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Kerbodyne SSTO Division: Omnibus Thread

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blowfish

blowfish

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8 hours ago, eddiew said:

Pretty big for 8 rapiers :)  also nice to see those wings used in the middle of a plane, not the back end... much better looking this way.

On the contrary, I'd call it a bit small for 8 RAPIERs.  But maybe as a tanker it's denser than most spaceplanes...

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Wanderfound

Wanderfound

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On 7 December 2015 at 3:27 AM, blowfish said:

On the contrary, I'd call it a bit small for 8 RAPIERs.  But maybe as a tanker it's denser than most spaceplanes...

Yup; change it to normal cargo and you could easily lose two engines.

The limiting factor is reaching takeoff speed before the runway ends [1]. If you're willing to strap some RT-10 RATOs on the wings, you can usually afford to ditch a couple of jets.y

 

[1] You could also add more wing area, but my sense of aesthetics stops me from trying to double up the non-modular large wings.

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Mens rea

Mens rea

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Hey Wanderfound,

I've been looking at your designs and trying to build an SSTO that can go anywhere without orbital refueling (via mining). However, after several days I still can't seem to be able to get it with significantly over 1000 m/s delta-V by the time I circularize. I tried more rapiers and aerospikes but that just made it heavier and ended up with even less dV. Nukes have turned out to be even more useless (<400 dV, probably because the damn thing is so massive). I feel like I've got my ascent decent enough that my design is now the limiting factor.

Ideally, I'd like around 2000 m/s dV left over even with a sub-optimal ascent profile so I have some room for error.

Forgot to take a screenshot of it but all my stability derivatives are green across the range of altitude and mach numbers I encounter. I've got some roll/yaw instability past the 20k mark, but nothing the vernors can't fix. I've got a little more wave drag at the tail than I'd like so that could be the problem. I've added pictures of the design as well as screenshots along my ascent in case I'm doing something obviously wrong.

Only non-stock part is an IR hinge for the drill. All the surfaces have mass/strength at 0.5.

 

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Wanderfound

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9 hours ago, Mens rea said:

Hey Wanderfound,

I've been looking at your designs and trying to build an SSTO that can go anywhere without orbital refueling (via mining). However, after several days I still can't seem to be able to get it with significantly over 1000 m/s delta-V by the time I circularize. I tried more rapiers and aerospikes but that just made it heavier and ended up with even less dV. Nukes have turned out to be even more useless (<400 dV, probably because the damn thing is so massive). I feel like I've got my ascent decent enough that my design is now the limiting factor.

Ideally, I'd like around 2000 m/s dV left over even with a sub-optimal ascent profile so I have some room for error.

You're a bit caught in the middle there; that's a Mk3 size ship built out of Mk2 parts. The Mk2 parts have nice aerodynamics, but they aren't very efficient from a mass POV; a Mk2 fuselage holds exactly as much fuel as a 1.25m rocket tank of the same length, but is heavier.

You do want nukes for a long-haul ship, but you have to build to accomodate them. This means lots of pure LF tankage; you should be carrying just enough oxidiser to lift your apoapsis out of the atmosphere, before shutting down the high-TWR rocketry and using the nukes to circularise. And not too many nukes; they're heavy.

Self-sufficient long range ships don't need to be huge. You can do them in a Mk2 airframe if you're willing to tolerate the inefficient small ISRU converter:

 

Or you can go big (this one's a bit old, I'd set up the intakes a little differently today):

 

Remember, if you're carrying ore tanks, you can launch with them full then process the ore into LF for your nuke once you're in orbit.

Major issues with ISRU ships are power generation and cooling. You need a bunch of the mobile panels, plus usually a fair amount of fuel cells. A couple of small radiators for cooling is usually plenty.

If you're planning on landing off-world, you need to build for it. Wide and long wheelbase landing gear plus drag chutes for atmospheric worlds; Vernor VTOL for low-g airless landing.

Also, though: don't aim to have huge range after reaching LKO. Aim to have maximum range after refuelling in LKO. Set up an orbital gas station, keep it filled with spaceplane tankers. Without an LKO refill, you're inevitably hauling masses of empty tanks across the solar system.

If you manage to hit LKO with 2,000m/s delta-V (not too hard, the demo ships above do it) you can skip the orbital refuel; just land on Minmus and start mining. That's a lot of time and hassle just to avoid having a basic space station around Kerbin, though.

It also looks like you're burning a bit more fuel than you should thanks to your ascent profile. I normally try to get my RAPIER ships up to maximum non-melted speed (about Mach 4.5) while just below 20,000m, then I climb up to the RAPIER jet ceiling (around 29,500m) before lighting any oxidising rocketry.

Some ships work best with a low-AoA low-drag float to 30,000m; others prefer a sharp zoom climb from 18 to 30km, sacrificing a bit of horizontal speed in favour of some vertical momentum at switchover. In either case, though, the fireworks stay unlit until the jets choke.

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Mens rea

Mens rea

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9 hours ago, Wanderfound said:

Remember, if you're carrying ore tanks, you can launch with them full then process the ore into LF for your nuke once you're in orbit.

If you're planning on landing off-world, you need to build for it. Wide and long wheelbase landing gear plus drag chutes for atmospheric worlds; Vernor VTOL for low-g airless landing.

Also, though: don't aim to have huge range after reaching LKO. Aim to have maximum range after refuelling in LKO. Set up an orbital gas station, keep it filled with spaceplane tankers. Without an LKO refill, you're inevitably hauling masses of empty tanks across the solar system.

If you manage to hit LKO with 2,000m/s delta-V (not too hard, the demo ships above do it) you can skip the orbital refuel; just land on Minmus and start mining. That's a lot of time and hassle just to avoid having a basic space station around Kerbin, though.

It also looks like you're burning a bit more fuel than you should thanks to your ascent profile. I normally try to get my RAPIER ships up to maximum non-melted speed (about Mach 4.5) while just below 20,000m, then I climb up to the RAPIER jet ceiling (around 29,500m) before lighting any oxidising rocketry.

Some ships work best with a low-AoA low-drag float to 30,000m; others prefer a sharp zoom climb from 18 to 30km, sacrificing a bit of horizontal speed in favour of some vertical momentum at switchover. In either case, though, the fireworks stay unlit until the jets choke.

Thanks a lot for the help! I really appreciate it. I'll definitely get some orbital refueling up. I had the game crash my pc and corrupt my longtime career save not too long ago so I've been moping and messing around in sandbox lately. Just a couple follow-up questions:

- What is the mass of your Mk 2 ship? Also how much LF/Oxidizer is on the Mk 2 and Mk 3 ships? (I can never figure out the right balance)

- Actually on that note, any good rules of thumb about balancing mass vs fuel vs # of engines?

- How many vernors would I need for VTOL on an airless world? Is there a good rule of thumb (like 1 per X amount of mass?)

- Regarding my ascent profile, I'm guessing I didn't build enough speed before rising past 20k? I've been struggling to keep my ships level enough to do that. I either drop back down or rise past too quickly.

- I'm guessing you're just using the radial tanks for ore and that's the converter under the radiator to save weight over the small inline tank (which I'm using)? Would I still be able to keep my large drill setup (so I don't have to worry about landing in a zone with >2.5% ore concentration) or would that be too much mass?

- Regarding the nukes, how are you circularizing that Mk 3 ship on just one? It never seems to be able to do it for me even for lighter ships. Is that again because of my ascent profile? (not enough speed early on so I need more burn later?). Also, even when I do circularize a 100-ton ship on a nuke with 4-5k LF remaining, it says that's only about 500 dV. Is the kerbal engineer dV calculation incorrect for nukes? (Now that I think about it, does it show dV for all stage engines, not just the active one?)

- Somewhat unrelated, but how do you get the blasted elevons onto the large Mk 3 wing? The sweep on that makes it near-impossible for me.

- Also, not a huge deal but I noticed my spaceplane rocking (like back and forth) along its x-axis, in vacuum, with no RCS/SAS and no input on the controls. I am at a loss as to anything reasonable that could be causing that torque. Also, my orbital distance is actually INCREASING, again without any thrust. I'm sure it's just...erm...solar wind or something :P

 

Also, just have to say I love all your designs. They're really awesome to look at and read about, and an inspiration to go out there and build something cool.

Edited by Mens rea
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Wanderfound

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4 hours ago, Mens rea said:

Thanks a lot for the help! I really appreciate it. I'll definitely get some orbital refueling up. I had the game crash my pc and corrupt my longtime career save not too long ago so I've been moping and messing around in sandbox lately. Just a couple follow-up questions:

- What is the mass of your Mk 2 ship? Also how much LF/Oxidizer is on the Mk 2 and Mk 3 ships? (I can never figure out the right balance)

- Actually on that note, any good rules of thumb about balancing mass vs fuel vs # of engines?

- How many vernors would I need for VTOL on an airless world? Is there a good rule of thumb (like 1 per X amount of mass?)

- Regarding my ascent profile, I'm guessing I didn't build enough speed before rising past 20k? I've been struggling to keep my ships level enough to do that. I either drop back down or rise past too quickly.

- I'm guessing you're just using the radial tanks for ore and that's the converter under the radiator to save weight over the small inline tank (which I'm using)? Would I still be able to keep my large drill setup (so I don't have to worry about landing in a zone with >2.5% ore concentration) or would that be too much mass?

- Regarding the nukes, how are you circularizing that Mk 3 ship on just one? It never seems to be able to do it for me even for lighter ships. Is that again because of my ascent profile? (not enough speed early on so I need more burn later?). Also, even when I do circularize a 100-ton ship on a nuke with 4-5k LF remaining, it says that's only about 500 dV. Is the kerbal engineer dV calculation incorrect for nukes? (Now that I think about it, does it show dV for all stage engines, not just the active one?)

- Somewhat unrelated, but how do you get the blasted elevons onto the large Mk 3 wing? The sweep on that makes it near-impossible for me.

- Also, not a huge deal but I noticed my spaceplane rocking (like back and forth) along its x-axis, in vacuum, with no RCS/SAS and no input on the controls. I am at a loss as to anything reasonable that could be causing that torque. Also, my orbital distance is actually INCREASING, again without any thrust. I'm sure it's just...erm...solar wind or something :P

The Mk2 shown above is this one: https://www.dropbox.com/s/bwiw7o0cs5lckkr/Kerbodyne%20Tourismo.craft?dl=0

That is about as small as you want to go for a long-range ship; you could go a bit bigger and still be in a reasonable Mk2 size.

For LF, you usually want about 400 units per jet engine just for the ascent to 30,000m/Mach4. Beyond that...if it's not carrying a nuke, the rest is balanced between LF and O in the normal proportions. If you are carrying a nuke, you want to bias it towards LF so that you have enough O to raise your apoapsis, but no more. Exactly how much that is varies greatly with the ship; draggy, heavy, low-TWR designs will normally have to keep the oxidising rocketry burning for longer, so may need a bit more O than a sleeker build. I generally find that one FL-T800 LF/O tank per RAPIER is enough, though.

56y1jnu.jpg

 

Regarding # of engines: you need enough jet thrust to reach takeoff speed before the end of the runway. That's it; if you can do that, you can go to space. However, the closer you shave that margin, the slower and trickier it will be to get the ship up to speed and altitude. So, if you've got the mass spare, a bit of extra jet thrust is a nice luxury to have.

Small Mk2 designs can get away with one or two RAPIERs; a large Mk2 might require three or four. Small Mk3's can manage with four RAPIERs, big ones need 6-8, huge ones can require a dozen or more.

DUnZ0Zt.jpg

 

Vernors...I use RCS Build Aid to help place them, and that also gives you TWR figures for them. Check the gravity of the world you're going to, do the math, make sure you've got >1 TWR in that gravity (i.e. if you're going to a moon with 0.2g, make sure you've got a Kerbin TWR of at least 0.2). It usually ends up being 4-10 Vernors, depending on the size of the ship. You can even get away with a bit less than 1 TWR; remember that the Vernors are just to soften the last bit of the landing. You do most of your deorbit on the nukes.

Don't worry about dropping back while building speed; it'll happen any time you get into a ballistic trajectory, which you will if you're climbing steeply. My normal pattern is to take off, then get into a climb steep enough that I continue accelerating, but only just. Once I get to about 15,000m I pull the nose down to gather speed. But usually what happens next is that the momentum of my climb will carry me a bit above 20,000m, then I'll drop back down to 15,000m or so in a shallow dive (keeping the nose in place just above the horizon the whole time).

euaQi6n.jpg

As I drop, the engines gain thrust, I gain speed and the atmosphere gains density; this reduces the AoA required for level flight until I stop descending and slowly begin to climb again.

c8mNEjS.jpg

Around this time, I'm generally above Mach 4, and I can then pull the stick back to throw myself into another ballistic arc that will take me up to 30,000m where the RAPIERs switch mode and I add the nukes (Whiplashes run out of puff around 25,000m, so you need to add the rockets a bit earlier with those). You just need to take care that you don't let yourself sink too far, or you'll start to overheat things. Keeping the nose about 10 degrees above the horizon usually does the trick, but it varies ship by ship.

With ore tanks and drill sizes, that's all up to you. You don't need big tanks if you're planning on converting it into fuel as soon as you dig it up. With drill sizes and things, it's not a bad idea to send a scanning probe ahead (or build one into the main ship) so you know where to drill. Nukes and landing gear make it easy to move around on the surface to get to the good bits of ore.

RFPFnNE.jpg

 

Nukes and timing: yes, if you're going to circularise on a low-TWR engine, you need to give yourself time to do it. This means both raising your apoapsis a bit higher, and getting as much lateral speed on as possible while doing it (i.e. shallow and fast; spaceplane style). Usually I'll keep the RAPIERs burning until the apoapsis is around 5-10 km below where I want it to finish, then I'll shut down the RAPIERs but keep the nuke lit all the way to the apoapsis (which will ideally have been raised those last few km by the nuke by the time I get there).

0KIwAZr.jpg

 

Fortunately, with a RAPIER/nuke design, you always have options. If you find that you've misjudged a burn and that the nuke is taking too long, just flick the RAPIERs on for a few seconds to make up the difference with a quick blast of high-TWR thrust. This is also very handy when landing.

The single nuke on that Mk3 is a bit extreme; you'd normally use 2-4 nukes on a ship that large. Small Mk2s are fine with one, larger Mk2s appreciate having a pair.

With your nuke dV: are you remembering to place fuel hoses? Unlike RAPIERs, nukes won't draw evenly from every tank on the ship.

QfxqKRV.jpg

 

Tricky elevons: disable angle snap, and you should be able to rotate them in 90 degree increments (i.e. WASD keys) to the correct alignment.

Rocking spaceplane: if SAS were on, I'd say use Kerbal Pilot Assistant's PID tuner to fix it and maybe stiffen the airframe with some struts. With SAS off, I'd guess that you've got a part clipping partially through a service bay or something and the game's collision mechanics are having a constant minor freakout.

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Wanderfound

Wanderfound

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Aerobatic demonstrator:

 

 

Craft file at https://www.dropbox.com/s/gayfy6bzd9tvcwa/Kerbodyne%20Dancer.craft?dl=0

 

 

6 hours ago, lordcirth said:

With the Solo R / Solo Three, are the wingtip tanks aesthetics, or do they actually add significant delta-v?  It looks to me like they'd cause a lot of drag.

You can get quite a lot of fuel in those tanks; the small LF and LFO tanks have the same mass ratios as the bigger tanks these days. Four Mk0 LF tanks equal one Mk1 LF tank.

They can increase drag, although careful placement will minimise that, but the increased fuel consumption is outweighed by the increased fuel capacity, especially once you get up above 20,000m.

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