Eve lifter question

[Anglave]

I've landed and returned Kerbals from Mun, Duna, Laythe, etc. I've put probe rovers on Eve and elsewhere.

My next goal is to land and return Kerbals from Eve's surface (probably at sea level), and the dV/TWR requirements are staggering. I can build a lander that can get back up to orbit (and I know not to carry my return-to-Kerbin vehicle and fuel down the well and back up again). But delivering the lander and transfer stage to LKO, and then to Eve orbit, is daunting. I've done a two-lift mission that docked in LKO (for my Laythe mission), so assembling the Eve mission in orbit is a possibility, but it's also something I've already done. I'm looking for a different challenge.

I was thinking of ways to decrease the lander size, and/or abuse Eve's atmosphere. I know that jet engines won't work there, but I was curious if a horizontal fixed wing rocket plane would potentially get me to orbit with less fuel?

One could, in theory, use lower TWR / higher efficiency engines, or fewer engines because the atmosphere will help hold you up. But I'm not sure about the trade off between lower TWR requirement vs. atmospheric drag and taking longer to ascend (and carrying the weight of the wings to Eve).

Does anyone have experience with such a project, and/or general knowledge about the efficiency of plane-type orbital ascent vs. traditional rocket ascent?

[EndlessWaves]

Does anyone have experience with such a project, and/or general knowledge about the efficiency of plane-type orbital ascent vs. traditional rocket ascent?

Back in 0.16/0.17 I did build a lot of craft trying to get better efficiency with wings (on kerbin) but I never got close - although it's somewhat hard to measure as wings decrease in efficiency as you ascend while rockets increase. Now there's better knowledge of atmospheric drag someone may be able to prove it one way or the other.

Assuming they are less efficient than an ion plane may be a possibility. It will fly on eve and even if it's half as efficient it's still far better than straight up with a conventional rocket. Getting sufficient payload AND sufficient rate of climb is of uncertain possibility though.

In short, if you're in any hurry then just accept you'll be lifting ~100 tons.

[numerobis]

The most efficient manned returns from Eve start just shy of 45 tonnes, starting from the highest plateau. Nobody was able to do better with wings: yes you can lift off with less engine mass, but you need wing mass.

The T30 and aerospike were the best engines, nearly equally good; and don't discount the worth of an SRB on the first stage.

[TheDarkStar]

It helps to start higher up; starting only a few km up lowers the required dV by several thousand m/s. Try to stay slow early on, because of the atmosphere, and try to make the most efficient ascent you can.

[tavert]

Stochasty got a 3-stage Eve plane to work using FAR, maybe he'll show up and post a link so we don't have to go digging through Challenges and figure out which thread it was in. I don't think anyone has done an Eve plane stock yet.

[Jason Patterson]

Starting from sea level, you're basically stuck with a big lifter. You can trim tiny bits of mass right when you launch (dropping parachutes and legs, for instance) but that adds to the mass that you have to lift into LKO in the first place. Practically speaking, the mission takes a 1000 ton or larger vessel to happen in a single launch.

It's not too terrible to carry the return vehicle down onto Eve's surface. The delta-v to get back to Kerbin is pretty low; a nearly empty fuel tank and any engine you've been using to lift off in the first place can probably do the job once you're in orbit, and honestly, when I've done Eve returns that included vehicle transfers, it somehow took away from the feeling of achievement involved (in much the same way that orbital assembly can.)

If you are fine with docking, you can launch your Eve ascent vehicle from the launchpad on Kerbin and fly it to low Eve orbit. Send a refueling vessel or two to fill up all of your empty boosters, then land it and lift off anew.

[Stochasty]

Stochasty got a 3-stage Eve plane to work using FAR, maybe he'll show up and post a link so we don't have to go digging through Challenges and figure out which thread it was in. I don't think anyone has done an Eve plane stock yet.

Tada! (Sorry for the lack of sound.)

In FAR, she'll make it back to orbit from sea level with a good flight plan.

I'm still working on the stock plane, but it's quite a bit harder.

One easy trick for the Eve return mission is to realize that any lander that can lift off from Eve can also lift off from Kerbin - and usually without staging. That's what I did with this plane: she's Kerbin SSTO capable if I don't use the staging, so she she put herself in orbit and then refueled. (I also used a tug for the transfer and the return trip.)

EDIT: Regarding the efficiency question.

In stock, lifting off from Kerbin, a rocket powered fixed wing SSTO can make orbit for almost exactly the same total delta-v as a vertically launched rocket. My best numbers for both types are in the neighborhood of 4.2km/s. However, a fixed wing rocket will have more mass for the same total delta-v, due to the need to carry the wings to orbit, so if mass is your primary criterion you are better off with a vertical launch. I don't know if the situation is similar on Eve, because I don't have a successful stock spaceplane design yet. I'm working on modifying the FAR design, but I'll need to add another 4k delta-v to have a shot at making it back from sea level, and that means almost double the fuel load.

In FAR, I have noticed a little bit of delta-v savings during winged ascent, but I don't have any fixed numbers yet. One thing I'm playing with is trying to find the minimum mass aerospike-powered vertical SSTO design (using a 1-man cockpit), and seeing if I can beat that mass with a winged design. I think that'd actually make an interesting challenge thread, but there don't seem to be enough people using FAR to get a good contest going.

One final thing: I've worked out a bit of the math behind winged ascent profiles. During the thick part of the atmosphere, the goal is to find the ascent angle which maximizes your vertical velocity. Assuming drag is proportional only to the square of the velocity and that you have enough wing surface that your angle of attack is near zero, your fastest climb rate is when your angle of ascent is equal to the Arcsin of two thirds your TWR. For a TWR of .8, this is about 30 degrees, rising to ~40 degrees at TWR of 1. For high TWRs, the optimum angle quickly shoots up from about 60 degrees around 1.4 TWR to vertical at a TWR of 1.5, meaning that beyond TWR of 1.5 wings are an absolute detriment.

One thing I haven't worked through the math on, though, is dependence of TWR on fuel burn rate, and therefore the optimum thrust for winged ascent which maximizes altitude per unit fuel used (this is a much harder problem than solving for constant TWR).

Edited May 17, 2013 by Stochasty

[birrhan]

Dude, I'm in the exact same boat. I'm trying to get the 3 man command pod down and back, and the smallest I've made a 9500 dv vehicle is 579 tons.

It is easy enough to get it into orbit around Kerbin. What's killing me is the weight--when I test the parachutes on kerbin the whole thing falls apart--too much stress, and the asparagus staging is refractory to stability. Frustrating, since that is ostensibly the easiest thing about Eve--the landing.

[tavert]

Practically speaking, the mission takes a 1000 ton or larger vessel to happen in a single launch.

I think that's pretty pessimistic. A 1-man mountaintop ascent vehicle is around 45 tons at the lightest. Doing the same from sea level will be bigger, not sure how much - maybe somewhere between 60 and 80 tons? An efficient Kerbin launch vehicle can get at least 15% payload fraction. Transfer and return engines and fuel won't add another 70 tons... And if you use jets for your Kerbin launch, your initial lifter can be lighter than the payload it carries, no refueling necessary.

I don't have a successful stock spaceplane design yet. I'm working on modifying the FAR design, but I'll need to add another 4k delta-v to have a shot at making it back from sea level, and that means almost double the fuel load.

It won't need to be 4k more from a mountaintop, will it? Or are none of the mountaintops suitable for spaceplane landing?

One final thing: I've worked out a bit of the math behind winged ascent profiles. During the thick part of the atmosphere, the goal is to find the ascent angle which maximizes your vertical velocity. Assuming drag is proportional only to the square of the velocity and that you have enough wing surface that your angle of attack is near zero, your fastest climb rate is when your angle of ascent is equal to the Arcsin of two thirds your TWR. For a TWR of .8, this is about 30 degrees, rising to ~40 degrees at TWR of 1. For high TWRs, the optimum angle quickly shoots up from about 60 degrees around 1.4 TWR to vertical at a TWR of 1.5, meaning that beyond TWR of 1.5 wings are an absolute detriment.

One thing I haven't worked through the math on, though, is dependence of TWR on fuel burn rate, and therefore the optimum thrust for winged ascent which maximizes altitude per unit fuel used (this is a much harder problem than solving for constant TWR).

asin(TWR*2/3) you say? Interesting, I'll have to play around with that... though the AoA ~ 0 approximation is not very valid for any planes I've built. Optimizing the trajectory taking fuel burn into account would probably have to be done numerically.

Dude, I'm in the exact same boat. I'm trying to get the 3 man command pod down and back, and the smallest I've made a 9500 dv vehicle is 579 tons.

You'd be better off just using 3 Mk1 pods and driving over the extra crew from the runway.

Edited May 17, 2013 by tavert

[Stochasty]

It won't need to be 4k more from a mountaintop, will it? Or are none of the mountaintops suitable for spaceplane landing?

It shouldn't, but I haven't been able to make things work yet. Unmodified she sits at nearly 8km/s delta-v, which should put an ascent from 6km altitude just barely within the realm of possibility in stock, but she doesn't fly as well in stock so I haven't managed a good landing yet. (Even in FAR, the landing is the hardest part of the trip; she touches down at over 100m/s deadstick, so you really need a flat surface. Beaches are ideal, but there aren't any beaches on a mountain top. )

[numerobis]

You could stage some wings, so you can land and take off at low speed, then pop off the excess wings when you're airborne again.

I've toyed with the idea of making a monstrosity using huge numbers of the delta-deluxe winglets. In stock, they have substantially better lift:mass ratio than any of the wings. But you'd have to get them lifting a heavy plane, so it seems painful even to fly given the ridiculously large part count you'd be stuck with.

[Anglave]

Thank you all (and particularly Stochasty) for your input!

It's unfortunate that the answer still seems to be "use a big vertical lifter". But I'm intrigued by some of the other suggestions. Hundreds of winglets? Staged wings for only the lowest part of the ascent?

[numerobis]

The design I'm thinking of is hundreds of wings with a mess of ion engines (and solar panels). From Kerbin you can launch with jets, and switch to the ions to transfer to Eve. Land by gliding. Lift off about local noon, using the mess of ions to push you to 20-30 m/s. Climb slowly (an hour or so) to about 1 atm pressure. Then switch to a vertical launcher -- that is, detach most of the ion engines and the wings. From such a high altitude you should be able to build something of rather more reasonable mass. It might be worth keeping the wings for the earliest rocket stage.

I have built no such thing, it's strictly on the drawing board. Testing would require building the vertical launcher, then sticking wings on it until it can lift itself on ion thrust to about 2 or 3km on Kerbin.

[Merinsan]

I have nothing to contribute to this discussion, but have been following it. I am now inspired to attempt an Eve return myself.

I expect this will keep me busy until resources are added to the game!

My gut feeling is that a straight up rocket with asparagus staging is the best approach. Having said that, last night I was messing around with a weird plane powered by 2 aerospikes.

I got it into orbit, well would have if I didn't mess up the launch profile. It seemed to use less fuel than I would have expected for a standard rocket, so I will investigate it a bit more tonight.

[Stochasty]

Well, I've done some more math, and things are not looking good in the efficiency department for spaceplanes. Turns out that, so long as my assumptions regarding angle of attack and drag hold, and so long as a plane follows the flight plane I laid out in my previous post (so, angle of ascent = asin(2*TWR/3)), then the equilibrium change in height per unit of fuel burned scales as TWR^(1/2) over the range 0<TWR<1.5, and as ((TWR-1)^1/2)/TWR for TWR>1.5 - so the peak efficiency appears to be a TWR of exactly 2, meaning higher thrust rockets are actually more efficient than lower thrust spaceplanes at digging their way out of the lower atmosphere. This seems counter-intuitive to me, but the math doesn't lie (unless I've made a mistake in my calculations).

Add to this the fact that spaceplanes will be heavier and have proportionally more drag due to wings and it appears that spaceplanes lose all the way around; a pity, because I enjoy messing with them.

Granted, I made a bunch of assumptions working this out, and I'm not a rocket scientist (well, okay, I am a physicist, just not of the rocket science variety); if someone (aka, Scott Manley) feels like vetting my calculations I'd be appreciative. I'm sure this has to have been worked out before; it's too easy of a calculation otherwise (which is why I'm a little concerned I've made a mistake somewhere, or that I've oversimplified things too much).

Edit: another interesting fact about this computation is that efficiency (defined as altitude gained per fractional mass burned) scales as ~m^(1/2), assuming that drag is independent of mass (which is true for cylindrical spacecraft where you add mass just by making it taller) or as ~m^(1/6) if drag scales as m^(2/3) (which it would for simple scaling of the ship without changing the density). Thus, larger craft are more efficient than smaller craft (although they will tend to be less structurally sound).

Edited May 17, 2013 by Stochasty

[numerobis]

I'm no longer a rocket scientist, but I'll happily bet your equations. I find that to be more fun than actually playing, strangely enough...

[Stochasty]

Hmmm. The hard part about this is how to test it, since all the calculations were assuming equilibrium velocity (so thrust = drag + gravity) at constant air pressure. That won't really be the case for high thrust rockets on Kerbin; you'll gain altitude quickly enough and pressure drops off fast enough that drag losses and gravity losses probably won't balance speed. Maybe test this out on Eve? Start at sea level, and compute fuel usage between 100m and 200m altitude for various TWRs (using the same rocket, so adjusting thrust as well as you can). You'll need to do this in FAR, since the mass dependence in the standard drag model will screw things up.

Edit: On second thought, you can probably make a similar test work on Kerbin, there'll just be more error in the numbers. Maybe compute fractional fuel use between 1000m and 1100m? (That should give enough room off the pad to hit near equilibrium velocity).

Edited May 17, 2013 by Stochasty

[Anglave]

... so the peak efficiency appears to be a TWR of exactly 2, meaning higher thrust rockets are actually more efficient than lower thrust spaceplanes at digging their way out of the lower atmosphere. This seems counter-intuitive to me, but the math doesn't lie (unless I've made a mistake in my calculations).

Experimental evidence suggests that (on Kerbin at least) a rocket plane is (very) approximately the same efficiency as a vertical ascent path. Meaning roughly the same mass at launch delivers roughly the same mass to orbit, for either strategy. We're just trading engine and fuel mass for wing mass. At least, that's what I thought you'd reported from earlier experiments.

How does that correlate with your math?

What is it exactly that we're trying to achieve? I'm interested by the theoretical (and experimental) exploration of space planes vs. vertical launch. But I need to know how we're measuring which is 'better'.

My goal is "A more feasible land and return mission to Eve." Which means a lower mass Eve ascent vehicle.

It probably also means a vehicle which is more rugged, able to survive atmospheric entry and landing.

And because of artificial game constraints, part count is also a concern.

I was really hoping that a slower, near-horizontal ascent, utilizing the lift force of wings, and requiring a much lower TWR would get a vehicle off Eve for much less than the standard rocket ascent.

We seem to be discovering that, while such an ascent may be possible, it's likely to require just as much mass (and probably more parts) than the standard lifter. An acre of wings and solar modules and ion engines is not a "more feasible" vehicle.

Fun to play with, yes. Easier to get to Eve, no.

[K^2]

Stochasty, yes, I've derived a general form for optimal ascent by minimizing a functional. Assuming exponentially decreasing density (which is true in KSP) the optimal ascent requires you to always travel at the free-fall terminal velocity. (This is the same as trivial result for constant density.) So until acceleration becomes significant in upper atmosphere, your TWR should be exactly 2 and you should be climbing vertically.

I have not been able to work out the optimal gravity turn yet. I need to write a very specialized program to solve for that, and I've been lazy, but for Eve it does not matter. Your ascent must start with vertical climb out of the atmosphere.

[Flixxbeatz]

I'll give you a clue: It's gonna be HUGE, will likely look more like a pod with tons of engines and tanks on it that a lander.

[Jason Patterson]

I think that's pretty pessimistic. A 1-man mountaintop ascent vehicle is around 45 tons at the lightest. Doing the same from sea level will be bigger, not sure how much - maybe somewhere between 60 and 80 tons? An efficient Kerbin launch vehicle can get at least 15% payload fraction. Transfer and return engines and fuel won't add another 70 tons... And if you use jets for your Kerbin launch, your initial lifter can be lighter than the payload it carries, no refueling necessary.

More along the lines of realistic. Launching from sea level with stock parts, I don't believe that anyone got under 130 tons or so in the old Smallest Eve Ascent Vehicle thread. You apparently haven't tried the thing you're attempting to give advice about. I would suggest that you do; it's amazingly difficult to get back to orbit from Eve sea level, especially with a manned vehicle (and a 3 man capsule is nuts.) Launching from a mountaintop allows you to cut at least 50% of the mass of the ship.

An efficient Eve ascent vehicle is not the same as an efficient Kerbin ascent vehicle. Launching from Eve sea level you are pretty much stuck with aerospikes (you can get away with LV-30's from the mountains, but the fuel efficiency hit from sea level is just too big.) Thus you've got mediocre lift/thrust and no realistic way to use something like an LV-N for any significant fraction of the vehicle's ascent, because they're just too heavy to carry and waste vast quantities of fuel for most of the ascent.

Going from 0m to 4000m (or 6000m, which is where the smallest ascent vehicles were launching from) involves burning upward at a velocity of 50-70m/s - call it 60 m/s average. There is almost no net acceleration, so we can ignore the delta-v requirement for going faster, but gravity drag and aerodynamic drag across that span are enormous. Eve's surface gravity is 16.7 m/s2, and on an ideal ascent, the acceleration from drag will match that, so you're looking at 3.4 km/s of delta-v to get from sea level to the top of the mountain from which they were ascending (2.2km/s to the high plateaus that are a much easier landing target.)

[Eleven]

I've been working on something to take off from Eve...it just keeps getting bigger and bigger. I was using a 3 man capsule. I think I'm going to scratch that and start from a 1 man capsule and see how that works. Does anyone have .craft files they want to share of their Eve landers and liftoff vehicles?

[Stochasty]

Stochasty, yes, I've derived a general form for optimal ascent by minimizing a functional. Assuming exponentially decreasing density (which is true in KSP) the optimal ascent requires you to always travel at the free-fall terminal velocity. (This is the same as trivial result for constant density.) So until acceleration becomes significant in upper atmosphere, your TWR should be exactly 2 and you should be climbing vertically.

I have not been able to work out the optimal gravity turn yet. I need to write a very specialized program to solve for that, and I've been lazy, but for Eve it does not matter. Your ascent must start with vertical climb out of the atmosphere.

Yeah. I was working assuming equilibrium velocity at all times, since for a sufficiently thick atmosphere (such as Eve) your ascent will be slow enough that the time rate of change of density will be negligible. It's interesting to know that exponentially decreasing density gives you the same result.

[Jason Patterson]

I've been working on something to take off from Eve...it just keeps getting bigger and bigger. I was using a 3 man capsule. I think I'm going to scratch that and start from a 1 man capsule and see how that works. Does anyone have .craft files they want to share of their Eve landers and liftoff vehicles?

It's bigger than you want, but here is a craft for a vehicle that uses only the parts available in the demo to get to and from Eve. Due to part number constraints that are built into the game (struts and fuel lines disappear after ~2000 parts) I had to land at 4km altitude and also had to transfer from my ascent vehicle to an orbiting return vehicle to make it work.

Here's video of the flight.

Unfortunately I no longer have the files of the crafts that got me to Eve and back originally. They're monsters as well, though less so than the one that I just linked.

[tavert]

Launching from sea level with stock parts, I don't believe that anyone got under 130 tons or so in the old Smallest Eve Ascent Vehicle thread. You apparently haven't tried the thing you're attempting to give advice about. I would suggest that you do; it's amazingly difficult to get back to orbit from Eve sea level, especially with a manned vehicle (and a 3 man capsule is nuts.) Launching from a mountaintop allows you to cut at least 50% of the mass of the ship.

I haven't done this exact combination of sea level manned, but I've done sea level probes and mountaintop manned. My sea level probe ascent vehicle is only 65 tons on the pad on Kerbin, about 36 tons landing at Eve sea level, and gets back to Eve orbit successfully. There wasn't as much competition to thoroughly optimize the sea level manned SEAV since the challenge didn't require a sea level launch. 130 tons is probably a good guess, but you can get that off Kerbin in way less than 1000 tons if you use (/ abuse) jets.

Edited May 18, 2013 by tavert