SSTO are all small?

[Jestersage]

I decided to finally make a SSTO-to-minmus, simply due to "everyone is doing it". However, based on the samples of good SSTO (they have forum posts in here or KerbalX, can go to Minmus and Mun in the same mission) usually can only carry a single kerbal (max 2), and unable to carry much (around 2t). Is that part of the design constrain?

Also, is it possible to make a SST-Minmus/Duna direct?

Lastly, why do some people use Rapier on top of NERV? Can't you just go Whiplash + NERV?

[Snark]

8 minutes ago, Jestersage said:

I decided to finally make a SSTO-to-minmus, simply due to "everyone is doing it". However, based on the samples of good SSTO (they have forum posts in here or KerbalX, can go to Minmus and Mun in the same mission) usually can only carry a single kerbal (max 2), and unable to carry much (around 2t). Is that part of the design constrain?

Big SSTOs are certainly possible.  They're a lot trickier to design, however, so I suspect that the prevalence of small designs is just 'coz they're easier to do.

9 minutes ago, Jestersage said:

Also, is it possible to make a SST-Minmus/Duna direct?

Definitely possible, but also very much a design challenge.  It's hard.

I speak not from personal experience, since I don't fly SSTOs much, but I've certainly seen posts and videos from people who have done it.

9 minutes ago, Jestersage said:

Lastly, why do some people use Rapier on top of NERV? Can't you just go Whiplash + NERV?

It's possible to do either one, but the Rapiers have a couple of advantages:  they can get up to a higher airbreathing speed before their thrust maxes out, and they can give some high thrust after airbreathing conks out.

[Jestersage]

9 minutes ago, Snark said:

I speak not from personal experience, since I don't fly SSTOs much, but I've certainly seen posts and videos from people who have done it.

It's possible to do either one, but the Rapiers have a couple of advantages:  they can get up to a higher airbreathing speed before their thrust maxes out, and they can give some high thrust after airbreathing conks out.

I see. TBH more interested in replicating the launch vehicles of Earth (hence the large, expensive launch vehicles that I shown in other threads since Saturn V is a 3-stage design, not 2; you will notice the few design questions are the prototypes USA/Russia tried to make, ala Lunex and SpiralOS)

9 minutes ago, Snark said:

It's possible to do either one, but the Rapiers have a couple of advantages:  they can get up to a higher airbreathing speed before their thrust maxes out, and they can give some high thrust after airbreathing conks out.

So use RAPIER as jets only?

[Snark]

6 minutes ago, Jestersage said:

I see. TBH more interested in replicating the launch vehicles of Earth (hence the large, expensive launch vehicles that I shown in other threads since Saturn V is a 3-stage design, not 2; you will notice the few design questions are the prototypes USA/Russia tried to make, ala Lunex and SpiralOS)

So use RAPIER as jets only?

No, use them as jets just as long as possible ('coz they get way better Isp that way), and then use them as a regular rocket engine for a bit of extra boost once the air gives out.  (NERVs are efficient but wimpy, they can have issues getting circular if the SSTO is too big.)

[wumpus]

I'm sure the mark III spaceplane parts exist for a reason.  I'm not too familar with spaceplanes (I've flown one, but prefer rockets and got cought up with career mode).  Bigger rockets can be vastly trickier to fly than small rockets, I'd expect big spaceplanes to be more of a handful then the little jobs.

[Empress Neptune]

I actually had more trouble building smaller rather than bigger. The issue was parts. In KSP, you build things Lego style, and minimum part size controls how finely you can make adjustments. Adding a single engine to a small ship can change the weight by 50%. On a large ship the change might not even be noticeable. My steps in improving my SSTO design were to find the appropriate weight fractions and then figure out the combinations of parts that let me achieve those fractions while also meeting my other constraints.

 

I've only [purposefully] built spaceplane SSTO's and never intended for them to go anywhere besides Kerbin orbit, but I have successfully flown one of them to Munar orbit. It had a crew of 4. I don't remember if it was carrying extra fuel on this flight, but even if it was, it probably had the capacity to carry an extra dozen, or two, tons.

[bewing]

Big spaceplanes are an interesting challenge, but they serve almost no purpose in a career game. Everything you could ever want to do on a destination celestial body can be done by two kerbals. So that's all you really need to carry. A spaceplane functions as its own rover, so you never need a rover if you have a spaceplane. If you want to attach some drills, an ISR unit, and an ore tank to a spaceplane, you can. So who really needs cargo capacity? And anyway, launching satellites and dedicated drills with rockets generally seems wiser. So, you end up with two kinds of spaceplane in the real game -- an efficient LKO ferry for as many kerbals as you can reasonably fit (mine carries 7), and an exploration science spaceplane/rover that carries one scientist.

[Van Disaster]

Biggest spaceplane I made which didn't use droptanks was about 550t loaded, I dragged station segments to orbit with it ( 250ish-t segments ). Never built a giant SSTO rocket but then I've never had to.

Large spaceplanes are still handy if you do a lot of orbital construction, which *is* useful for career ( at least if you use certain mods ) but generally no, not for just completing the stock tree at least.

Little spaceplanes otoh are marvellous. Recover the whole thing? who cares what it costs then.

Edited July 13, 2016 by Van Disaster

[Jestersage]

As part of my continuous research, here's a question I been having In Kerbal: If SSTO is so great, why do we still use Rocket/Vertical launchers, other than the fact that it's easier to fly and construct?

Also, it's a principle of mine not to fly any mission with anything less than 3 Kerbals once past LKO. Personal Agency policy.  Perhaps I really need to install RSS...

Edited July 13, 2016 by Jestersage

[juanml82]

3 hours ago, wumpus said:

I'm sure the mark III spaceplane parts exist for a reason.  I'm not too familar with spaceplanes (I've flown one, but prefer rockets and got cought up with career mode).  Bigger rockets can be vastly trickier to fly than small rockets, I'd expect big spaceplanes to be more of a handful then the little jobs.

The problem with MK3 spaceplanes, IMHO, is the lack of 2.5m Rapier-like engines (or turboramjets, for that matter). So you need to spam rapiers and intakes... it can be done, but it's a mess. I use the MK4 mod and the MK2/MK3 expansion mods for the engines and the additional adapters.

1 hour ago, Jestersage said:

As part of my continuous research, here's a question I been having In Kerbal: If SSTO is so great, why do we still use Rocket/Vertical launchers, other than the fact that it's easier to fly and construct?

Also, it's a principle of mine not to fly any mission with anything less than 3 Kerbals once past LKO. Personal Agency policy.  Perhaps I really need to install RSS...

Vertical launchers are easier to fly and construct, and have a far larger payload ratio.

Also, landers work best when they are short and wide while spaceplanes should be long and narrow. If you want to fulfill that contract that asks for a base for 10+ kerbals on the Mun or you want to take 12 kerbonauts to peek into solar orbit... yeah, you could use a spaceplane. But landing a spaceplane in the Mun isn't easy, and packing enough dV and crew space for getting 12 kerbals just outside Kerbin's SOI and return is far easier with vertical rockets. Also, landings and reentry are simpler.

And if you want to put large payloads in orbit, it's not even a question. The widest you can put with an SSTO has to fit an MK3 cargo bay and the payload mass is way more limited. With vertical rockets, you can send anything into orbit as long as you're adding enough boosters and fuel

[bewing]

1 hour ago, Jestersage said:

As part of my continuous research, here's a question I been having In Kerbal: If SSTO is so great, why do we still use Rocket/Vertical launchers, other than the fact that it's easier to fly and construct?

 

Because the Panther is barely adequate as a spaceplane engine. So you really need a whiplash (or rapier, depending on your design). For which, you need a Tier3 R&D building, and 550 or 1550 spare science points laying around.

But to launch a very nice rocket, you need Tier1 everything, except for a Tier2 lauchpad. And barely any science points at all.

 

[Van Disaster]

Panther is ok for lighter spaceplanes ( and first stage rocket boosters if you mix with an actual rocket motor ) but obiously not ideal. All a jet is doing is saving some dV though, you can use Junos or do the entire thing with rockets if you want, you just need to check it's saving anything.

B9 has 2.5m SABREs - in fact they were the original bi-mode engines - as well as the parts stock took Mk3 from, so all you need for giant craft.

[Empress Neptune]

10 hours ago, juanml82 said:

The problem with MK3 spaceplanes, IMHO, is the lack of 2.5m Rapier-like engines (or turboramjets, for that matter). So you need to spam rapiers and intakes... it can be done, but it's a mess. I use the MK4 mod and the MK2/MK3 expansion mods for the engines and the additional adapters.

Vertical launchers are easier to fly and construct, and have a far larger payload ratio.

Also, landers work best when they are short and wide while spaceplanes should be long and narrow. If you want to fulfill that contract that asks for a base for 10+ kerbals on the Mun or you want to take 12 kerbonauts to peek into solar orbit... yeah, you could use a spaceplane. But landing a spaceplane in the Mun isn't easy, and packing enough dV and crew space for getting 12 kerbals just outside Kerbin's SOI and return is far easier with vertical rockets. Also, landings and reentry are simpler.

And if you want to put large payloads in orbit, it's not even a question. The widest you can put with an SSTO has to fit an MK3 cargo bay and the payload mass is way more limited. With vertical rockets, you can send anything into orbit as long as you're adding enough boosters and fuel

I don't think the lack of large engines is that big of a problem. You can use the existing adapter parts to make tri/quad clusters of rapiers (or anything) and just pretend that it's one engine.

 

I don't use spaceplanes to land on other bodies as a self imposed constraint, but it's doable. VTOL is a lot easier on low gravity worlds, but having a VTOL system and long range will cut into payload. I use spaceplanes as a method to get into Kerbin orbit, and they work fine for that.

 

Spaceplane payload aren't limited to fitting the MK3 cargo bay:

You can lift external loads or integrate the 3.75 m fairing into your design. It's more complicated than building a rocket, but not impossible. The attempt I made above only took a few tries to figure out.

 

I don't play career mode, but I do have my own rules for sandbox and I like the ability to recover spaceplanes. The one pictured above is very easy to land at KSC, so it's essentially a zero cost vehicle. It will carry 85 tons to orbit and should be able to carry a probe/lander to reach any stock KSP planet into orbit.

[FyunchClick]

15 hours ago, Snark said:

Big SSTOs are certainly possible.  They're a lot trickier to design, however, so I suspect that the prevalence of small designs is just 'coz they're easier to do.

Reading this topic, I wonder if there's not some very hard limit, specifically with SSTO planes, which is that when you make your plane twice as big, your lift surface is squared, but your volume (and therefore mass) is cubed (is that a valid expression? if not: wing surface goes up by a power of two, but volume goes up by a power of three).

If I'm right, that means that you have to add a disproportionate amount of wing to lift your additional mass, and at some point adding more wing may even add more mass than the additional lifting surface accounts for.

[Corona688]

I don't think so.  Technically you could bolt two spaceplanes together wingtip-to-wingtip and get one spaceplane with twice the mass and cargo capacity.

In fact, most of the really big SSTO spaceplanes I've seen in KSP are butterfly-like constructions of many apparent planes bolted together.

[Van Disaster]

That would be true - if they were solid. Wings in particular are very light, but most parts of a plane are hollow. If you fill all the space with fuel, that's your own fault Drag is the problem with fat spaceplanes, and the game getting a bit creaky.

[Empress Neptune]

33 minutes ago, FyunchClick said:

Reading this topic, I wonder if there's not some very hard limit, specifically with SSTO planes, which is that when you make your plane twice as big, your lift surface is squared, but your volume (and therefore mass) is cubed (is that a valid expression? if not: wing surface goes up by a power of two, but volume goes up by a power of three).

If I'm right, that means that you have to add a disproportionate amount of wing to lift your additional mass, and at some point adding more wing may even add more mass than the additional lifting surface accounts for.

If you double the plane, lifting area goes up by 2 and volume goes up by 2. The cube/square thing is what happens when you scale a dimension of uniform shape. You don't necessarily have to do that with spaceplanes.

 

I just use the method I outlined in my first post. Find the mass fractions, these fractions are the same for any size ship. Then just find the parts that meet those fractions.

[FyunchClick]

18 minutes ago, Empress Neptune said:

If you double the plane, lifting area goes up by 2 and volume goes up by 2. The cube/square thing is what happens when you scale a dimension of uniform shape. You don't necessarily have to do that with spaceplanes.

To some extend, you can't really escape it I think. If you're going from a MK1 to a MK3 fuselage for a spaceplane, you're scaling the dimensions of a uniform shape, in this case a tube. Take the comparison of the MK1 to the MK3 fuselage, which just about triples its diameter and length (eyeballing it from the screenshot there for the length, the MK3 seems a little bit shorter than x3 the MK1 but that could be an off ratio on the image), but according to the wiki it weighs between 12 and 15 times as much (wet or dry), not 9 times. So it's more like 3x3x1.5 than 3x3x3, but still substantial.

Also, looking at this, I now realize there's some inefficiencies at upscaling. First of all, I noticed the MK1 fuselage is about 14% more efficient in storing fuel for it's weight when compared to the MK3 (storing 1400 units of fuel per ton of dry mass compared to 1600 for the MK1 according to those wiki pages).
Also, wings theoretically scale linearly with lift in KPS as you construct them from parts that are equally thick on small and large planes (not considering the FatS and airliner wings), but then again you need to strut a lot of them together, adding drag that has to be compensated for with thrust and fuel, so in practical terms it's also not a pure linear upscale there either (although I'm not sure if struts are still as horribly draggy as they were once so it may be trivial).  
Finally, if you have to add adapter parts to place additional engines like you said in another post, that also adds dead mass, so there again engines don't scale up truly linearly in practice either.

All of this was besides my original point though, which was purely about looking at it from a dimensions point of view. I think you are absolutely right that mass fractions will scale up and down linearly, and if you work the problem from that angle, you should arrive at a workable solution, taking into consideration that there is some additional inefficiencies to account for when upscaling beyond a certain point.

[AeroGav]

I have a stock 5 crew minmus SSTO - could do with updating for 1.13 (could use a service bay, some radiators to deploy from within and moving the solar array inside the bay)  , but it still works.

https://kerbalx.com/AeroGav/Astrojet-Citation

The problem with building large pure stock craft

1. there is a shortage of wing parts of sufficient size.  

2. spaceplane parts generally are just smaller than the largest available rocket stuff. This means you need more parts to make something the same size,  so your creation becomes too wobbly.   Wobble is even less desirable in wing/control surface parts than it is in rocket body parts,   but you can't use struts because they cause huge drag.     Spaceplanes need to have low drag above all else, it's worth sacrificing more weight for less drag.  And unlike a rocket, which might shed some of its struts as boosters fall off, a plane wears its struts full time.

3. fuel transfer problems.  If you use only Rapier engines, this is a non issue because they drain evenly from every tank even in rocket mode.

However, Rapier have the worst vacuum ISP in rocket mode of any engine,  they are no good for an interplanetary spaceplane.   For that you need NERV, but they drain fuel from the front tanks first, unbalancing your plane, and won't pull fuel from the wing tanks etc.   So, you either need a lot of fuel ducts (massive drag, due to game bug).   Or transfer manually, which gets unmanageable with too many engines or tanks.

Very easily solved with mods ( GPOspeedfuelpump, or tac fuel balancer)   but pure stock it's a major headache.

 

Re - engines

I build a lot of stuff in the 30-40 ton class.

With 2 NERV and 1 Rapier,   I just run the Rapier in air breathe mode all the way to cutoff at 29km, even though it is unable to lift me beyond 23km on its own.  After 23km, I start the 2 nukes up,  which have 800ISP,  but might as well keep the Rapier going at 3200ISP till it quits , no matter how little power it's making.

I did try taking some oxidizer, and running a bit of closed cycle mode from say 29km to 34km.   However,  i actually had slightly less delta V once i reached orbit than if just left the oxidizer tanks empty.   Yes, you launch with more total fuel , yes, you have better TWR when the Rapier is going in closed cycle mode,  but you're also squandering liquid fuel at 300ISP when it would have given you 800 run through the nukes,  and you also use more LF getting all that oxidizer up to 29-34km so you can burn it in the first place.

It's also harder to fly - you're heavier, so you climb slower for a given speed and get hotter, closer to blowing up.   Getting through the sound barrier is harder.   It's harder to hold a nice steady flight profile when your TWR is fluctuating all over the place like that - underpowered - rapiers in closed cycle - waaahhh! massively overpowered ! splutter ! out of ox 5 seconds later ! now underpowered again !

 

Now there's other combos

 

2 Rapier, 1 Nuke

This is what my very first spaceplane(s) used

Because you have more RApier, you can get higher on pure airbreathing, but not much higher because thrust halves with every 1.5km up there.  So at 24km you can go no higher and must switch the rapier over to closed cycle to keep going up - one nuke on its own is not enough.   This means you miss out on  milking air breathing mode on your rapier all the way to 29km.     In closed cycle, you are masssively overpowered,  ideally you'd only switch one RAPIER to rocket mode and keep the other air breathing, that'd give plenty enough power along with the single nuke, but obviously you can't on a 3 engine ship due to asymmetric thrust .

 

2 Whiplash, 2 Terrier, 1 Nuke

I'll soon be in a position to try this in my latest career game.  I think it could be the best yet.

2 nukes gives plenty of power to push a 30 ton ship the rest of the way to orbit once the air breathers quit, but that's 6 tons of reactor weight you're carrying everywhere you go.    If I only had one nuke, that'd save 3 tons.  Once in orbit , that'd be plenty.    In the climb to orbit,  above 35km drag is under 20kn, so the 60kn from a single nuke would be fine.   But when the jet engines first conk out, you're seeing drag numbers of 50-70kn.     

So my idea is

0. Start the Nuke when the Whiplashes start to struggle.

1. Shortly afterward, start the Terriers to maintain climb rate as jet power declines further.

1.  Jettison the Whiplash when they flame out.  They weigh 1.8 tons each, that will boost my delta V a lot.  They only cost 2000 credits per engine, unlike the Rapier (6000) and nuke (13000?) so are cheap enough to throw away.

2. Jettison the Terrier pods when empty.  Only bring enough fuel to reach 35km or so.  Terrier are also very cheap engines (400 cost?)

3.  The nuke takes you the rest of the way to orbit then on to wherever you're going.

Before you start hollering whats the point of an spaceplane if it throws away 4400 Kredits worth of engines every flight,  this should give enough delta v to fly to duna and back, comfortably,  fly around a bit in its atmosphere and possibly land a few times or drive over the surface a bit.  Any rocket that can do the same will cost more than 4400 kredits.  A lot more.

Edited July 13, 2016 by AeroGav

[Jestersage]

Another question I want to confirm: VSSTO is just not as efficent as the Standard Spaceplane SSTO, right? My understanding is that SSTO also used the lift from the lifting surface to achieve orbit.

Edited July 13, 2016 by Jestersage

[Empress Neptune]

5 hours ago, FyunchClick said:

To some extend, you can't really escape it I think. If you're going from a MK1 to a MK3 fuselage for a spaceplane, you're scaling the dimensions of a uniform shape, in this case a tube. Take the comparison of the MK1 to the MK3 fuselage, which just about triples its diameter and length (eyeballing it from the screenshot there for the length, the MK3 seems a little bit shorter than x3 the MK1 but that could be an off ratio on the image), but according to the wiki it weighs between 12 and 15 times as much (wet or dry), not 9 times. So it's more like 3x3x1.5 than 3x3x3, but still substantial.

Also, looking at this, I now realize there's some inefficiencies at upscaling. First of all, I noticed the MK1 fuselage is about 14% more efficient in storing fuel for it's weight when compared to the MK3 (storing 1400 units of fuel per ton of dry mass compared to 1600 for the MK1 according to those wiki pages).
Also, wings theoretically scale linearly with lift in KPS as you construct them from parts that are equally thick on small and large planes (not considering the FatS and airliner wings), but then again you need to strut a lot of them together, adding drag that has to be compensated for with thrust and fuel, so in practical terms it's also not a pure linear upscale there either (although I'm not sure if struts are still as horribly draggy as they were once so it may be trivial).  
Finally, if you have to add adapter parts to place additional engines like you said in another post, that also adds dead mass, so there again engines don't scale up truly linearly in practice either.

All of this was besides my original point though, which was purely about looking at it from a dimensions point of view. I think you are absolutely right that mass fractions will scale up and down linearly, and if you work the problem from that angle, you should arrive at a workable solution, taking into consideration that there is some additional inefficiencies to account for when upscaling beyond a certain point.

There are some problems that make scaling slightly off from 1:1, but these shouldn't be deal breakers. My Mk3 planes use a lot of Mk2 parts because of the poor Mk3 fuel mass ratio. The advantage of the Mk3 is that they allow for more solid aircraft (less need for struts) and easier fuel management. I don't use Mk3 for much besides those two things.

When it comes to wings I used the Big S wings as much as possible. I really think all wings should have fuel capacity as it's extremely common in real aircraft. The Big S wings also provide a good amount of lift even for large aircraft. My ~40 ton payload SSTO uses 4 of them, while my 85 ton payload lifter uses 8. My lighter SSTO's use 2 no matter how small. I only needed struts on my 85 ton SSTO and the performance hit was marginal. Without KER it was hard to reliably notice.

I agree with most of what you said though. There are some things that you need to consider as you go larger that cost efficiency. If there is a hard limit that arises from these inefficiencies, you probably won't see it unless you build a truly enormous plane.

12 minutes ago, Jestersage said:

Another question I want to confirm: VSSTO is just not as inefficent as the Standard Spaceplane SSTO, right? My understanding is that SSTO also used the lift from the lifting surface to achieve orbit.

This depends on your TWR mostly. If you can quickly rise vertically and transition to a low angle, that will be efficient. Fuel burnt on the runway and accelerating to climb speed doesn't really get you any closer to orbit. Also keep in mind that wings are dead weight at high altitude so they can hurt you as much as they help lower in the atmosphere. The biggest advantage from wings is probably better control on reentry and landing.

[bewing]

1 hour ago, AeroGav said:

 Any rocket that can do the same will cost more than 4400 kredits.  A lot more.

Heh. Not necessarily. If you take the same spaceplane, take off the whiplash, take off the terriers, take off a couple LF tanks -- then attach a trio of kickbacks under it: That costs maybe a thousand kredits less overall. The kickbacks get you all the way up to 35km with lots of upward velocity (but much less horizontal), and you end up in LKO about the same as before on one nuke. But if you can recover those kickbacks ... then you come out a couple thousand more ahead.

22 minutes ago, Jestersage said:

Another question I want to confirm: VSSTO is just not as inefficent as the Standard Spaceplane SSTO, right? My understanding is that SSTO also used the lift from the lifting surface to achieve orbit.

I think you mean "efficient". HSSTO spaceplanes get atmospheric lift, but in return they experience a lot more atmospheric drag. It ends up being pretty equal overall, and more a question of aesthetics and part costs. So it depends on exactly how you measure "efficient".

[FyunchClick]

25 minutes ago, Empress Neptune said:

When it comes to wings I used the Big S wings as much as possible. I really think all wings should have fuel capacity as it's extremely common in real aircraft. The Big S wings also provide a good amount of lift even for large aircraft. My ~40 ton payload SSTO uses 4 of them, while my 85 ton payload lifter uses 8.

Actually, I'd loved to have an even bigger wing option. My last creation was 240 tons, using 12 of 'em and smaller wing parts to fill the gaps. :D It'd snap the wings off when pulling up on takeoff, or when pulling out of the shallow dive it needed to go supersonic, if I didn't strut everything down. I started out with some airliner wings incorporated but those would blow up due to the heat on ascension.

I realize it is horribly inefficient as SSTO go with all rapiers and lots of modules I don't need, but I wanted TWR in vacuum to see if I could somehow make a Tylo takeoff work with it (A: no).

[Jestersage]

1 hour ago, bewing said:

I think you mean "efficient". HSSTO spaceplanes get atmospheric lift, but in return they experience a lot more atmospheric drag. It ends up being pretty equal overall, and more a question of aesthetics and part costs. So it depends on exactly how you measure "efficient".

Able to actually orbit given similar mass/fuel is good enough for me.

As of now, my SSTO plan have about the same success as the Soviets. Ironically got myself a Lunex....

[Van Disaster]

Spaceplanes also get to use vastly more efficient jet engines for a large chunk of their orbital velocity. I put a mission in orbit earlier for the fuel cost of the first stage if it had been a conventional rocket launch. I do use FAR though and stock air is still pretty syrupy.