Taking off from Eve: tips and techniques

[Cirocco]

Hello fellow kerbonauts! As I was browsing the forums, I noticed that I couldn’t find any tutorials or posts summarizing tips for taking off from Eve. There’s a lot of scattered information, but nothing that I could find that bundles the info together. So after successfully building my own lander able to get into stable orbit from Eve sea level, I figured I’d write down what I learned for future kerbonauts.

This post will be broken up into two main parts: tips, which are broad hints/guidelines on how you could build and what you could do, and techniques, which are practical things you can (and almost always should) do when building your rocket and performing the launch.

Note that all these tips and techniques assume a vanilla version of KSP (which is what I use. The only mod I use is hyperedit for field testing).

TIPS:

1) Try to land on a site high above sea level. The most important thing to remember about Eve that it’s not the additional gravity that will cripple your launches, it’s the atmosphere. Eve’s atmosphere is thick as pea soup at sea level and the drag is absolutely murderous at lower altitudes. Landing on a mountain can save you several thousand delta-V

2) Use Aerospike engines. Again, Eve’s atmosphere is a killer. At sea level it is 5 times thicker than Kerbin’s. And because (according to my research) the Isp of an engine scales linearly downwards as atmosphere goes up, the ISP of most engines drops well below 100 at Eve sea level. (note: Isp = specific impulse, the efficiency of your engine. The higher the Isp, the less fuel consumption. Example: mainsail has 330 seconds Isp in vacuum, 280 in 1 atm = difference of 50 Isp. In 5 atm, this means a difference of 5*50 or 250 seconds Isp compared to vacuum, so an Isp of 80 at Eve sea level. This is absolutely terrible.)This means that most big engines will drain even very large tanks in a matter of seconds, whereas aerospikes are almost unaffected by atmosphere changes. One exception is the new dual exhaust liquid fuel booster, which has an Isp of 180 at Eve sea level. This is still a pretty bad Isp, but not as horrible as everything else and the engine provides a LOT of thrust, which is what you need at the lower altitudes. Which brings us to …

3) Maintaining terminal velocity is more important than engine efficiency at lower altitudes. Because Eve’s atmosphere is so thick, the drag is immense and you need to get your butt out of that lowest atmosphere layer (this means the first 5 to 10 km) ASAP. The terminal velocity doesn’t seem very high (50 and a bit m/s) but because the drag is so high, the thrust required to get to that velocity is massive. I’d recommend using the new liquid fuel boosters added in 0.23.5 in your lower stages. If you’d prefer not to use such big engines on a lander, make sure you pack a LOT of aerospikes. My design had 19 aerospikes in the first stage and kept those running for quite some time.

4) In lower altitudes, traditional asparagus staging is not your friend. The traditional 2-way symmetry asparagus staging (dropping 2 tanks and associated engines at a time) is great for fuel efficiency, but drops a lot of thrust rather quickly. This is a major problem at low altitudes on Eve because as stated before, you need a LOT of thrust in the early stages. Therefore, consider using another staging method or some other way to compensate for the loss of thrust early on. Compensating for loss of thrust early on is SUPER important and in my opinion the biggest reason why a lot of landers fail is because they simply don’t have the thrust to punch through the lower atmosphere fast enough.

5) Do not take any weight back up with you that you do not need. Any weight that’s not helping you get back up should be ejected from the lander prior to takeoff. This includes, but is not limited to: landing gear, science gear (gather the data and store it in the command pod, then leave the experiments), parachutes, lights, etc. Any excess weight at all, especially in the later stages of the rocket, will make it more difficult to get back into orbit, and it’s damn difficult as it is. Make sure that all ejected items fall completely clear of the lander and don’t fall on top of it. They’ll still add their weight and also throw you completely off balance.

6) You don’t need a lot of SAS. Just enough to keep the rocket from spinning out of control is enough. You don’t need her to be super responsive in the early stages, you’ll just be going straight up for at least the first 20 km anyway.

That’s about it for the general advice, on to the practical techniques!

TECHNIQUES:

1) Gravity turns on Eve should be done high. The atmosphere will kill you otherwise. You can initiate about a 20-30° turn at around 25-30 km and keep slowly turning up to almost full horizontal at about 50km. At around 50km, the atmosphere is thin enough for it not to matter much anymore. Below that it still has a considerable impact

2) You can easily simulate Eve’s gravity on Kerbin by putting any engines you fire on 60% thrust. I won’t bore you with the calculations, but 59% and a smidge thrust in 1 gravity equals 100% thrust in 1.6 gravity. Note that this does not account for drag or for the change in Isp due to thicker atmosphere. Engines at 60% will consume a lot less fuel than ones at 100%. But if your lander can take off on Kerbin with its engines set to 60%, it WILL take off on Eve, just slower. For a somewhat realistic simulation: launch your test lander on Kerbin with 60% thrust and stage when your tanks have about 40% fuel remaining in them. Try to get to a 100km orbit and you should have a rough idea of whether or not you have a chance on Eve itself. This technique isn’t perfect, I think it is actually slightly harder to do it on Eve proper, but it will still give you an idea of whether or not you’re on the right track and which stages of your lander need work.

3) Unless you are compensating thrust somewhere else, NEVER put more than 1600 units of fuel on one single aerospike engine. A single aerospike will just barely have enough thrust to lift two 800 fuel unit tanks on Eve. I personally always took 1200 units of fuel (one large 1.25m diameter tank and one half-sized 1.25m diameter tank) per aerospike as my golden rule. Maintaining a max of 1200 units of fuel per aerospike at any point in time is a neat shortcut that gives you considerable thrust and really helps save you time when designing landers.

That’s all I got. Hope it helps you guys out in trying to brave the purple hell that is Eve. On the bright side, if your kerbals get stranded there at least they’ll be stuck in a very pretty place.

May your engines never overheat, your Isp always be high and your TWR always be greater than 1.

Cirocco

[wasmic]

Just one fault with your tips here: Isp is not extrapolated above 1 atmosphere of pressure. This means that the Mainsail will never go below 280, even on Eve at sea level. Between 0-1 atmospheres, Isp scales, but not above 1 atmosphere.

[Cirocco]

Just one fault with your tips here: Isp is not extrapolated above 1 atmosphere of pressure. This means that the Mainsail will never go below 280, even on Eve at sea level. Between 0-1 atmospheres, Isp scales, but not above 1 atmosphere.

wow, really? Never knew that. Well that throws half of my reasoning down the drain and could have saved me a crapton of worries when designing my lander

whelp, back to the drawing board, ignore this thread folks!

[-Velocity-]

I'm working on an Eve lander at the moment. I was going to start a new thread, but then I searched and found this one.

Thanks for the tip about the science experiments, I forgot that I could just leave them on the surface after having a Kerbal transfer the results! Right now, I'm sitting at 10 km/s delta-V, but leaving the science experiments behind ought to squeeze enough extra delta-V out of my final stage to allow me to launch from near Eve sea level.

The Eve TWR on the first several stages of my lander are about 1.7 to 2.2, so that ought to keep me near terminal velocity. The lander weight is about 310 tons (including the landing system), if I remember correctly.

If the atmosphere is 5X thicker than Kerbin's, then assuming that I land near(ish) sea level, that means I need even fewer parachutes, than Kerbin, right?

The way my landing system works right now, I've got ten "landing pods" attached to decouplers on my spacecraft. The "landing pods" contain small fuel tanks, five or six parachutes, an aerospike "landing assist" engine, and sepatrons. When landing, I pop the chutes, and when I get near the ground, I fire up the landing assist engines to reduce the effective weight of my spacecraft right before touchdown. When it's time to take off, the "landing pods" are decoupled and the sepatrons fire to carry them safely away from the spacecraft.

Anyway, thanks again for the tip about the science experiments, that ought to be the last little "umph" I need to guarantee I can make orbit, even from near Eve sea level. Maybe I'll post some images later. First, I have to finish the lander, put it into orbit with my heavy lifter, build and put into orbit a Kerbin-Eve-Kerbin NTR transfer stage, rendezvous them, and get the lander to Eve!