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MarkvirsII

Cant get into orbit

Question

SO I built a ssto spaceplane but I cant get it into orbit since It likes too do mid air flips and stuff and forsome reason does not have enough thrust but its almost the same spaceplane I copied from a video just some changes  here and there to make it more easier too fly but it still does flips and the thrust is not enough

 

So what changes should I make too it?

 

http://prntscr.com/ndun6d

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Flips could be centre of lift too far forward, you don't have it switched on in your screenshot.

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2 hours ago, MarkvirsII said:

So what changes should I make too it?

Probably the only change you need is how you fly it. You're probably trying to fly it like a spaceplane, when it's really built more like a horizontally launched rocket. There's a number of things that are 'wrong' on it for a spaceplane, but it also carries enough engine power and fuel to make it to orbit despite the design errors.

Try this:

  • Full throttle until it rolls off the runway (with the main gear so far back you won't be able to pitch up before that anyway).
  • Then pitch up to 10-15 degrees and hold there. Don't try to adapt pitch from here on, it will only make the nose dip down.
  • Stage Nerv when you stop accelerating.
  • Let the rapiers auto-switch to closed cycle, or manually switch if you have set an action group for it. At this point adjust pitch slowly to prograde and set SAS to follow prograde.
  • Cut throttle when you reach 80km apoapsis.
  • Circularize at apoapsis.

After flying a rebuilt copy of your plane like this, I still had enough fuel in orbit to do a Mun flyby, return to Kerbin and land - although the last part was very slow on just the Nerv.

 

As for making this more into a 'real' spaceplane:

  • CoL is too far behind CoM. This is the main cause of the problem you describe where it dips down and it takes too much effort to pitch up again. There's a lot of wing at the very back of the plane which causes this. There has to be wing sections clipped into the back end of the body too that we can't see on the images, to make the CoL so far back. Remove those, and add some more wing towards the front end. Your goal should be to get the CoL very close behind the CoM.
  • Wings have no angle of incidence. The wings have been attached 'straight', which means they don't provide any lift when flying prograde. They need to be angled up a bit, so they generate lift while the plane's draggy body keeps pointing prograde. You can use the rotate tool, while holding shift for fine adjustment, and a single 'tick' will add a 5 degree angle. This may not be optimal but it's close enough for your first attempts. It will make the CoL drop even farther back though, so they wings will need to be ofset a bit towards the front.
  • I count three Mk2 monoprop tanks in that fuselage. That's really two and a half too many, in a tank type that is less than optimal. Cut them out entirely, or exchange them for regular fuel, and add two FL-R25 mono tanks on one of the engine pod sets instead. Better yet, exchange them for a long Mk2 cargo bay, and now your plane can carry stuff up. Two small mono tanks can go in the cargo bay, and are plenty for any orbital maneuvers.
  • The engines look to be 'uncapped' - they have no nose cones on the back node. Due to how KSP calculates drag, this adds unnecessary drag. Add some nose cones on their back, then offset them into the engines so they don't block the exhaust. Drag is a key factor in being able to make orbit for a spaceplane, so it's important to optimize for it.
  • The fuel is unbalanced. Always build your spaceplanes with empty tanks first, and then place/fill tanks in such a way that the CoM stays mostly in the same place. That way the handling of your plane remains predictable regardless of how much fuel you have left.
  • RCS/vernor thuster placing is... chaotic. It looks to have been placed for looks instead of for function, and it will likely not work correctly when you need it in orbit. Also, they add drag, so you need to ask yourself if you really need them, and if so, how to use as few of them as possible.

This size/weight craft (~50t) should not need 4 rapiers to get to orbit, especially if you remove the excess monoprop. The single Nerv on the other hand makes for very slow orbital acceleration. Two and two would probably make a better mix.

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Posted (edited)

The important bit is that to be aerodynamically stable center of mass needs to be in front of center of lift (the blue sphere), which isn't pictured. And for easy take off the main landing gear should be positioned just a little behind the center of mass.

Keep in mind that planes are MUCH harder to design and fly than staged rockets, especially SSTO's. If you haven't gotten to orbit yet, I might start with a rocket.

Edited by BadOaks

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Posted (edited)

Huh, that's weird that it's flipping. Flipping usually happens when the CoL is in front of the CoM. Normally you actually want the CoL closer behind the CoM than what you have there. If anything, I would think your plane should be lawn-darting into the ground (the opposite problem).

16 minutes ago, MarkvirsII said:

THis counts as a safe landing still? http://prntscr.com/ndv7fe

Well... normally the phrase is "any landing you can walk away from is a good landing" but I think we can accept "swimming away" this one time.

Edit: Here's a good tutorial for plane design that might help you troubleshoot.

Edited by BadOaks

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WHen it flips it just dips down out of control and I waste fuel trying to get it up 

 

But can it reach orbit even?

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Depends on the engines and fuel mass ratio. SSTOs have very tight margins, which is why they are the most challenging craft to build. But first focus on making something that's flyable. Orbit comes later!

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Posted (edited)

My SSTO is very similar and also has big problems in the territory where wing lift starts failing and engine control takes over.

Also with running out of fuel if everything doesnt go just right.

Two things you could check out.

1. Timing of switching the engines from airbreathing to closed cycle. No good advice here, I usually get 1000m/s at maybe 30° upwards and switch when engine thrust drops below 100.

2. Check out what controls are used in what control surfaces. I always have problems with slanted wings, and when yaw is enabled for too many flaps.

 

Also, I've yet to build SSTO that'd be actually useful for something. They pretty much can just reach orbit and come back, no payload or anything.
"Normal" rockets with boosters are much more usable. Or spaceplanes with boosters, then you can also afford payload.

Edited by Jarmo
btw, ssto is useless

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13 hours ago, Jarmo said:

Also, I've yet to build SSTO that'd be actually useful for something. They pretty much can just reach orbit and come back, no payload or anything.

"Normal" rockets with boosters are much more usable. Or spaceplanes with boosters, then you can also afford payload.

My Mk2 spaceplane could lift about 6 tonnes or so to lko with enough fuel margin to do orbital rendezvous. I didn't have rapiers either as it was in career. I mainly used it for shuttling tourists and crew up and down though as it is much cheaper than using staged rockets for that.

I recall having a lot more wing than the OP though, huge delta wings in fact to give enough lift. They also meant I could glide land it fairly easily.

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I wonder if body lift isn't having a pretty major impact ... it's the only thing I could see making enough of a difference to cause flipping.

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On 4/19/2019 at 4:24 AM, Flibble said:

I recall having a lot more wing than the OP though, huge delta wings in fact to give enough lift. They also meant I could glide land it fairly easily.

Agreed, I would suggest adding a lot more wing (in addition to @swjr-swis''s other good suggestions).  You may have tried to compensate for low wing area by adding more raw power in the form of Rapiers.  This puts a lot of mass at the back, exacerbating the lawn-dart issue others have described.  But it also hurts your fuel efficiency, and hence your effective range and payload.  

Are you planning on docking this plane with anything (I can't always tell if there's an inline docking port).  If not, I'd strongly consider getting rid of all monopropellant stuff.  If want/need RCS for attitude control, the Vernors you have are probably the simpler option.  Personally I just abuse reaction wheels, though.

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On 4/18/2019 at 3:53 PM, swjr-swis said:

CoL is too far behind CoM. This is the main cause of the problem you describe where it dips down and it takes too much effort to pitch up again. There's a lot of wing at the very back of the plane which causes this. There has to be wing sections clipped into the back end of the body too that we can't see on the images, to make the CoL so far back. Remove those, and add some more wing towards the front end. Your goal should be to get the CoL very close behind the CoM.

 

I agree with this being the likely big culprit, along with there being (what looks to be) not enough pitch authority. The ship probably doesn't have enough lift to brute force the nose around. Then, when the nose starts to drop, it takes full pitch to pull it back up. With the control surfaces deflected, it's likely also stealing a significant portion of the aircraft's lift and causing a lot of induced drag. It becomes a self feeding cycle once you get into the nose fight.

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