[KSP] Battle Report 2

The sequel no one has been waiting for.

As in the past, busy periods in school inhibit me from being as prolific a blogger as I'd like. Given that this is the case, it occurs to me in retrospect that it made little sense to frame my KSP Battle Reports in terms of a serialized rather than stand-alone narrative. But late is better than never, and if I learned one thing from Battlestar Galactica, it's that I'm not a man unless I finish what I started, or something like that. At any rate, here's part two!

Last time we built and tested a crew capsule which would be capable of small orbital operations and transfers between vessels. A logical next step would be constructing a landing unit. This will not be a large lander capable of planetary landing and relaunch; the plan is to limit these test missions to the Kerbol system's moons, so a smaller lander will be sufficient for these purposes. It must, however, have enough a thrust-to-weight ratio sufficient to relaunch after landing on Eeloo, which has a surface gravity of 1.69 m/s^2.

Here's what I came up with:

She may not look like much, but she's got it where it counts, kid.

+ Show Spoiler [Stats] +

+ Show Spoiler [Part Breakdown] +

(Weights are given in metric tons. Where applicable, total and dry masses are given in the format [total]/[dry] t.)
1 FL-T 400 tank, 2.25/.25 t
3 FL-T 800 tanks, 4.5/.5 t
3 fuel hoses, .05 t
3 LV-909 engines, .5 t, 50 kN thrust
3 modular girder segments, .125 t
1 Rockomax X-200-16, 9/1 t
6 struts, .05 t
3 LT-2 Landing Struts, .1 t
1 Rockomax Adapter 02, .08 t
1 SC-9001 Science, Jr., .2 t
2 Mystery Goo (TM), .15 t
4 1x6 photovoltaic panels, .0175 t
4 docking ports, .05 t

(all stats except part count assume a single crew capsule atop the lander)
35 parts
total mass: 30.635 t
dry mass: 8.635 t
thrust-to-weight ratio: 4.90 m/s^2 (17.4 m/s^2 dry)
mass ratio (total mass/dry mass): 3.55
Isp: 390 s
delta-V: 4845 m/s

I elected to use 3-way rotational symmetry, since this would provide the smallest mass ship that could still stand upright (after all, we still have to push this thing all the way out to Eeloo!). Her thrust-to-weight ratio is easily sufficient to lift off of Eeloo, so the limiting factor here is fuel. Can this lander perform a powered landing, take off, and get back into orbit on just 4845 m/s delta-V?

Calculating the delta-V cost of landing and launching is difficult, particularly since it depends heavily on the skill of the pilot, so the easiest way would be to simply try it and see if the fuel holds out. But hw can we test the lander without flying it all the way out to Eeloo and trying to land, possibly stranding our astronaut on the surface?

I present to you Kerbin's closer and larger natural satellite, Mun:

Kerbal astronomer: "Mun's surface appears rather ragged and beat up."
Mun: "You should see the other guy."

Mun and Eeloo have very similar properties. Their masses are 9.76e20 kg and 1.11e21 kg, respectively, and their equatorial radii are 200,000 m and 210,000 m. Mun's surface gravity is 1.63 m/s^2; Eeloo's is 1.69. Therefore if the lander can handle a de-orbit, landing, launch, and re-orbit on Mun, it should be able to do so on Eeloo, as well.

So we put a massive rocket under the landing module large enough to put it in orbit of Mun, and then perform the landing and re-orbit. Remaining fuel in the lander can be used to return home. Sounds safe enough, right?

A giant pillar of explosives. What could go wrong?

As it happens, the launch vehicle turned out to be a bit less stable than hoped. It made it all the way into orbit, and then:

It may not be obvious from this picture, but as you can see from the part and staging summary at the left, the connection between that fuel tank and the ship actually broke. As a result Jebediah has a big rocket behind him pushing him, but he can't actually control it. He can throttle down, but it's no longer receiving his commands. He can try to turn, but since the connection is broken, it will likely spin him out of control. Then if he's lucky it will fly past him and off into space. If he's not, it will crash into the side of his ship and cause an explosion large enough that space programs will have to track and avoid his debris cloud for decades to come.

Instead Jebediah elects to wait until the fuel runs out. This is not an ideal position from which to start pushing up towards Mun, but when life gives you an out-of-control rocket burning prograde to higher-than-orbital velocities...

Eventually the rocket finishes and drops away behind. Now the lander stage is all that remains of Jebediah's rocket, and while it was only designed to de-orbit, land, and re-orbit, it will now have to push itself up to the Mun's altitude, circularize on a Munar orbit, land, launch, and return home. The bad news is Jebediah's chances are looking a bit slim; the good news is if this vessel makes it home, it definitely has enough delta V to handle the Eeloo mission.

Jebediah finishes what the rogue thruster started and raises his apoapsis to 11,400 km. Unfortunately, he did not have the opportunity to time his orbital transfer so he would connect with the moon, so he'll have to just raise his orbit and wait until a Mun encounter pops up on the flight computer. The first orbit around is a miss.

Almost as soon as he passes his apoapsis, however, the flight computer shows a startlingly close encounter. On his current course he would not only enter Mun's sphere of influence, but crash straight into it.

That seems... suboptimal.

Jebediah adjusts course until the flight computer predicts a low flyby rather than a collision, and adds one more mark to the tally of times the flight computer has saved his life. Soon enough the Mun appears on the horizon as a black spot against the ambient light of space.

Jebediah freaks out at a lot of things, but in this case, it's because of his past career as a pirate.

Once he reaches the closest point in his flyby, Jebediah starts slowing the ship down until a low orbit is obtained. He's eager to de-orbit and begin the landing, but mission control informs him that standard procedure requires that he wait until he has orbited around to the day side before attempting a landing.

Briefly after sunrise, Jebediah begins his deorbit, aiming for a large equatorial crater as a landing site. Soon he is low over the surface with about half of his fuel reserves already spent on maneuvers that were supposed to be performed by a previous stage.

On the bright side, look at that view!

Nonetheless, he performs the landing without incident. The lander is tall, but still stays upright on a reasonably steep slope. Jebediah quickly gets out and begins performing important science.

Now Jebediah begins launch and re-orbit, worrying that the maneuver will be as expensive on fuel as getting here was. As it happens, it's not. Jebediah is more of a reaction times and coordination kind of a guy than a rocket scientist, so he wasn't quite aware of this, but being reduced to half fuel is not so much of a drop as you might think. The reason has to do with an often-misunderstood but fairly intuitive principle of rocket science.

If I have a rocket full of fuel, and I begin spending that fuel to push myself in different directions, the fuel isn't very efficient when I start out. That's because most of its energy isn't being used to push me; it's being used to push all that fuel I'm lugging around. Of course, once I use fuel it's not with me any more, so as time goes on, less of my fuel is being spent on accelerating the rest of my fuel, and more of it is being spent on accelerating me. So when half of my fuel is gone, my fuel is now almost twice as effective as it was before.

So Jebediah launched himself into orbit, avoiding any glance at the fuel gauge. Once high enough up, he finally forced himself to look – and was pleasantly surprised.

Phew!

During his descent he had been at a little under half fuel; Now on re-orbit he's still a bit over a third! Jebediah is certain some mysticism is at work, and briefly checks to make sure Elijah isn't on the ship somewhere making sure his fuel doesn't run out. Unable to find an explanation, he thanks whatever deity saved his hide from the apparently imminent destruction, and begins thrusting to exit the Mun's sphere of influence and return home.

Of course, this lander was never designed for a home trip. It was intended for non-atmospheric landings and launches, not breaking Kerbin atmosphere at well over 2000 m/s. On the way in the solar panels shatter, and Jebediah curses that same deity that saved him for playing with his life so recklessly. His parachutes probably cannot support this much weight, particularly since so much fuel is still left in the tanks.

Jebediah understandably hopes to end up as something other than a very expensive fireworks display.

Finished cursing the gods for his untimely death, Jebediah decides to take a look around and see if there's anything he can actually do about it. He sees the "Undock" button and remembers that mission protocol called for detaching from the landing module before re-entry. Embarrassed, he detaches and decides never to let mission control know about that little panic moment.

How do you ALWAYS get the flag backwards in the picture?

With his safe landing, Jebediah Kerman has proven that the landing module with its 4500 m/s of delta v is not only sufficient, but far more than necessary for an Eeloo landing. Next is to design a transfer stage large enough to actually carry him there.

[KSP] Battle Report 2

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