Chapter 2. Mün Mission

In the first chapter we learned to send a payload into low Kerbin orbit. Here we take a much harder mission. Some just build a huge rocket and send it. I admit Mün is close and not very demanding, so this may work. But still there are chances to screw things up, and you may not want to build a monster.

Lunar Orbit Rendezvous

Nothing new here if you have read about it or Apollo program. To get basic ideas, watch how to fly to Mün by Scott Manley and read the basics of Apollo program profile.

LOR was proposed first by Yuri Kondratyuk, who even worked in my city about 80 years ago. The idea is to land only a specialized module on the Moon, and leave the returning ship in lunar orbit to save fuel. How much fuel it saved – we’ll calculate in the next chapter. The lander had to rendezvous with the main ship. Here’s the mission profile:

_images/Apollo-Moon-mission-profile.png

As you see, in this mission, there are 7 steps that require thrust, and we have to design a ship for all of them. I suggest the simplest way: design it from end to start.

Procedure for Complex Missions

Step 1: Assemble the ship for the end of the stage. Here we have the very last part: a command pod and a parachute.

_images/tn_pic-chap2-command_pod.jpg

Step 2: look at the 𝛥v map and estimate the maneuver or remember a previous mission. To return from Mün orbit we only need 230 + 80 m/s (I usually spend 260 m/s), and the rest of the path has red arrows which means we brake for free. Assemble a ship for this maneuver. Here’s a new, more accurate Delta-V map.

_images/tn_pic-chap2-delta-v-map.png
_images/tn_pic-chap2-return_ship.jpg

Step 3: check if it can do the previous maneuver. Maybe it’s worthwhile to just extend the tank. This ship, for instance can also land and take-off, if we add struts.

_images/tn_pic-chap2-lunar_csm_final.jpg

And so forth. Now the question is if we need a separate lander.

Saint Grail of Lander

How small can a lander be? If we look into stock ships, there’s this one.

_images/tn_pic-chap2-mun_huge_lander.jpg

The docking port hints that it should dock with the command module after taking off. But look: it’s overpowered for Münar landing. The TWR of Apollo spacecrafts was 0.33, or 2 in lunar gravity, something manageable. Here we have 1.6 or about 10 in local gravity. You can add some more tanks, and it will easily fly all the way to Mün and back! Nothing burns in atmosphere in KSP yet (v0.22), so... why not add parachutes and land it on Kerbin? (I wonder what that small booster is for.)

What about a smaller lander? I tried some designs, and here are two of them.

_images/tn_pic-chap2-screenshot1481.jpg

One-piece lander and ...takeoffer o..0 TWR on Mün will be about 1.7

_images/tn_pic-chap2-screenshot1480.jpg

A more complex lander, with asparagus staging. Mün TWR about 2.5

And then I tried a radical approach: use two single-kerbal landing cans together. You can either stack them or tie laterally as I did. They weigh 600 kg, 1200 together, while the big landing can weighs 2500 kg. We save extra 1300 kg and a good deal of fuel.

_images/tn_pic-chap2-mun_small.jpg
_images/tn_pic-chap2-screenshot1076.jpg
_images/tn_pic-chap2-screenshot349.jpg
_images/tn_pic-chap2-screenshot1110.jpg

It’s like a razor blade: very tiny, very easy to control (it can roll on the ground with reaction wheels), and it has exactly as much fuel as needed. When landing I kept the speed as 1/33 of altitude, i.e. I divided by 100 and multiplied by 3.

To fly it you should know what you’re doing, as the margins account for human imprecision, but not for stupidity. Also don’t apply full thrust, because it’s overpowered and depletes fuel in seconds.

_images/tiny_lander.jpg

If not for the requirement to land 2 kerbals, I’d have sent this one. Has enough fuel and good safety margins, can roll itself too. Cute, isn’t it?

I guess, to go further down you need a space bicycle™ (I take this name first!), basically, a small rocket with external command pods.

All together the lightest rendezvous configuration weighs 13.49. There’s also a single-piece ship that can fly there and land.

_images/tn_pic-chap2-screenshot1464.jpg
_images/tn_pic-chap2-mun_single_lander.jpg

Notice: the lateral tanks will be dropped long before landing. I had to redesign it – the last ascent stage also sends it to Mün.

But anyway, lander did not save that much. Why did Apollo program bother with complex rendezvous? On the real-life Moon, landing round trip takes 3200 m/s, hence savings are a lot more significant. Also, the Command-Service module was too big to spend fuel on. I’ll expand on this in the next chapter.

Complex Staging on a Spreadsheet

Now look again at the ship and the lander.

_images/screenshot1464.jpg _images/lunar_csm_final.jpg

I want to know if it’s capable of flying to the Mün on it’s own. The plugin can’t calculate the performance, but we can do this manually. Let’s calculate everything again from the end to start.

First, we can’t use dry mass, because it assumes empty RCS tanks, and I usually have them half or 2/3 full. So substract fuel mass from full mass to get the worst case. 10.5 - 4 = 6.5 tonnes. Now let’s calculate how much fuel we spend on returning.

The maneuver requires 275 m/s, but let’s add a 100 for rendezvous and corrections.

6.5 \times (e^{\frac {375} {390 \times 9.81}} - 1) = 6.5 \times 0.103 = 0.668 t

Add this fuel and lander full mass to this:

Part Mass
Command Module 6.5
Returning fuel 0.668
Lander (full) 2.87
Total 10.04

Let’s calculate the same formula for this mass

10.04 t \times (e^{\frac {1345} {390 \times 9.81}} - 1) = 4.230 t

Together with the previous fuel, 4.230 + 0.668 = 4.898. We have to add a 1-tonne tank.

This is quite tedious, but works in Excel (Open Office Calc). Dry weight of the 1-tonne tank is 125 kg, so CM mass grows to 6.625. Return fuel is

6.625 t \times (e^{\frac {375} {390 \times 9.81}} - 1) = 6.625 \times 0.103 = 0.682 t

The ship and lander now weigh 10.177, so we need

10.177 t \times (e^{\frac {1345} {390 \times 9.81}} - 1) = 4.287 t

Add the return fuel: 4.287 + 0.668 = 4.970. Almost exactly within the capacity of 5 tonnes.

_images/tn_pic-chap2-csm.jpg

Results

Before I show the final rocket, here are some non-optimal configurations.

_images/tn_pic-chap2-screenshot1483.jpg

166 t

_images/tn_pic-chap2-mun_optimal_can.jpg

143 t

And here are my best Münar rockets. You can download both of them: the signle ship and the one with the lander.

_images/tn_pic-chap2-best_rocket.jpg

99 t. Those quadruple LV-T30 aren’t for fun: they have better characteristics than Skippers!

_images/tn_pic-chap2-best_rocket_orbiting.jpg

In orbit, after orbit adjustments.

_images/tn_pic-chap2-single_lander.jpg

109 t. Single lander was just 10 tonnes behind.

_images/tn_pic-chap2-Apollo-ground.jpg

The Mün rocket from an old tutorial weighs more than 600 tonnes.

As you see, single lander’s extra tonne grew to 10 on the ground. Still it’s quite a small rocket. For comparison, here’s an old tutorial recorded in older versions, with a rocket that weighs 600 tonnes. Without planning tools you start making bigger margins, and rocket grows quickly.

I guess this rocket was hard to turn. Meanwhile, the small ship is no difficult to turn around, in fact it turned with reaction wheels alone. The RCS is needed only for docking. Another good reason to budget things. Some screenshots from my flights.

_images/tn_pic-chap2-best_rocket_orbiting.jpg
_images/tn_pic-chap2-mun_arrival.jpg
_images/tn_pic-chap2-mun_arrival2.jpg
_images/tn_pic-chap2-mun_circularize.jpg
_images/tn_pic-chap2-mun_undocking.jpg
_images/tn_pic-chap2-mun_landing.jpg
_images/tn_pic-chap2-mun_landing2.jpg
_images/tn_pic-chap2-mun_eva.jpg
_images/tn_pic-chap2-mun_eva2.jpg
_images/tn_pic-chap2-mun_jumps.jpg

A traditional jumping photo.

_images/tn_pic-chap2-mun_boarding.jpg

Kerbals can board with jetpacks, no ladders needed.

_images/tn_pic-chap2-mun_rendezvous.jpg

Docking

_images/tn_pic-chap2-mun_escape.jpg

Escaping: to enter Kerbin atmosphere, only 250-260 m/s are required if you shoot to escape exactly backwards from Mün.

In the next chapter, I’ll explain how to calculate these things without too much trial and error. Fly on budget and enjoy flying!

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See also

One Giant Leap
a photo gallery of Apollo program (with comments in Russian)