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Please someone unveil the secret of long trip SSTOs


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Hi, i've been playing KSP longtime, before 1.0, and surfing the forum sometimes i see here and there people talking about his SSTOs that can land Duna and return, for instance. 

I find quite challenging making a SSTO spaceplane that can orbit and land without spinning, once in a forgotten savegame i managed to create one able to land on minmus (or was just orbit?) and back, but never dreamed about interplanetary travel with a SSTO, i looks like magic to me.

So, what advices and tips can you give me an us to achieve that high objective?

thanks

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Don't use MK2 parts. Either go lean and mean with MK1, or just build a gigantic MK3 monster with a billion tons of fuel. After that, it's a matter of reducing drag as much as possible, getting to LKO efficiently, and of reducing mass.

 

 

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17 hours ago, juvilado said:

I find quite challenging making a SSTO spaceplane that can orbit and land without spinning ...

So I'd say you want to run before learning to walk. Chances are that whatever you may be doing wrong with LKO only spaceplanes will also be issues with your interplanetary spaceplanes. 

Also in that sense,  a iterative approach is usually a good way to go. Whatever you learned with LKO only spaceplanes will be useful for interplanetary vessels also,  no need to reinvent the jet. 

Finally,  decide what your objectives are (e. g.  Scientific exploration of duna)  and start designing.  If you find a problem you can't solve yourself (e. g.  difficult to land on Duna,  crash every time)  post a picture and ask for help. 

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It's helpful to think back to the Rocket Equation.  There are three things (and only three things)that will determine your delta-v: specific impulse, wet mass (fuel), and dry mass (everything else).  So the short answer is: use nukes, bring as much liquid fuel as possible, and bring as little of anything that's not LF as possible.  On that last one, you want the absolute minimum of engine mass and control surfaces that you can survive with.  Of course, you still have to get the thing to orbit, so you have to strike a bit of a balance with atmospheric performance.  But if want super-long range, you're not going to have something that performs like a fighter-plan in atmosphere.  

A few more specific tips:

  • Cut mass ruthlessly.  Figure out your mission requirements in advance and don't take anything that isn't absolutely necessary for the trip.  Cut engines to the bare minimum needed, and be prepared to live with sluggish performance (e.g., split your burns into multiple passes).  Unless you expect CommNet problems, a probe core / reaction wheel / passenger cabin may save mass compared to a cockpit.  Use the smallest landing gear that will handle your plane's weight.  Drain monopropellant and don't use RCS thrusters.
  • Cut drag ruthlessly.  If you're following the "wet mass good, dry mass bad" concept, your plane will be underpowered and will need to have good aerodynamic design to punch through the dreaded transonic barrier.  KSP's aerodynamic model is a little counter-intuitive sometimes, so you may want to read up on the details.  But generally you want to be as streamlined as possible, with minimal stuff sticking out the sides of your plane.  Make sure all your parts connect at the same form factor (i.e.,  1.25m to 1.25m, or Mk. 2 to Mk.2).  If you need to transition, use an adapter.  Giving your wings a little angle of incidence (i.e., rotating the front end of the wings up a few degrees) is one nice trick to help with drag.  
  • Use as few control surfaces as possible, since they add both mass and drag.  Place your control surfaces as far from the center of mass as possible, which will increase their effectiveness and allow you to use fewer or smaller surfaces.  Canards are often good if your COM is closer to the rear of your plane. 
  • Design iteratively. If your plane makes it to orbit, ask yourself if you can add some more LF and lose any other parts (including engines).  Try that, and if you can't make to orbit, iterate back the other way.  A good overall measure of your performance is how much delta-v you have left in the tank once you're in LKO.  If a design gets you to orbit with more delta-v, it's a step in the right direction.  
  • Big-S wings and strakes are crazy good.  They have the same lift: mass ratio as the other wings, plus come with free LF storage.  And they have a very high heat tolerance to boot.  Conversely, the swept wings are twice as heavy as other wings so they should be avoided like the plague.  FAT wings have fuel storage and large surface areas, but they have terrible heat tolerances and are prone to breaking when stressed. 
  • Assuming you use Rapiers, try to minimize the amount of time you use them in closed-cycle mode.  They're much less efficient than nukes, and their oxidizer will not be useful for your other engine modes.  Turn on your nukes while the Rapiers are still in jet mode, but losing thrust, and plan to ride the nukes all the way to orbit.  If all goes well, you may be able to only use the closed-cycle Rapiers for a few seconds to give you a little extra push.  Only bring the bare minimum of oxidizer needed for this little push.  Hopefully this will fit in some of your adapter parts, so you won't need a dedicated LFO tank.  
  • Ascent profiles are a topic among themselves, but I think the most important part is to get your plane going as fast horizontally as possible at the point your jets give out.  
  • Pay attention to the thermal tolerances of your parts, especially if you plan to aerobrake (say, at Laythe, or at Kerbin on the way back).  Every bit of delta-v you can save while aerobraking is more range for your mission.  
  • Design (and test) your plane for reentry and landing.  In particular, make sure your plane does not become unstable after your fuel has burned off.  This is a very common issue with planes that have all the heavy engines at the rear.  
  • Get good at gravity assists, especially if you're heading to the Jool system.  They can save hundreds of m/s of delta-v.  
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Interplanetary / Grand Tour SSTOs:

1) You need NERVAs. Some folks go for pure LF Turbojet/Nuke builds, but I find that RAPIER/Nuke ships (or RAPIER/Panther/Nuke ships; the extreme thrust vectoring ability of the Panther makes it ideal for off-centre mounting with minimal angling required) are much more versatile and pleasant to fly. The RAPIERs weigh a bit more, but their higher air breathing top speed more than compensates. Additionally, a few seconds of closed-cycle boost is good for getting through the 30-40km altitude region and up into the low-drag upper atmosphere where the nukes can easily finish the job.

2) Fuel mix: you want just a smidgeon of oxidiser to help boost your apoapsis, and then you want to stuff in as much LF as you can possibly carry. Fuel-carrying wings are useful; so are NCS nosecones. If you've got any spare room in a cargo bay, stuff the gaps with small LF tanks.

3) Body shape: you can do Mk1 interplanetary ships, but for a Grand Tourer you'll need ISRU. That means either Mk2 or Mk3. There's nothing wrong with Mk2 fuselages, but you do need to build appropriately: they require wing incidence to minimise fuselage drag. Angle the leading edge of the wing up a couple of degrees so that the nose stays on prograde during the speed run.

4) If you're going interplanetary, that means rough landings. For vacuum worlds: belly vernors for low-g VTOL. For atmospheric worlds: wide-set landing gear and drag chutes for post-touchdown braking.

5) Make sure that everything in your cargo bays is actually recognised by the game as being shielded by the bay. ISRU drills are particularly bad for this; if they're mounted near the edges of the bay the game is likely to treat them as unshielded, and they create a huge amount of drag.

6) You want to crank the speed up to 1,500m/s while below 20,000m (beware overheating), hit the 20,000m mark with as much vertical speed as possible (without dropping below 1,500m/s horizontal speed), turn the nukes on at 20,000m and keep the RAPIERs in air breathing mode until 29,000m. Burn all of your oxidiser after switching mode (save a little if you're using vernor VTOL) and circularise on the nukes.

7) Heat tolerance matters, and you're only as strong as your weakest link. Don't leave low-temp parts like science gear exposed to the airflow.

8) Fuel cell arrays are the simplest way to power ISRU rigs. Yes, they burn fuel, but if you're mining that doesn't much matter. Carry an RTG or a bit of solar to handle power generation when not mining. Fuel cells also double as batteries; you don't need both.

Some demonstration ships:

https://kerbalx.com/Wanderfound/Kerbodyne-Pathfinder

 

5UxKfup.jpg

https://kerbalx.com/Wanderfound/Kerbodyne-Aqua

 

 

 

All of those have sufficient range for interplanetary trips. The latter two are ISRU equipped and can go direct from KSC to Minmus, the first wants a refuel in LKO to get the most from it. The latter two are also seaplane-capable, but a similar design without the aquatic gear would work equally well.

You'll maximise your range if you strip your jet thrust down to the bare minimum, but it is possible to tour in a hot rod if you want:

That one is an oldish ship now; the design isn't as polished as I'd manage these days.

Edited by Wanderfound
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On 6/4/2017 at 3:10 AM, bewing said:

Don't use MK2 parts. Either go lean and mean with MK1, or just build a gigantic MK3 monster with a billion tons of fuel. After that, it's a matter of reducing drag as much as possible, getting to LKO efficiently, and of reducing mass.

 

 

Sorry but mk2 is just too perfect for space plane, so I would disagree.  But the rest is spot on.  It takes practice to get a good range out of an ssto.  But you can also think outside the box.  For example, this is technically a ssto, but I earn some of my dv back by leaving some weight behind once in orbit.

https://kerbalx.com/ForScience6686/MSS

I reconnect with the wing segment for reentry, so all parts come back.

Edited by ForScience6686
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15 minutes ago, ForScience6686 said:

Sorry but mk2 is just too perfect for space plane, so I would disagree.  But the rest is spot on.  It takes practice to get a good range out of an ssto.  But you can also think outside the box.  For example, this is technically a ssto, but I earn some of my dv back by leaving some weight behind once in orbit.

https://kerbalx.com/ForScience6686/MSS

I reconnect with the wing segment for reentry, so all parts come back.

Wow, that concept is just fantastic!

12 hours ago, Spricigo said:

So I'd say you want to run before learning to walk. Chances are that whatever you may be doing wrong with LKO only spaceplanes will also be issues with your interplanetary spaceplanes. 

Also in that sense,  a iterative approach is usually a good way to go. Whatever you learned with LKO only spaceplanes will be useful for interplanetary vessels also,  no need to reinvent the jet. 

Finally,  decide what your objectives are (e. g.  Scientific exploration of duna)  and start designing.  If you find a problem you can't solve yourself (e. g.  difficult to land on Duna,  crash every time)  post a picture and ask for help. 

I might be a little exagerated... 

I just made a SSTO spaceplane able to land on minmus and back in one piece,

9thdn5.png

but i find really difficult to optimize a vessel to do the same for Duna without refuelling...

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Well duna doesn't take much more, plus you can use it's atmosphere to save you some dv.  Ssto space planes are not an easy task, but you're almost there.  Some tips though, you only need the shock cone intake, the rest are just hurting you.  I would also guess you could do it with just one rapier in that design.

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9 hours ago, ForScience6686 said:

Well duna doesn't take much more, plus you can use it's atmosphere to save you some dv.  Ssto space planes are not an easy task, but you're almost there.  Some tips though, you only need the shock cone intake, the rest are just hurting you.  I would also guess you could do it with just one rapier in that design.

Correct me if im wrong, but won't i need as much air as i can to feed the jets? If not, they won't hav enough power, so the plane won't catch enough speed?

Thanks

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1 hour ago, juvilado said:

Correct me if im wrong, but won't i need as much air as i can to feed the jets? If not, they won't hav enough power, so the plane won't catch enough speed?

Thanks

You don't need as much as you might think, especially when you get going faster. I saw a craft once with 2 precoolers and 2 intakes feeding 6 rapiers, and it did fine. So on that, you probably only need the shock cone intake.

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1 hour ago, juvilado said:

Correct me if im wrong, but won't i need as much air as i can to feed the jets? If not, they won't hav enough power, so the plane won't catch enough speed?

Nope.

You want enough air to keep the jets at 100% air supply until they reach their altitude ceiling (about 29,500m for RAPIERs); any more is a waste. In early versions of KSP there was an air hogging exploit, but that hasn't been the case for years.

My example ships upthread are running two, three and five jets respectively. The first two are fed by a single ramscoop/nacelle combo, the third just has a pair of shock cones. All of them can easily get up to cockpit-melting speed and extreme altitude.

One shock cone or intercooler will feed three RAPIERs; one ramscoop will feed two. Any excess intake is just wasted mass and drag.

Edited by Wanderfound
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1 hour ago, juvilado said:

Correct me if im wrong, but won't i need as much air as i can to feed the jets? If not, they won't hav enough power, so the plane won't catch enough speed?

Thanks

Each engine can only accept air from one intake, at most. So the most you can possibly theoretically use is two on your plane. And as said above, the shock cone all by itself is plenty for your two engines.

 

Edited by bewing
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Thanks, You have been of great help!

With your advices, I refined the spaceplane and managed to orbit LKO with 2K dV left, so thats a great improvement over previous designs.

Now let's aim for a bigger bird...

 

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