Testing Question

[Zad Fnark]

Hi Guys (Gals?)

In my never ending pursuit for a lightweight Eve lander, I tend to run a lot of tests.

I know the glorious target is 12,000 dv.

Is there a particular orbital height for Kerbin which would be an equivalent dv height of 100 km for Eve? One where if I can get to this height on Kerbin, I should be able to successfully get off of Eve.

Thanks

ZF-

[Streetwind]

There is not, because your time spent in ascent determines the height you can reach with identical on-paper dV capabilities. That means, on Eve 12,000 m/s will only be enough if you fly a proper gravity turn at terminal velocity - if you keep your ascent as short as possible without suffering too much atmospheric drag. Heck, I could take a certified working Eve lander and fail to reach a 75km Kerbin orbit with it. You can effortlessly waste a near-infinite amount of fuel by just floating vertically up at sightseeing speed

What you want to do is not test, but calculate. Figure out how much dV your craft has by solving the rocket equation. That will get you to your target with less trouble... in fact, if you're good at math, you can input the desired dV of each stage into the equation and then reverse it, solving for the mass faction you need for that stage. Sounds complicated, but will still be faster than trying to fly repeated, lengthy ascents in an attempt to suss it out by trial and error.

Though if you really wanted to, you could do this:

- Download HyperEdit

- Put your test vessel onto the launchpad

- Launch into low Kerbin orbit, circularize 75km

- Use HyperEdit to teleport the ship back onto the launchpad

- Launch into low Kerbin orbit a second time, circularize at 75km

- Use HyperEdit to teleport the ship back onto the launchpad

- Launch into low Kerbin orbit a third time!

It's okay if you fall a little short there. If you manage to get most of the way there (apoapsis outside the atmosphere, and at least 1000 m/s vessel speed when reaching that apoapsis), then your craft has about 12,000 dV (on account of needing 4500 dV to launch into low Kerbin orbit every time). Of course, I'm not sure if you'll have the TWR on the later stages... but then again Eve has brutal gravity, so all your stages but the final one are probably going to be somewhat beefy!

[The_Rocketeer]

An alternative approach would be to scale the lander to match Eve's gravity.

Basically, start with your standard lander, make a note of the mass of each stage. Then add parts to each stage as inert payload so that each stage now weighs 5 x more than it did originally but you still have the same fuel and thrust.

I'm well aware this is a far from perfect model. I'd improve it myself but I haven't slept in 47 hours and my brain is refusing complex tasks. As I see it, you'd need to further adjust for the shallower atmosphere, the lower density, the increased drag (due to additional parts) and possibly some other things that I'm too dozy to think of.

Even after all that it probably won't be right-on. Nonetheless, you should be able to apply a number of modifications to approximately model an Eve ascent from a Kerbin ascent. Mass multiplier is the biggy.

[GoSlash27]

An alternative approach would be to scale the lander to match Eve's gravity.

Basically, start with your standard lander, make a note of the mass of each stage. Then add parts to each stage as inert payload so that each stage now weighs 5 x more than it did originally but you still have the same fuel and thrust.

I'm well aware this is a far from perfect model. I'd improve it myself but I haven't slept in 47 hours and my brain is refusing complex tasks. As I see it, you'd need to further adjust for the shallower atmosphere, the lower density, the increased drag (due to additional parts) and possibly some other things that I'm too dozy to think of.

Even after all that it probably won't be right-on. Nonetheless, you should be able to apply a number of modifications to approximately model an Eve ascent from a Kerbin ascent. Mass multiplier is the biggy.

All this, and you should probably add some parachutes as well to help simulate the the density of Eve's atmosphere below 10 KM.

If you're looking to test your lander/ ascent without building an analogue, the easy solution is to hyperedit it to Eve orbit in sandbox and give it a go.

Best,

-Slashy

[Zad Fnark]

Thanks,

I'm trying to keep things light, but things to spiral out of control. I've been running tests on Kerbin, using the SRBs on my booster and shutting everything down to test the landings. Can't seem to get under 10 m/s at this point. Eve has the denser atmosphere, but the higher gravity, so I'm not sure if it's a wash at this point, or if the atmosphere wins.

ZF-

[GoSlash27]

The atmosphere wins on Eve. It's very easy to land on Eve and very difficult to get back off.

Best,

-Slashy

[Streetwind]

Atmosphere absolutely wins. Eve is hell. You might as well be falling through honey.

This is also partly why you need almost 3 times as much dV to take off, even though gravity is only 1.7 times as high and gravity losses cease to matter halfway through a typical ascent. If you end up down at sea level, it can feel like you're trying to lift the entire planet with you when trying to get off this godforsaken rock. You just... won't... accelerate...!

[Laie]

All this, and you should probably add some parachutes as well to help simulate the the density of Eve's atmosphere below 10 KM.

Not necessary / too complicated.

An engine's ISP never gets worse than its "atmospheric" rating. A mainsail in the lower atmosphere of Eve will have an ISP of 320; a Mainsail on the launchpad on Kerbin will have 320.6 -- big deal. The difference is that on Kerbin, the ISP improves immediately with every meter of altitude, while on Eve it will be stuck at 320 until you reach about 10.5-11km -- that's where Eve's atmosphere is just as dense as Kerbin's at sea level. The situation will improve not quite as quickly, however. While on Kerbin you can expect to have almost-vacuum ISP at 10km, on Eve this won't happen until 20-25km.

What I'm trying to get at: you can test your lifter if you can tie it down on the launchpad: put on launch clamps and don't release them for the first few minutes. Just how many minutes? Now it's getting tricky. I recommend that you browse the entries in the Eve Rocks Challenge, note what TWR the ships have and how long it takes them to get to 15 kilometers. (All are Asparagus-staged, often many small stages where TWR doesn't change all that much over the trip; also, many people have left all kinds of MechJeb and KER info panels open. There's a lot of data in those galleries.) Compare to your own TWR to determine how long it will take you.

So, put your vessel on the test stand and let the engines run at sea level for as long as it would take you to reach 15km on Eve, plus a safety margin of at least ten and no more than twenty seconds. Cut the engines. Check remaining delta-v: if you have about 5000m/s left, you'll be good. Just remember that your TWR has to be pretty high for almost all the way: out of those 5000m/s, you should have an Eve-TWR of about two-ish for the first 2500m/s, around one for the next 1500m/s, and only for the last 1000m/s may it be lower than one. If your TWR is less, you'll need more delta-v, but not all that much more: 6000m/s should do if you reach the 15km mark with an Eve-TWR of 1.5.

Pro tip: also test your lending gear. Put some extra weight on your lander and have it plop down on the grassland weast of KSC.

Totally unrelated: is it Eve's atmosphere? Eves' atmosphere? Eves atmosphere? Eve her atmosphere?

[KITTYONFYRE]

Totally unrelated: is it Eve's atmosphere? Eves' atmosphere? Eves atmosphere? Eve her atmosphere?

Eve's atmosphere. Atmosphere of eve.

[GoSlash27]

So, put your vessel on the test stand and let the engines run at sea level for as long as it would take you to reach 15km on Eve,

^ This is an awesome idea!

-Slashy

[Laie]

Can't seem to get under 10 m/s at this point. Eve has the denser atmosphere, but the higher gravity, so I'm not sure if it's a wash at this point, or if the atmosphere wins.

Atmosphere wins. But. With stock atmosphere/chutes, your vessel has to be ~3% chutes by weight:

http://forum.kerbalspaceprogram.com/threads/31405-Parachute-guidelines

That's two radial chutes for every ten tons of mass. On many Eve lifter-landers, the parachutes and landing gear amount for about 1/3rd of the entire part count.

If you're constrained by part count, I recommend a half-powered landing: terminal velocity on sea level is about 50-60m/s. A perfect suicide burn would consume about 150m/s of delta-v, but is very treacherous and unforgiving. BUT: a very small number of parachutes will suffice bring that down to 20m/s. The parachutes will also ensure that you're pointed straight up and don't drift, so you don't have to care about steering and may concentrate on the throttle. For the latter purpose, a single drogue chute will be enough.

A parachute-buffered powered landing can be done even if you have clumsy fingers. If you account for 300m/s, that should really be safe. Just check how many parts you need to hold that much extra fuel. It will be a lot less than for the parachute-only approach. Landing will be softer, too.