FAR SSTOs

[viktor19]

I have already made two functional SSTOs with FAR and B9 installed but I'm asking you for some general tips on SSTO building, particularly stability on reentry, fuel amount and balance.

Any help is appreciated!

[Wanderfound]

See posts #2 & #3 in the Kerbodyne thread linked below, particularly this tutorial: http://s1378.photobucket.com/user/craigmotbey/Kerbal/Tutorials/Hangar%20to%20Landing/story

[Pecan]

For a SSTO rocket (or 'tail-sitter' jet-launched vehicle without wings) aim for 500m/s more deltaV than required to get to orbit and include 2 - 3 drogue parachutes. De-orbit from LKO should take ~200m/s dV, the rest is for a soft drogue-assisted powered-landing.

[LethalDose]

I have already made two functional SSTOs with FAR and B9 installed but I'm asking you for some general tips on SSTO building, particularly stability on reentry, fuel amount and balance.

Any help is appreciated!

• Take as little LF and Oxidizer as you can. That way you don't have to lift excess weight into orbit.
  
• Move most of the fuel into the forward tanks as needed on re-entry ("lawn dart"). You can shift the fuel towards the aft if you need to more pitch authority during landing or atmospheric flight.
  
• You can also lock the fuel flow in the forward tanks during ascent to get the the CoM forward.
  
• Try to build vessel so that your tanks are close to the CoM of the vessel to reduce how the mass shifts during flight.
  
• Try to put your pitch control surfaces as far forward and aft from the CoM as possible to maximize pitch authority.
  

I think that's the most generic advice I can give. If you post your vessels, we can give more specific advice.

[Empiro]

I've found that under FAR, the following general design elements are helpful:

-A full-moving tailplane (like the F-22) is extremely helpful in maintaining pitch authority. A canard can also help, but I rarely have found the need for both.

-I like to keep the wings mostly flat at a 0 degree angle, because it makes mounting things easy, but I've found that having small wing parts angled upwards at the tip (i.e most wing parts flat, but the part at the very tip is angled up) greatly improves stability at high altitudes and speeds

-I often wind up using a fairly large tailfin to reduce sideslip tendency. It makes the craft look somewhat ugly, but it's what works

-For cargo-bearing SSTOs, I like to put fuel tanks in front and behind the cargo bay. While this increases how much the CoM may shift during flight, you also have greater control by transferring fuel around. Ideally, your CoM when fully fueled, totally empty, with payload, and without payload (and all combinations thereof) are all close to the same place.

-Use airbrakes. It saves you time when landing, and gives you far more control over your speed. It lets you have much steeper re-entry angles without risk of burning up or breaking apart.

[LethalDose]

-Use airbrakes. It saves you time when landing, and gives you far more control over your speed. It lets you have much steeper re-entry angles without risk of burning up or breaking apart.

Mind if I ask how you use them (Altitude, speed, situation, settings, etc.)? Every time I've tried air brakes, they always seem to get pulled right off the wings. I'd like to learn how to use them 'right'.

[Hodo]

With B9 you can actually get away with placing fuel alongside your cargo hold which is pretty nice for balance issues.

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This craft the cargo hold is actually pretty well centered in between the fuel tanks.

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This craft on the other hand had its fuel tanks fore and aft of the craft hold with TAC fuel balancer I am able to keep the CoM pretty much where I want it, which is nice.

But it also has canards and a delta wing design. The trick is to use decent airbraking before landing or even getting down to the lower atmosphere.

I highly suggest one mod you should get for your SSTO building needs is RCS Build Aid. It will show you your dry and wet CoM at the same time. You can even configure it for different fuel loads and to show your average CoM. It will even tell you your CoM movement distance which is nice also.

RCS Build Aid

Edited January 12, 2015 by Hodo

[Wanderfound]

Mind if I ask how you use them (Altitude, speed, situation, settings, etc.)? Every time I've tried air brakes, they always seem to get pulled right off the wings. I'd like to learn how to use them 'right'.

We'd need more details: what parts, what speed, what altitude, what were the FAR mass tweakables set to, etc.

Offset spoiler/flaps (one up, one down) work well as stock-FAR airbrakes:

[viktor19]

We'd need more details: what parts, what speed, what altitude, what were the FAR mass tweakables set to, etc.

Offset spoiler/flaps (one up, one down) work well as stock-FAR airbrakes:

http://i1378.photobucket.com/albums/ah120/craigmotbey/Kerbal/Challenges/Akademy%20Awards/Kerbodyne%20Atlas/screenshot16_zps5f7ce745.jpg

Um...yellow plane? How?

*scratches his head*

[LethalDose]

We'd need more details: what parts, what speed, what altitude, what were the FAR mass tweakables set to, etc.

That's exactly the information I'm asking for from the people who do use them...

Edited January 12, 2015 by LethalDose

[Wanderfound]

Yellow plane = Kerbpaint.

For me, to build airbrakes I use nornal FAR control surfaces set as maximised opposed spoilers. Place them behind CoM for stability, make them the same strength as the rest of your wings. Haven't torn one off yet.

If they're too far forwards they may cause instability problems at hypersonic speeds, but they'll still generally be usable at subsonic speeds. Stick 'em up back and you can use them at whatever speed and altitude you like.

[viktor19]

Yellow plane = Kerbpaint.

Thanks! Note taken.

[LethalDose]

For me, to build airbrakes I use nornal FAR control surfaces set as maximised opposed spoilers.

Can you be more specific about what you mean by "Maximized opposed spoilers"? Is this placing spoilers above flaps to increase drag?

And in retrospect, it was flaps that were getting ripped off during flight. It's been > 1 year since I tried using spoilers or flaps, so I really can't give examples of where they were problematic.

[Alshain]

Can you be more specific about what you mean by "Maximized opposed spoilers"? Is this placing spoilers above flaps to increase drag?

And in retrospect, it was flaps that were getting ripped off during flight. It's been > 1 year since I tried using spoilers or flaps, so I really can't give examples of where they were problematic.

In FAR you can control which direction spoilers will deflect. Air brakes create drag without altering lift (unlike real spoilers which alter lift), so you need equal and opposite. What he is saying is put spoilers in the same place that go in both directions. If you want an example, download the Harrier from my exchange thread in my signature and use the action group to toggle the air brakes (listed in the info dialog). I put NASA Shuttle-like air brakes on that one, you can put them horizontally on the wings as well but you have to be really careful doing that, if you don't place them just right, you will alter lift.

Edited January 12, 2015 by Alshain

[Van Disaster]

In FAR you can control which direction spoilers will deflect. Air brakes create drag without altering lift (unlike real spoilers which alter lift), so you need equal and opposite. What he is saying is put spoilers in the same place that go in both directions. If you want an example, download the Harrier from my exchange thread in my signature and use the action group to toggle the air brakes (listed in the info dialog). I put NASA Shuttle-like air brakes on that one, you can put them horizontally on the wings as well but you have to be really careful doing that, if you don't place them just right, you will alter lift.

To further this - don't overlap control surfaces any more than you'd overlap wings, because you'll get biplane effects even if neither surface projects above or below the wing profile. Note CoL:

Better to use neighbouring pairs, which needs a bit of care if the control surfaces are swept. Alternatively I set the brakes to deflect up & use flaps to balance, which saves a set of surfaces.