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SSTO pitching up when thrusting at max?


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I created an SSTO which works fine in Kerbin atmosphere, has plenty of fuel to get into orbit, however, when trying to circularise the orbit and using the nuclear engines in orbit, the SSTO keeps on pitching up, even when all control surfaces are turned off. Any help please?

The SSTO has a mass of 56.3k kg, 4900 units of liquid fuel and 880 units of oxidiser, with 4 rapier engines and 2 nuclear.

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Hello, and welcome to the forum!  :)

1 hour ago, Jager798 said:

I created an SSTO which works fine in Kerbin atmosphere, has plenty of fuel to get into orbit, however, when trying to circularise the orbit and using the nuclear engines in orbit, the SSTO keeps on pitching up, even when all control surfaces are turned off. Any help please?

The SSTO has a mass of 56.3k kg, 4900 units of liquid fuel and 880 units of oxidiser, with 4 rapier engines and 2 nuclear.

From the symptoms, my guess would be that the thrust vector isn't lined up with the CoM, e.g. if the engines' CoT is lower (more ventral) than the CoM.  But that's just a guess, it's pretty hard to tell without seeing the craft.  Could you post a screenshot?

(Certainly the control surfaces have nothing to do with the problem, since you're out of atmosphere and therefore they have zero effect on anything.)

Instructions for how to post a screenshot in spoiler, in case you're wondering.  :wink:

Spoiler
  1. Take an in-game screenshot using F1.  It gets saved to the "Screenshots" folder of your KSP installation.
  2. Go to www.imgur.com.  (You don't need to make an account or anything, this works fine anonymously.)
  3. Click on the green "new post" button up top.
  4. Drag your image into the "drop image here" box that it pops up.
  5. As soon as the post is complete, right-click on the image on the imgur page and choose "Copy Image Location".
    • Important, not the URL of the page on Imgur.  You need the URL of the image, i.e. it should end in ".png".
  6. Paste that URL into your forum post here.  It'll get automagically converted into an in-line image.

 

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Even without a screen shot, I'm 99% sure it's a COM/COT alignment issue. 

While you may have built it with the correct alignment in the SPH, you may have designed it so the fuel is consumed asymmetrically.   So when you get to orbit, the COM is no longer in the same place you put it. 

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Yep it's a CoM/CoT problem for sure. Something everyone designing spaceplanes will bump into sooner or later. It's called "thrust torque."

There are three ways to address this. They can be combined.

  1. Adjust your design so that the CoM/CoT line up. This usually means a rocket-like design with everything symmetrical around a central axis, except light essential things like the rudder and landing gear. This is preferable but not always possible, if you want to do something with your plane.
  2. Make it so you can change the CoT in flight. For example, if your craft is pitching up, switch off your lower row of RAPIERs as you switch the others to closed cycle; you can do both in one action by binding this to an action group. You won't need all that thrust while up there anyway. Fine-tune by adjusting the thrust limiter by hand if necessary.
  3. Add something to compensate for the thrust torque. Relatively small torque can be cancelled out by reaction wheels (although in these cases it's usually possible to tune the engine position to make the problem go away altogether). More powerful alternatives include putting attitude control jets (e.g. a pair of vernors pointing up and down) or a rocket engine on a gimbal (e.g. Swivel, Vector) on the tip of the tail. This works very well on designs that have long tails, and the extra fuel expenditure is a rounding error.
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Thanks guys for the help. Seems like it was the CoT that was slightly lower than the CoM as you suggested. I have adjusted the engine layout so the CoT is now inline with the CoM and will see if it works later when I have time. Thanks for your help as I am kinda new to SSTOs.

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If you don't want to redesign, consider if you need all your vacuum thrust. My primary crew ferry is a mk 1 fuselage designed for high efficiency single RAPIER use. The low mounted wings and gear result in significant thrust torque when dry. Reaction wheels can overpower it, but SAS doesn't use them correctly. The circularizeation/rendevous burn still has minimal torque due to excess fuel; it always dumps some surplus fuel at station unless I make a mistake or more than 1 rendevous. I just deorbit at half-power and the torque is more manageable. Any design technique to manage the thrust torque for 1 low accuracy burn would be more costly for the higher risk mission portions!

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  • 3 weeks later...

@Jager798 One thing you must consider is that the center of mass might change as you consume fuel. Fuel acounts for a large percentage of your total mass. So if your tanks are not aligned with your overall craft,  the center of mass will shift during flight as the fuel drains - sometimes dramaticly!

Most rockets have a gimbal angle. In the first stages of the flight the CoM shift may occur within the scope and limits of your rocket engine's gimbal,  and the compensating gimbal will counter the CoM shift. But if the CoM shifts further than the gimbal angle,  you will FOR SURE have an uncontrolable plane - one that no control surface is able to counter.

A good tool to check both your FULL and EMPTY CoM is a mod called "CoM helper",  found in spacedock. It will clearly point the path of the CoM as fuel is depleted,  and help you design better craft.

A final note: maybe your spaceplane can be salvaged as it is. Maybe the empty CoM is overpassing the gimbal limit of your engine by just a little bit. Did you consider offsetting the angle of the engine UPWARDS, just a little bit?  Check the historical spaceshuttle: the engines point UPWARD a little bit,  just for that reason. Maybe you could twist them upward,  test it,  and keep twisting until the entire CoM shift is within the engine's total gimbal angle. It will only not work if the twisting required is so high,  that the CoM will over-excede to the other side,  making your nose point down right after takeoff!

Also: some engines have higher total gimbal angles than others. Consider that.

One last resort: redesign your plane,  so that the fuel tanks are more in line with the overall fuselage. This way,  CoM tends to remain in the same place as fuel is depleted. Maybe that should have been the first resort anyway!

 

Edited by Daniel Prates
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