A few random questions

[uncle_jew]

Hello,

don't want to open several threads for trivial questions. So here are all questions at once:

1) Engine performance on Duna - does "ASL" or "Vacuum" Isp apply?

I want to build a small reusable Duna lander. I understand the Duna atmosphere is much thinner than on Kerbin - so if I use e.g. the LV-909 "Terrier" engine, will it produce the "vacuum" Isp of 340s, or the much worse "ASL" Isp of 85s?

2) Purpose of rovers

Apart from the fun of driving around, do rovers (as opposed to static landers) have any advantages?

They don't drive very fast, so except at KSC, you will not want to drive them to other biomes.

3) Droppable heat shield - do you need a decoupler?

I always use a decoupler to drop the heat shield prior to landing. But once, I right clicked somewhere on the lander and got an option to "drop heat shield" (or something like that). Couldn't reproduce this since - so can you discard a heat shield without using a decoupler, and if yes, where (and when) are you supposed to click?

4) When is a heat shield needed?

Is there any way to determine when a heat shield is required and when not? E.g. when entering a Duna atmosphere with 1300m/s with a lander consisting mostly of a Rockomax X200-16 fuel tank, do I need a heat shield or will the fuel tank absorb the delta-V without too much heating?

5) Aerobraking - how to aim?

How do I determine the proper periapsis for aerobraking (without landing)? E.g. I am approaching Duna with 1400m/s - what is the proper aerobraking altitude to go into orbit?

Thanks

[Sanic]

1. The Isp will go down very little due to the extremely thin Dunan atmosphere. It'll probably be around 310, though this is just my estimate. I'm probably wrong, but there is no way in a million years you're going to get anything less than 230 Isp.

2. Rovers are meant to traverse biomes. Besides that, they're pretty much pointless.

3. You don't need a heatshield for Duna.

4. At 1300 m/s a heatsield is nothing but dead weight.

5. I would go 15 km, but extraterrestrial atmospheres are buggy right now so that's really just a guess.

[capran]

1) Engine performance on Duna - does "ASL" or "Vacuum" Isp apply?

I want to build a small reusable Duna lander. I understand the Duna atmosphere is much thinner than on Kerbin - so if I use e.g. the LV-909 "Terrier" engine, will it produce the "vacuum" Isp of 340s, or the much worse "ASL" Isp of 85s?

In my experience, Duna's atmosphere is so thin, that you can just use the vacuum Isp and thrust, unless you're landed, but even then it won't be too much lower than vacuum.

2) Purpose of rovers

Apart from the fun of driving around, do rovers (as opposed to static landers) have any advantages?

They don't drive very fast, so except at KSC, you will not want to drive them to other biomes.

This is true for stock, though some mods do give bonuses or special missions for rovers. Plus, if you plan on making a base somewhere other than in orbit, it helps if each module is motorized. Treat each module (say a hab module or a science module) as a rover and dock them up. Just need to be sure the docking parts are all at the same height, or it can be very tricky to get them to connect.

3) Droppable heat shield - do you need a decoupler?

I always use a decoupler to drop the heat shield prior to landing. But once, I right clicked somewhere on the lander and got an option to "drop heat shield" (or something like that). Couldn't reproduce this since - so can you discard a heat shield without using a decoupler, and if yes, where (and when) are you supposed to click?

Since 1.0, it doesn't seem necessary to decouple the heat shield. I wouldn't bother, I don't think they weigh much.

4) When is a heat shield needed?

Is there any way to determine when a heat shield is required and when not? E.g. when entering a Duna atmosphere with 1300m/s with a lander consisting mostly of a Rockomax X200-16 fuel tank, do I need a heat shield or will the fuel tank absorb the delta-V without too much heating?

You only need heatshields for re-entry on Kerbin, Eve, and Laythe. I suppose if you're feeling lucky you can try an aero-capture on Jool, but I've yet to make that work in 1.0.x.

Fuel tanks generally explode, so it's best to drop them before you re-enter. Put a decoupler between your fuel tank and heatshield.

5) Aerobraking - how to aim?

How do I determine the proper periapsis for aerobraking (without landing)? E.g. I am approaching Duna with 1400m/s - what is the proper aerobraking altitude to go into orbit?

Thanks

Trial and error. For Kerbin, when I come in from Mun or Minmus, I try to make my periapsis around 30 KM. For coming in from outside Kerbin SOI, though, it takes some practice to get it right, and it depends on how much faster you are going than the 3000 m/s you usually get from Mun-return. If you aim too low, you'll burn up; if you aim too high, you won't slow down enough to capture. If that happens, and you still have some fuel left, you can try a retrograde burn when still in the atmosphere, maybe that'll be enough.

Make sure you get in the habit of using quicksave (F5) and quickload (F9)! Unless, of course, you're playing "hard" mode.

[Snark]

Agreed that rovers are mostly useless, but there are a few places they can be handy. For example, there's a type of contract which is "take seismic readings on <planet> at <location>." These have to be done on the surface, and they're usually for 3 different locations that are located fairly close together (within a few kilometers). A rover is very handy for driving around to the different spots to collect the science. You could do it with a hopping lander, too, but at least the rover doesn't burn fuel while doing it.

It can also be handy to use a rover with an ore detector to drive around to pinpoint the "sweet spot" to drop your miner.

If you go in for bases, fuel trucks can be useful: you have your miner/refinery/fuel-storage ship, and if you have a roving fuel truck, then incoming ships don't have to land right exactly next to the base (without hitting it), which can be tricky-- as long as the incoming ship lands within a couple of kilometers, you can send the fuel truck to top them off. (If you're planning on doing this sort of thing, I highly recommend KIS/KAS mod; stock KSP is stupidly difficult to dock together two things that are on the surface, and KIS/KAS has handy fuel ports that let you link together nearby craft with a pipe.)

As for deciding on aerobraking altitude: yah, it's pretty much trial and error (especially since the "right" altitude will vary from ship to ship, and even a very tiny difference in Pe can make the difference between "fly off into space" or "get into orbit" or "go down to land"). One thing that can be very useful for aerobraking is airbrakes. Reason: they're controllable. You pick your altitude, and you can put the brakes on and off as you see fit as you watch your projected trajectory in map view. "Oh, my Ap is dropping too fast, I'll turn off the brakes." Makes it easier to adjust the amount of aerobraking after you've already hit atmosphere and thereby committed yourself.

Edited October 31, 2015 by Snark

[FlyingPete]

Rovers useful in practical terms for doing contract missions where you have to run an experiment on the surface at two or three different points. They're usually close enough together that it pays to land near one, then rover from one site to the next. They're also good for when you're doing actual exploration of the place for the purposes of seeing what's around. In practice it's only on Minmus where you're likely to be in the vicinity of several biomes that you can drive between. However Minmus presents its own difficulties in actually controlling the thing due to the low gravity.

[Superfluous J]

I never use rovers anymore. The fuel you "save" by driving it around is far more than wasted getting it to the location. Just make your ship able to make those small hops. It won't take that much more fuel.

If it's close enough to use a rover, it's close enough to hop without spending much fuel.

[celem]

I used to have a droppable heatshield too but cant find it anymore. I guess somewhere out there is a modpack where the author added a decoupler to their custom shield. I, like the others here, dont drop the kerbin return shields, i'll often blow away aerobrake heatshields however. I dont drop the return shield because its weight is fairly negligible when looking at parachutes. Its weight is significant however when looking at overall craft design, and I will often remove some of the ablator resource in the VAB prior to the mission to shave mass. Coming in from Munar orbit with a multipass re-entry i will usually remove 1/3 to 1/2 from the 1.25m

The other tactic is to leave the payload inside its aeroshell from launch at kerbin until after it has arrived on site and completed it's aerobrake. I use this one a lot in RSS. If there are components you need to protect at kerbin launch, but will then need to use along the interplantary stages (or docking ports for LKO refueling) then you can nest the aeroshells, having a major launch shell, then a second aerobrake shell inside.

As for predicting aerobrakes, I use the Trajectories mod a lot, be advised that you will never be properly aerocapturing at Duna without some kind of burn, its atmosphere is just too thin and too low for you to bleed enough speed on the first pass without coming in seriously low and hot, expect to have to help a little.

Edited October 31, 2015 by celem

[uncle_jew]

Wow, I didn't realize you could shave off the ablator in the VAB...

be advised that you will never be properly aerocapturing at Duna without some kind of burn, its atmosphere is just too thin and too low for you to bleed enough speed on the first pass without coming in seriously low and hot, expect to have to help a little.

Hmmm. After a Hohmann transfer orbit, I arrived at Duna with ~1300m/s delta-V. I then aimed my ship to pass 8km above its surface - this produced enough aerobraking to land without doing any burns (except immediately prior to touchdown, since I didn't attach enough parachutes). Aerobraking got my speed down to ~450m/s, after which I deployed the parachutes.

If there is enough air resistance to land, there must be enough air resistance to aerobrake too... or am I getting this wrong? Or is the margin for error simply too thin?

[Superfluous J]

Yeah I aerocapture at Duna all the time. 13-15 km is perfect. Never needed any heat protection either.