Has it been proved, mathematically, that an Eve SSTO is impossible?

[mr_yogurt]

I am most curious if it's been proved mathematically impossible (using stock KSP parts) or if it's just insanely close to impossible so that nobody's been able to do it.

[Rocket Farmer]

No. It's not possible to get the TWR required with 8,000+ DV (up to 12,000 at sea level).

TWR easy, 8,000 DV not too bad. Both together........

[GoSlash27]

According to my spreadsheet there isn't anything that can generate over 8000 m/sec DV while simultaneously exceeding 17M/sec acceleration in the stock part set. Even with a payload comprised entirely of physics- free parts, you're still gonna have to stage to get off of Eve.

[GoSlash27]

... although if you take advantage of the loopholes in the physics engine, it *is* possible.

Couple an Ion drive with infiniglide, and it becomes possible. Likewise a Kraken drive could possibly pull it off.

But none of that is mathematically provable.

[turkwinif]

I am most curious if it's been proved mathematically impossible (using stock KSP parts) or if it's just insanely close to impossible so that nobody's been able to do it.

May I direct you towards this thread? http://forum.kerbalspaceprogram.com/threads/70180-Attempting-to-go-to-space-the-Kerbal-on-ladder-engine-An-experiment-report This guy technically made an Eve SSTO.

Edited July 5, 2014 by turkwinif

[KvickFlygarn87]

No. It's not possible to get the TWR required with 8,000+ DV (up to 12,000 at sea level).

TWR easy, 8,000 DV not too bad. Both together........

But, the required TWR is lowered by the fact that YOU HAVE WINGS. Look up "Lift-drag ratio"

[qromodynmc]

But, the required TWR is lowered by the fact that YOU HAVE WINGS. Look up "Lift-drag ratio"

But you know,at some point wings are effectless..

Maybe things would be different if jet engines worked on eve.

[Red Iron Crown]

Non-exploit SSTO from Eve was mathematically impossible before 0.23.5. I think it still is, but it just might be possible to do it with an ion glider now that they have more thrust and the required electrical system can be massless. It's harder to do the calculation for a spaceplane, so I'm not 100% sure if it's possible or not.

Of course, with exploits like ladder force or infiniglide it's not at all impossible.

[cantab]

The problem with using wings is they add weight, lowering your mass fraction and thus your achievable delta-V.

[GoSlash27]

The problem with using wings is they add weight, lowering your mass fraction and thus your achievable delta-V.

True, but they allow you to use a much more efficient engine without worrying about t/w. I'm thinking that if there's a legit SSTO for Eve, it'll have wings and be Ion powered.

[Epthelyn]

True, but they allow you to use a much more efficient engine without worrying about t/w. I'm thinking that if there's a legit SSTO for Eve, it'll have wings and be Ion powered.

Maybe for the upper atmosphere, but I doubt a purely ion powered vehicle could reach orbit around Eve without abusing infiniglide in some way.

Still, nothing wrong with a rocket engine for the first few kilometers, levelling out then gliding whilst very slowly accelerating with ion engines; it's all 'one stage' as long as the engine comes along with you. That might work.

[GoSlash27]

This discussion has my wheels turning...

We would have to define "mathematical proof" of what is possible and what is not. The case of a vertical launcher is fairly straightforward; A lifter generating 16.7 m/sec at the start can lift a fixed mass of fuel and tankage. This represents the upper limit of what is possible with a single stage composed of an infinite number of engines.

If it is at all possible, it would involve the aerospike (although you can use the same method to check other possibilities).

The aerospike generates 175 KN, which translates to roughly 10 1/2 tons at 1 G on Eve. Subtract the mass of the engine (1.5t) and that leaves 9 tons of tank & fuel, 8 tons of which is fuel. Our "dead weight" is therefore 2 1/2 tons and our fuel is 8 tons. Plugging that into the rocket equation generates less than 5,500 m/sec delta vee. Not enough, so we can confidently state that a vertical lift SSTO from Eve is mathematically impossible.

The aircraft is a different beast, since thrust is no longer a going concern. Here we assume no mass other than the Xenon tanks and try to guesstimate how much wing we need to lift them.

The absolute limit of an infinite number of xenon tanks and a single PB-ION is 69,000 m/sec delta vee. Easily over the limit. Reversing the rocket equation, we can figure the minimum mass ratio that will still do the job. For 8,500 DV, it's 1.114, which means that 11.4% of the total mass of the vehicle needs to be Xenon. Accordingly, 4.8% of the vehicle's mass needs to be Xenon tanks, thus 16.2% is xenon and tanks, while 83.8% is not.

Assuming zero mass other than engines and wings and assuming a pair of delta wings for each engine (why not?), this is .39T per engine/ wing combo. This would require 63.2 kG of xenon and tank. But of course our tanks weigh 120 kg, so we're looking at 2 engines and 4 wings per tank of xenon. This combo should be just enough to pull it off, and it looks like there's plenty of leeway to juggle that and even add payload/ control surfaces/ etc.

I'm gonna call it possible, though watching something that slow get to orbit would be about as exciting as watching paint dry.

Thoughts/ corrections/ criticism?

-Slashy

[Red Iron Crown]

GoSlash27, that's a good analysis, but I think you'd need more than 8,500m/s of dV for a plane. They don't need the higher TWR but they do lose more dV to atmospheric losses. Still, it looks like it could be possible.

[GoSlash27]

GoSlash27, that's a good analysis, but I think you'd need more than 8,500m/s of dV for a plane. They don't need the higher TWR but they do lose more dV to atmospheric losses. Still, it looks like it could be possible.

Yeah, I think you're right. It still looks highly doable, tho'. Especially since structure, batteries, and solar panels are all massless and the wings provide a convenient surface for them (panels on top and batteries beneath). This suggests that swinging more towards wings is the way to go, so you don't run out of juice during the ascent.

I'm thinking it would (very) slowly climb up out of the atmosphere and then limp into orbit, but you'd better bring a Snickers, 'cuz you ain't going anywhere for a while!

*edit* crap! I grossly overestimated the DV for Ion engines! That's what I get for quoting figures off the top of my head instead of verifying them

The Ion engine generates an absolute maximum 36,000 DV, not 69,000. The mass ratio to make 8,500 is 1.229, not 1.114. So that definitely tightens up the margin. A sane DV budget to pull it off is nearly half the theoretical maximum, so it looks a whole lot less doable. Can't say for sure it's impossible, but it doesn't look promising IMO.

Edited July 5, 2014 by GoSlash27

[GoSlash27]

After wasting a day playing with ion gliders, I've decided that it's impossible.

Gliders don't simply "limp into orbit" if given enough DV as I had assumed. They merely reach an equillibrium point of lift vs weight and thrust vs drag and hang there.

Barring loopholes in the physics engine, SSTO is impossible from Eve with current technology.

[cantab]

I recall one of the challenges ion gliders face is that in KSP drag goes with speed squared, while lift just goes linearly with speed. So the higher you climb, the more speed you need to generate enough lift to stay up, but that means you incur more drag. As you found, eventually the drag equals the thrust and you can go no higher or faster.

Ion gliders are marginal enough on Kerbin, and Eve's considerably higher gravity will make them even harder to do there. You do have the advantages of the more intense sunlight and slower day, but considering that .23.5 has massless solar panels and batteries, they will be minor helps.

I shan't say it's impossible, but it's a tall order.

[Red Iron Crown]

After wasting a day playing with ion gliders, I've decided that it's impossible.

Gliders don't simply "limp into orbit" if given enough DV as I had assumed. They merely reach an equillibrium point of lift vs weight and thrust vs drag and hang there.

Barring loopholes in the physics engine, SSTO is impossible from Eve with current technology.

How high/fast were you able to achieve? I'm wondering if you carried up a bit of rocket fuel and a lightweight motor to kick you into a trajectory out of the atmosphere if it might be possible. Would you mind sharing your best design?

[GoSlash27]

How high/fast were you able to achieve? I'm wondering if you carried up a bit of rocket fuel and a lightweight motor to kick you into a trajectory out of the atmosphere if it might be possible. Would you mind sharing your best design?

Checking back on my notes....

All gliders used 1 ion engine, 6 small control surfaces and an okto 2 controller. All items other than the wings and xenon tanks were massless.

#1) 2 tanks, 2 swept wings. I wasn't able to hang enough solar panels and batteries to burn all the xenon, so no result recorded.

#2) 2 tanks, 4 swept wings. 8,037 DV. It made 27 kM altitude and 838 M/sec.

#3) 4 tanks, 4 swept wings. 13,070 DV. It made 28 kM altitude and 550 M/sec.

#4) 4 tanks, 6 delta wings. DV not calculated (I figured it wasn't an issue). It made 33kM and 720 M/sec.

I tried various AoA profiles and strategies, but I always ended up hung in the same balance in the end.

Best,

-Slashy

[GoSlash27]

Hmm...

Maybe I should try *no* wings! Just cubic struts with small control surfaces. It will probably have to go really fast to take off, but that would minimize drag...

[cantab]

That might work, since IIRC the control surfaces are bugged and can generate loads of lift. Disable all the axes on the control surfaces so you don't have infiniglide effects confounding things.

[-Velocity-]

Squad probably increased ion engine thrust so that people wouldn't die from old age before their ion maneuver was finished. I greatly disagreed with the change (the highest thrust ion engine every created was like, 5N, and now, KSP's ion engine produces what- 2 kN? Or is it even higher? I can't remember). The main reason I disagreed though was that it makes ion engines TOO good- you can even use them as lander engines now, for small bodies. Honestly, ion engine airplanes are even more of an exploit, as ion engines only function in a vacuum IRL, and I really don't think that Squad introduced ion engines for people to use them on airplanes. They're supposed to be low thrust, high Isp engines for efficient interplanetary probes.

I think that ion engines only work in the atmosphere because of the (incorrect) way that the KSP Isp system works. Engines (incorrectly) have constant thrust, but their fuel flow changes with atmospheric pressure. Thus, if Squad implemented an engine with an Isp of 0 in the atmosphere (IRL, ion engines have an Isp of 0 under atmosphere), then an ion engine would have an infinite fuel flow rate in the atmosphere (or even, depending on how Squad calculates fuel flow in their code, the fuel flow rate could even go negative). The game engine might just CTD when faced with infinite or negative fuel flow rates. At the very least, if you accidentally enable your ion engines while in the atmosphere, you will INSTANTLY use ALL your fuel.

Anyway, aside from exploit-like ion engines and "true" physics exploits, I don't see how an Eve SSTO is possible. You could perhaps make a fully reusable balloon launched or airplane launched system, with a few mods. You'd need to have a high dV rocket you could launch from above the atmosphere. I did make a big, nuclear powered, electric-propeller-driven airplane that I flew on Eve, but I never finished the rest of the launch system.

Edited July 7, 2014 by |Velocity|

[GoSlash27]

Trying this as a scaled test. I can add more solar panels and batteries without adding any weight or drag, and *surprise* it actually does fly!

I'll see what this does and then go from there.

Edited July 7, 2014 by GoSlash27

[GoSlash27]

Duplicate post

[-Velocity-]

Try putting a command chair on that and seeing if a Kerbal can ride it, lol.

[GoSlash27]

Full scale test. I may have to wait until pre-dawn if I run out of daylight and batteries before xenon.