Spaceplane for Eve

[Conte_Vincero]

Hello, I'm planning a manned mission to Eve, and was wondering about building a spaceplane for it, however I'm having some issues. The first of these is choosing the right engine, I am currently experimenting with Aerospikes and they seem to work well, but does anyone know if I should be using something else. Secondly the ascent profile, most spaceplane ascent profiles are created expecting you to use jet engines for the early stages, however as I can't use jets I'm struggling to work out the optimum. A 50 degree climb seems to work well, but should I stay low and then try to climb almost vertically? Answers would be very greatly appreciated, please go as technical as possible!

[cantab]

First up bear in mind you're still going to have to stage, at least in stock. There's been lots of discussion and research on the possibility of stock non-glitch-exploiting Eve SSTOs, but nobody's made one yet.

I suspect that without jets the ideal ascent will be similar to a regular rocket. Firstly ascend out of the thickest atmosphere using minimum fuel per metre of altitude gain, then climb and gain speed. Unless you want to make an aerodynamic landing somewhere afterwards, you're best off dropping your wings when their weight exceeds their usefulness.

[Pecan]

No need to be technical, it's all very simple:

Without jets you want to get up and out of the thick atmosphere and crushing gravity as soon as possible.

Going up as soon as possible means keeping an EVE TWR of 2+.

Wings are just extra mass and drag that'll keep prevent you getting TWR, velocity and height up.

So ... build a spaceplane without jets or wings. It's called a rocket and a rocket ascent profile is the one to use.

http://forum.kerbalspaceprogram.com/threads/85196-Eve-SSTO-is-impossible%21

[Red Iron Crown]

As cantab mentions, you will definitely have to stage away some empty fuel tanks at least to have enough dV for the ascent. Minimum dV for ascent from Eve's highest point is about 8,500m/s, more like 12,000m/s from sea level. Neither is possible in a single stage while maintaining enough TWR to escape Eve's extreme gravity conditions.

Aerospikes and 48-7Ss seem to be the popular choices for Eve ascent, the lower Isp of the 48-7S is offset to a large degree by its high TWR. For a spaceplane you can get away with an Eve-relative TWR of less than 1 and depend on your wings to provide enough lift to get you up, but you will want to climb out of the thickest part of the atmosphere fairly quickly (for a VTOL rocket ascent it works well to get to 30,000m or so before beginning the gravity turn).

[Conte_Vincero]

OK, thanks for the replies, Staging is going to be interesting as I plan to fly it out of Kerbin's atmosphere and then dock it with a tug for the voyage. Regarding climbing out of the atmosphere though, I'm trying to work out what my optimal AOA is to minimise fuel consumption. I think I've found a helpful text book, but if anyone knows if anyone has done such calculations before please let me know

[Jimbimbibble]

That textbook is going to be useless for stock because stock KSP aerodynamics are not realistic at all. I think that a 45 degree AoA would be most useful for stock KSP, but that's going to be hard to maintain. The real advantage of having wings is your initial TWR can be much lower. A rocket needs an Eve TWR of 1.5+ at launch while a plane can get away with a lot less, which means you can pack more fuel for the same amount of rocket power.

[cantab]

Bear in mind KSP's stock aerodynamic model isn't much like reality. I suspect most aspects have been covered, though I don't recall anything specific on fuel-efficient climbs.

[Red Iron Crown]

That textbook is going to be useless for stock because stock KSP aerodynamics are not realistic at all. I think that a 45 degree AoA would be most useful for stock KSP, but that's going to be hard to maintain. The real advantage of having wings is your initial TWR can be much lower. A rocket needs an Eve TWR of 1.5+ at launch while a plane can get away with a lot less, which means you can pack more fuel for the same amount of rocket power.

A rocket only needs an Eve TWR of 1 or more. I've built several with Eve TWR of ~1.3 that worked fine, lower is possible but less dV efficient.

[GoSlash27]

As cantab pointed out, you're not going to get to Eve orbit without staging unless you're exploiting a glitch. I'm working on a practical solution to that now, but it's liable to be a while.

Best,

-Slashy

[Conte_Vincero]

OK, so update! I found an old forum thread which basically stated that for optimal efficiency you must keep the angle of attack at around 25 degrees. This means having the angle between your marker and the yellow trajectory marker on the navball at 25 deg. What I'm doing at the moment is setting my AoA to 45 degrees and then adjusting the throttle accordingly. However there are some more complications with velocity, and I may have to do some more digging to work out the best velocity to climb at.

[cantab]

You might also want to consider the angle of incidence for the wing - how it's tilted compared to the fuselage and, assuming they're mounted straight, the engines. Most people build their planes with zero angle of incidence which works fine on Kerbin and Laythe, but by altering it you can vary the angle of attack independently from the angle between engine thrust and prograde.

[Conte_Vincero]

Yeah, but that's once I've got a working model. I've had to ditch Excel and switch to Matlab

[Pecan]

25 degrees AoA, which'll probably give you a climb-angle of 20 degrees or less, sounds incredibly low for Eve. That'll keep you low in the thick atmosphere and high gravity for a long time - exactly what you don't want. Do yoiu have a link to that thread?

[Conte_Vincero]

http://forum.kerbalspaceprogram.com/threads/29788-How-to-calculate-lift

I downloaded the plugin and verified it to be correct. Hopefully once I've got my Matlab script working then I'll be able to work out what the best approach is.

[Pecan]

Post 10 of that thread does indeed suggest that the optimal lift is from a wing AoA of 25ish degrees. That's in Kerbin's atmosphere but I won't quibble about whether it's true for Eve or not. The thing is that however much lift your wings are producing you won't be climbing very fast and you'll run out of fuel long before you get anywhere useful. You need your engines to be lifting you, not your wings. That's why wings are just wasted mass and a steep rocket ascent is better.

[Conte_Vincero]

I disagree, According to the drag calculations in the thread and on the wiki, Wings actually reduce your drag! Because an average of all parts is used and wings have extremely low drag, the net result is lower overall drag! I've put the figures into my calculations so far and it checks out, so I'm just gonna verify this ingame.

EDIT: Checked doesn't work rechecking calculations. I do feel that Spaceplanes are the most efficient as you are essentially getting lift for free from the wings, but anyway I'm pushing on to get my model working so I can run simulations

Edited August 15, 2014 by Conte_Vincero

[Pecan]

Drag also increases with mass in stock KSP, and it was wasted mass I was objecting to. I'm not going to get into an argument about the aerodynamic model though, I'll just wish you good luck. Yes, you're maximising your atmospheric performance but in every other way you're making things worse for yourself. Slower climb, higher drag, higher gravity, lower Isp; ir would be a strange profile that turned that into a fuel-efficient ascent.

[Red Iron Crown]

I disagree, According to the drag calculations in the thread and on the wiki, Wings actually reduce your drag! Because an average of all parts is used and wings have extremely low drag, the net result is lower overall drag! I've put the figures into my calculations so far and it checks out, so I'm just gonna verify this ingame.

I think you are misinterpreting the formula a little bit. The coefficient of drag decreases when you add low drag parts like wings, but the drag area actually increases. The coefficient of drag is a mass-weighted average of all the parts and frontal area is calculated as proportional to mass, so it is useful to think of each part having an actual drag amount that gets added to the whole. In stock aero, more parts equals more drag, always, full stop. Even if the drag coefficient goes down a bit, the mass by which it is multiplied goes up.

[Conte_Vincero]

No, I understood that, my preliminary calculations suggested that the increase in area (mass) was compensated for by the decrease in drag.

[Red Iron Crown]

No, I understood that, my preliminary calculations suggested that the increase in area (mass) was compensated for by the decrease in drag.

It's not. It uses a mass-weighted average for the coefficient of drag, so you can simply calculate the drag for each part and sum them.

Edit for moar math:

Drag is calculated by the formula:

F_d = 0.5 * p * v² * d * A

Where:

F_d is the drag force

p is the ambient air pressure

v is the speed

d is the drag coefficient

A is the cross sectional area

We can ignore the p and v² terms as they are the same no matter the design. So we're really interested in the terms d * A, which aerodynamicists call the drag area.

Let us assume a two part ship, with part one having mass m1 and drag coefficient d1 and part two having mass m2 and drag coefficient d2, and calculate drag area:

d in KSP is calculated as the mass-weighted average of the parts, so:

d = (m1*d1 + m2*d2)/(m1+m2)

A in KSP is (controversially) calculated as mass * 0.008, so:

A = (m1+m2) * 0.008

Multiply these together to get d*A, the drag area:

d*A = (m1*d1 + m2*d2)/(m1+m2) * (m1+m2) * 0.008

We can cancel the total mass terms, (m1 + m2), to get:

d*A = (m1*d1 + m2*d2) * 0.008

So drag area is calculated by summing the products of individual part masses and their drag coefficients, then multiplying by 0.008. If we add a third part with m3 and d3, the calculation becomes:

d*A = (m1*d1 + m2*d2 + m3*d3) * 0.008

No matter how small d3 is, it will always increase the value for d*A. (Assuming m3 and d3 are positive and non-zero, which they always are.) So adding parts always increases drag.

(Except for the physicsless parts, which have no effect on drag or mass.)

Edited August 15, 2014 by Red Iron Crown

[Geschosskopf]

We've been assuming stock here to say spaceplanes on Eve can't reach orbit. What about mods?

Has anybody yet done a Karbonite-burning spaceplane on Eve? Karbonite allows getting fuel out of the atmosphere as you're flying through it, at least on some planets. If this works in Eve's air, AND if the Karbonite-burning jet engines have the right combination of stats, AND you design the whole thing carefully to take advantage of this, then a Karbonite SSTO Eve spaceplane might be possible. Being able to refuel during flight would reduce the amount of mass you need to start with, which is the main problem with leaving Eve.

I intend to test this out this weekend.

[Red Iron Crown]

We've been assuming stock here to say spaceplanes on Eve can't reach orbit. What about mods?

Has anybody yet done a Karbonite-burning spaceplane on Eve? Karbonite allows getting fuel out of the atmosphere as you're flying through it, at least on some planets. If this works in Eve's air, AND if the Karbonite-burning jet engines have the right combination of stats, AND you design the whole thing carefully to take advantage of this, then a Karbonite SSTO Eve spaceplane might be possible. Being able to refuel during flight would reduce the amount of mass you need to start with, which is the main problem with leaving Eve.

I intend to test this out this weekend.

Interstellar's thermal turbojets SSTO from Eve easily, as I understand it. Multistage stock spaceplanes work fine for Eve ascent, even allow very low mass solutions (see this thread for some) but it's hard to tell how much of an effect unintentional infinigliding has.

Haven't seen anyone try with Karbonite parts, it'll be interesting to see your results.

[smart013]

"Might be possible" with mods? Might? in my last career game with b9 and ksp interstellar i had a antimatter propelled vtol / ssto spaceplane capable of launching vertical on kerbin, flying to and landing vertical on eve, return to eve orbit and fly back to kerbin. No kethane or karbonite needed. The essential combo were the atmo breathing, antimatter propelled turbojet engines from kspi. Once the antimatter from jool orbit was provided these badboys were fantastic. I think getting karbonite from the atmosphere is more or less the same as burn eve atmo as intake air so this should be no problem.

In my new careermod with deadly reentry there may be new issues to be overcome, but with interstellar eve is yours

We've been assuming stock here to say spaceplanes on Eve can't reach orbit. What about mods?

Has anybody yet done a Karbonite-burning spaceplane on Eve? Karbonite allows getting fuel out of the atmosphere as you're flying through it, at least on some planets. If this works in Eve's air, AND if the Karbonite-burning jet engines have the right combination of stats, AND you design the whole thing carefully to take advantage of this, then a Karbonite SSTO Eve spaceplane might be possible. Being able to refuel during flight would reduce the amount of mass you need to start with, which is the main problem with leaving Eve.

I intend to test this out this weekend.

Edited August 15, 2014 by smart013

[Geschosskopf]

In my new careermod with deadly reentry there may be new issues to be overcome, but with interstellar eve is yours

Yeah, I guess I knew that, I just never think about KSPI because I don't use it myself these days. I appreciate it as a fine example of the higher levels of the modding art but after giving it a fair trial, I decided it just doesn't suit my personal tastes. With KSPI, ever imaginable spacetravel feat eventually becomes not only possible but largely trivial. That outcome is of course the whole point of developing god-like powers, but I prefer to wrestle with difficulties.

So what I'm hoping for with Karbonite is that with some clever tinkering and careful flying, I can just barely beat Eve within certain limited parameters. If it turns out to be a trivial task, I'll be disappointed.

[cantab]

Drag also increases with mass in stock KSP, and it was wasted mass I was objecting to. I'm not going to get into an argument about the aerodynamic model though, I'll just wish you good luck. Yes, you're maximising your atmospheric performance but in every other way you're making things worse for yourself. Slower climb, higher drag, higher gravity, lower Isp; ir would be a strange profile that turned that into a fuel-efficient ascent.

The potential benefit of wings comes from requiring less engine thrust, and thus less mass and drag from those engines. If you can replace a ton of engine with less than a ton of wing that's where your gains can come from. Whether that's enough to overcome the impact of staying lower for longer is another matter.

Am I right in recalling that in stock wings also have less regular drag for their mass, but have an additional drag component associated with their lift generation?