO.

If you're looking to maximise speed and altitude on a spaceplane, you want to be shutting down engines as you ascend to concentrate the remaining air. By the time I crack 25,000m, the RAPIERs are turned off and don't come back on until it's time to start burning oxidiser. My smaller planes generally have a single turbojet on the centreline that is their sole source of high-altitude thrust.

The RAPIERs are only used for the initial climb to 20,000 and the final push to orbit. You just don't need a lot of thrust for high altitude level flight; two turbojets are enough to get my 100 ton spaceplane to Mach 4.5 at that altitude. The poster should also be shutting down engines as the air thins; the eight turbos may be handy for getting off the ground, but they just aren't that useful at altitude.

Adding more intakes is usually a waste of time; keeping more engines running isn't going to make you significantly faster, because once you approach Mach 5, the turbojet thrust (which reduces greatly at these speeds, no matter how much air you feed them) begins to be outweighed by the intake drag. Hypersonic is rocket and scramjet territory.

[Taki117]

I'm surprised no one has pointed this out. It's your relationship between your CoM and CoT. Your CoT is mugh higher than your CoM, meaning that, while you are in the low atmosphere and have lots of control authority you can overcome the natural pitch down force of the engines, but as you climb you loose control authority as the air gets thinner, until you reach a point where you can't pitch up anymore because your control surface input is enough to counteract the torque of the thrust, but not enough to over come it.

To see what I'm talking about try this: Take off as normal, and when you start to level out stop inputting control commands (Hands off the keyboard/joystick) If your plane pitches nose down you will see what I'm talking about, if it doesn't then there is something else at work.

[Wanderfound]

Looking at the plane, adding to the valid points above re: air, CoT and excess weight:

Almost all of your control surfaces are on the wing; the only surface I can see that's placed to control pitch is that tiny tailplane. Because the wing is close to CoM, it doesn't have a lot of leverage for pitch. Mid-plane control surfaces are primarily for roll; pitch is handled from the ends of the plane.

You want to get some chunky horizontal control surfaces (best if they're all-moving; AV-R8’s or similar) mounted as far as possible from CoM. Elevators on the tail or canards on the nose (preferably both).

Build a larger tailplane from wing pieces, put standard control surfaces on the back of it and AV-R8's on the tips. Then use the right-click tweakables to crank their control authority to maximum.

Edited July 30, 2014 by Wanderfound

[Jimbimbibble]

Another thing is the horizontal stabilizer is quite small for a plane of that size. It really should be 3X as large with some big control surfaces. Try using delta wings with the standard control surface instead of those winglets.

[Hodo]

Hola Kerbonauts!

I've recently built a C-17 (plane) and wanted to try and turn it into an SSTO. I've made some progress, like adding rocket engines, but the thing can't pitch up past 12km.

I shall keep testing and messing with control surfaces. Ideally I don't want to add much more to this other than control surfaces due to it's bulk, but I'm open to suggestions.

Let me know if anyone anywhere has ideas I'd appreciate it.

Thanks!

First your CoM is to far ahead of your CoL, this is causing your nose to become heavy and pitch down.

Additionally your CoT is way above your CoM, which is pushing your nose down on top of the other issues.

Fix- Move your CoM closer to your CoL but not behind it.

Fix 2- Bring your engines down more inline with your CoM this way your CoT is going through your CoM.

The other problem is you lack proper pitch control surfaces. You have two tiny little winglets trying to pick the nose up on that massive craft. That is like putting the legs of a ant on a hippo and asking it to jump.

Finally, the control surfaces on the wings really wont do anything other than help with roll, they won't help with pitch because they are to close to the CoM of the craft, it is like putting the wheels that turn your car in the middle of your car, your car will just not turn as well if at all.

While others suggest or assume you are using FAR/NEAR, I don't see that you are. Not going to tell you to get it, but it does help in designing aircraft, as it actually makes the atmosphere in KSP respond like an atmosphere not chilled tomato soup.

[MarvinKitFox]

I'm surprised no one has pointed this out. It's your relationship between your CoM and CoT. Your CoT is mugh higher than your CoM,

Taki, do you need glasses?

I quote from my waaaaay earlier posting

"However, your centre of THRUST is well above the COM."

[WDude4k]

I actually have Something around 36 of the ram air intakes. They are mounted on the 3 inner fuel tanks. I Suppose I could always add more though.

[Wanderfound]

You don't need more. All they're doing is adding to the drag and making it harder to keep the nose up. Ditch at least half.

[Hodo]

You don't need more. All they're doing is adding to the drag and making it harder to keep the nose up. Ditch at least half.

So true, the last thing you need is MORE intakes.

[FacialJack]

As i can see, and by my experience (which is small ) im agrre wiz the lack of air intake, if thats really ure pb, that mean that when u reach 12km high, u dont have enough air intake for generating trust. on my SSTO, i put someting like 30 airintake for 8 engine (same than ures, i dont like rapier engine) and wiz that, i can go up to 35km, and reach orbital speed at this altitude, then engage nuke for going higher and orbit. 4 doing that, theres a trick wiz connector, put a connector anywhere, put an airintake on it, an place the connector where u want, not the air intake. By tthis way, ull be able to make a "rail" of air intake, where lot of airintake are close to the same position (not reallistic, but very usefull, and not an Alt-F12 solutions (sry 4 my Eng, im Fr..)

Edit :

Replace 2 of ure engine by low altitude one (the first one we can acces in career), and sry, i hadnt read that u still have 36 airintake

On that pics, u can see engine section of one of my bird, notice the engine extreme right and left, wizout them, its very hard to pitch up at low altitude

Edited July 30, 2014 by FacialJack

[WDude4k]

I wanted to update where I'm at having read all the comments.

I went ahead and moved some engines around. I've moved 4 of them into the cargo hold, and added a 5th in the very center (for the added benefit of letting it go till it flames out).

As shown, the CoT is much closer to the CoM and I compensate the rest of that gap by turning off the outer most engines as I gain more altitude.

My problem now is at about 32km. Even with the available jet engines (1-3 depending on throttle) + nukes firing I'm losing altitude. I can't turn the behemoth too vertical or it starts flipping, plus I don't think I have enough force with those engines to do a straight vertical push anyway.

Any Suggestions?

[Wanderfound]

My problem now is at about 32km. Even with the available jet engines (1-3 depending on throttle) + nukes firing I'm losing altitude. I can't turn the behemoth too vertical or it starts flipping, plus I don't think I have enough force with those engines to do a straight vertical push anyway.

Any Suggestions?

Nukes don't have enough thrust for getting to orbit, and air-breathing engines are largely ineffective over 30,000m; even if you can keep them running, their power decreases as you climb.

Ditch the nukes (unless you feel you absolutely must have them for long-range interplanetary stuff) and swap out some of the turbojets for RAPIERs or Aerospikes.

[WDude4k]

Would you call me crazy if I told you I was planning on taking this puppy out to duna? I'd like to keep some nukes for deep space delivery.

[Hodo]

Would you call me crazy if I told you I was planning on taking this puppy out to duna? I'd like to keep some nukes for deep space delivery.

The nukes are fine, you just have thrust balance issues now. Keep messing with it till you get your CoT running through the center of the CoM.

Otherwise you will continue to have pitch control issues.

[FacialJack]

Would you call me crazy if I told you I was planning on taking this puppy out to duna? I'd like to keep some nukes for deep space delivery.

Not at all !! but duna atmo is very thin, not enough for lifting, or wiz moremoremore wings than on Kerbin

Edited July 30, 2014 by FacialJack

[buzz66boy]

I don't know if you've solved it yet, but here's my input.

Noticeable issues:

1. Your COL behind your mass is causing a downward pitch at high velocity

2. Your COT is above your COM making the downward pitching worse.

Normal Solutions:

1. Move your COM up and to the rear (and/or)

2. Move COL forward

3. Move wings down so COT is behind COM

Alternate Solutions:

1. Make it a Bi-plane (with jets on upper and lower wing mirrored like)

2. Make a canard on the front, kinda like a tail section, this will give you more control and bring COL forward

Try and keep the COL and COM inside eachother.

Also I use nukes a lot in SSTO designs, so it's just finding the balance between payload, number of jets and nukes.

Edited July 30, 2014 by buzz66boy