🚧 PAGE IS UNFINISHED - READ AT YOUR OWN RISK 🚧

Written for RP-1 v4.3.0.0

[!IMPORTANT] This guide will use the Target Intercept Planner (TIP) and Principia mods. If you are not using these mods, then this guide should still be helpful, but some of the steps may not be the same as what you would do. Mainly, the process of getting a lunar intercept is significantly different in Principia compared to stock patched conics, and the UI of TIP is somewhat different than Lunar Transfer Planner (LTP) but are similar enough for the basic usage to be the same.

Introduction

Upon taking the "Early Lunar Probes" program (unlocked after completing a Light/Heavy Satellites contract) the first two contracts that will be presented to you are "Lunar Flyby (Uncrewed)" and "First Lunar Impactor". However, it is quite easy to complete both of these contracts in the same flight, which is what this guide will do.

To complete both of these in one flight, we just need to hit the moon with a craft with an antenna that can transmit science back home. All of the other requirements can be satisfied by doing this (a lunar impactor will naturally be over 40 kg, be moving faster than 2450 m/s, etc).

Tech

I would consider the bare minimum research needed to be this:

You can use tech beyond this of course, but this is what this guide will use and should match up with the tech of anyone attempting these contracts.

The most important tech here is "Lunar Range Communications". You will need "Comms Tech Level 1", which is something you must manually purchase to use.

Additionally, you must upgrade both the Tracking Station and Mission Control once each. This will significantly extend your communications range (+14 UHF dBi) and, more importantly, unlock patched conics and maneuver nodes, making the planning of the mission actually possible.

The Impactor

We'll start by designing the actual impactor first, then extend on it.

First, we'll start with a procedural avionics core. We'll make this a Near-Earth. This guide will mark avionics cores by making them foil. Don't worry about it's stats for now, we'll set them later.

We'll add some experiments to this, which we can do by clicking the "Configure Experiments" button. We want experiments that don't take more than a few hours to run, since experiments that take longer than that will generate almost no science before we hit the moon. This leaves us with these experiments:

Communications (or: RealAntennas for Dummies)

Right click on the avionics core, and open up the "Communications" tab. We're going to add an external antenna to this, and there's no benefit to having multiple antennas on our impactor, so let's remove the built-in antenna from the avionics core.

Now, let's add a Communotron 16 to the craft.

Up until this point, we got away with not needing to think about antennas and communications, but once you start doing lunar-range missions, antennas become a VERY significant design consideration.

Let's start with the two most important settings on the antenna: the "Transmit Power (dBm)" and "RF Band".

Starting with the latter, the RF band determines what radio frequency the antenna receives/transmits on. Right now we have two bands unlocked: VHF and UHF. They have the following advantages and disadvantages:

• VHF: Lower frequency. It has a lower max bandwidth (13 bps at tech level 1) but less attenuation (the signal drops off with distance slower). Ground stations are worse at transmitting/receiving low-frequency bands like VHF.
• UHF: Higher frequency. It has a higher max bandwidth (32 bps at tech level 1) but more attenuation (the signal drops off with distance faster). Ground stations are better at transmitting/receiving high-frequency bands like UHF.

Notably, DSN stations (which are what you'll connect to at lunar-range) are SIGNIFICANTLY more receptive to UHF than VHF, enough to offset the increased attenuation, which means that generally speaking, UHF will have better range when phoning home compared to VHF. That's why we'll make this antenna a UHF antenna.

Now onto the Transmit Power. Increasing this will increase the range of the antenna as well as the bandwidth possible at a given range, at the cost of requiring more power to actively transmit. Be careful with this slider, because these effects are exponential; increasing dBm by 10 will result in a 3.16x increase in range (inverse-square), but a 10x increase in power draw. At the maximum of 60 dBm, transmission can use kilowatts of power, which will drain even large batteries quickly. We want to set this as low as possible while still getting an acceptable bandwidth.

Now, how do we know how much power we should pump into the antenna? Click on "Antenna Planning".

Here, we can put in a few factors (station tech level, our antenna, ground station, distance) and it'll give us back what bandwidth we'll get in those conditions.

Here, we want the ground station level to be 1, the primary antenna to be the communotron we just placed, the peer to be one of the DSNTrackingStations (should be the "Best Station"), and we want to click on the "moon" button in the remote body presets. It should say "no connection" now, because the antenna isn't getting enough power.

[!NOTE] If you're using Principia, you may want to adjust the distances generated by this. The max distance should be around 406.7 Mm and the min distance 356.4 Mm.

Now, we can change the power slider until we get a good bandwidth. For now, let's set it high enough so that it actually connects, which is when all 4 of the numbers are not 0. For me, that's 33 dBm.

Except, what actually is a good bandwidth?

Each of the experiments on our craft is transmitted, and the amount of bandwidth each experiment needs to transmit is exactly equal to that experiment's Data Rate. For us, those data rates are:

Experiment	Data Rate
Telemetry Analysis	1.5 b/s
Barometer	1.0 b/s
Thermometer	1.0 b/s
Mass Spectrometer	1.1 b/s
Early TV Camera	1.1 B/s (8.8 b/s)
TOTAL	13.4 b/s

This means that in order to transmit all of the experiments on the craft simultaneously, we must have at least 13.4 bps of bandwidth. That's a lot higher than the 1 bps we're currently getting!

Let's increase the power to the antenna until the Tx rate is at least 13.4 bps. We only need the Rx rate to be not 0, so we can ignore it.

For me, getting to the correct bandwidth requires 45 dBm! The bandwidth increases in power-of-2 steps, so this is 16 bps of bandwidth.

The listed active power draw is a whopping 411.52 W! We're only going to use about 84% of that bandwidth at most (using less bandwidth than the maximum saves on power), so it won't get that high; we're going to be using "only" 342 W of power...

[!TIP] Setting the "Active Transmission Time" slider will affect how much power is used for power simulation in the VAB. You can set this to simulate how much power the antenna uses. The Kerbalism windows uses this to give an estimated battery life of your craft.

If we drop the Early TV Camera, our max bandwidth drops down to 4.6 bps. Losing out on science sucks, but it'll cut down the power draw of the antenna down to roughly a third, and we'll get A LOT of science very soon from lunar orbiters, so this is a fair tradeoff.

This is just barely above the 4 bps bandwidth step, so we can choose to either be bandwidth-bottlenecked and transmit science at 87% the speed it is generated, or we can add a little more power to get to 8 bps and have full bandwidth. Going up to 8 bps requires 42 dBm, which at 4.6 bps transmission rate results in 120 W of peak antenna power draw. Going to 4 bps requires 39 dBm, which at max transmission speed is 104 W of peak antenna power draw. I'd say that going up 16 W is worth it for the increased science, so I'll go with 42 dBm on my antenna.

Now, once we're at the moon, we'll be able to transmit our science with worry!

Building the Impactor

In order to hit the moon, we'll need to actually be able to get to it first.

Let's start by assuming we're able to get our current craft to low Earth orbit. It's a good idea to design the impactor, and then later build a launch vehicle that can bring that impactor to orbit.

Looking at a Δv map, we'll find that it roughly takes 3150 m/s of Δv to go from low Earth orbit to a moon transfer. The burn that will do this is called the Trans-Lunar Injection, or TLI.

At this tech level, the best engine for this is the XLR35-RM-1, a config of the XLR11 (the one with the 4 tubes) that is unlocked in the Early Rocketry node. If you have it unlocked, the RD-0105 is better, but this guide will use the XLR11. The AJ10-37 is also unlocked at this tech level, but it has poor reliability, so use it at your own risk.

Let's attach an isogrid modular tank and put the XLR35 on it. We'll resize the tank to hit the required Δv later.

We also want some RCS. Let's put on another isogrid tank (I use a procedural tank because it's smaller, but it doesn't matter too much) and make it tiny (10 mm works). Then, let's attach the RCS thrusters; I'll use the 28/45 non-angled non-elbow version, and attach three of them right next to the engine. Set the RCS config to HTP, then fill the empty tank with it.

Now, let's make the main fuel tank bigger. Remember, we need 3150 Δv to do the TLI, but it's safer to aim for 3200 Δv. The XLR35 also uses cryogenic fuel! We're going to be in low-Earth orbit for anywhere between a few minutes and an hour and a half, so there may be fuel boiloff! Boiloff is a deep rabbit hole, but in this case we can add about 100 extra Δv to account for it. This leaves us with a target of around 3300 Δv.

I settled on this:

Finally, we need to update the avionics core. Right now, it has no controllable mass, and it's battery is too big. Let's fix that.

This impactor is just short of 3 tons, so let's make the avionics weight limit 3t as well. Also, we can reduce the battery capacity to 20 MJ. We'll fine tune the battery capacity later, but this is a good starting point.

Now, we have a new problem: the avionics core itself is drawing a massive 119 W of power! And unlike the antenna, it's going to be drawing this constantly!

We can fix this by clicking the "permanently disable avionics" button in-flight. Problem is, this is our only avionics core, and if all avionics cores are permanently disabled like this, then our experiments and transmission will also shutdown!

In order to fix the fix, we can add a second avionics core, this one a tiny science core. It doesn't need a battery, and we can take out its built-in antenna. Make it as small as possible.

This science core doesn't actually do anything other than make sure the entire craft doesn't shutoff when we disable the Near-Earth avionics. Just like that, we dropped the 119 W power draw.

All of these changes overall made the impactor a bit lighter, so now our Δv is a lot higher. Let's make the fuel tank smaller again. This leads to the impactor's mass being low, so we can reduce the weight limit further, which reduces how big the tank needs to be... After some fine tuning, we end up with this design:

[!NOTE] This is pretty heavy for a lunar impactor; with later tech you can make an impactor that's a lot lighter than this. This is an example of "cavemanning": doing a mission with a lower tech level than you probably should. Here, cavemanning is used to create a lowest-common-denominator example, but realistically you should probably have a little more engine tech for doing this.

Testing the Impactor

We can skip the tedious step of actually launching the impactor to orbit, by starting with just the impactor in orbit. However, we can't just put the impactor into a circular orbit and call it a day.

Here, we can use the mod Transfer Intercept Planner (or Lunar Transfer Planner). This will tell us the optimal orbital parameters for a TLI.

[!NOTE] TIP can be used in the VAB, but it's pretty buggy. I recommend going out of the VAB to get a TIP readout.

Press the + button below the "Required Δv" field, and make sure the button to the left of the + button says "Next Window". Set the flight time to whatever sounds reasonable (this guide will use 3.5 days).

This tells us that in 15 hours and 27 minutes, we'll have a launch window with a 28.53° inclination (we're launching from the cape) and 358.28° LAN. If we make it to this orbit, then we'll be able to perform a 3138.77 m/s Δv burn somewhere on our orbit, and about 3.5 days after that burn we'll be in the vicinity of the moon.

We can simulate this by plugging the appropriate values into the orbital parameters in the "Start in Orbit" mode of the simulate window.

Let's simulate! If everything worked as intended, the impactor should now be in orbit around the Earth on a roughly similar plane to the moon.

In order to actually perform the burn, follow the following steps:

[!NOTE] This process uses Principia, but if you're using stock patched conics, this will be completely different!

1. Make sure the engine and RCS are staged, RCS is toggled on, and the antenna is extended.
2. Open the Principia window by clicking its icon in the toolbar.
3. Click on "Flight plan...".
4. Click on "Create flight plan".
5. Click on "Add maneuver".

6. Toggle on "Show on navball".
7. Make sure that "Maneuvering frame selection" is set to Earth-Centered Inertial.
8. Set "Δv tangent" to the TIP Δv readout.
9. Set "Plan length" to be long enough for the mission (4 days should be long enough)
10. Switch the "Plotting frame selection" (main Principia window, do NOT change "Maneuvering frame selection") to Moon-Earth Orbit (MEO). The flight plan window should say "Maneuver frame differs from plotting frame".

[!NOTE] If you're using RSS, it'll be called MEO. If you're using Sol, it'll instead be called LEO (Luna-Earth Orbit). This guide will use MEO, but any screenshots will use LEO.

11. Double-click on the moon to focus on it.
12. Increase the "t initial" value until the trajectory passes near or into the moon.

[!IMPORTANT] The probe must impact on the side of the moon facing the Earth in order for science to be transmitted!

Congratulations! You're now on track to do a lunar impact. All that's left is to do the burn.

MechJeb has a useful tool called "Maneuver Planner" that lets the craft automatically perform a maneuver burn.

To execute the burn, just press the "Execute next node" button, and MechJeb will do the rest for you. Make sure that "RCS Burn" is TURNED OFF.

After MechJeb timewarps to the maneuver, aligns you with the node, and completes the burn, you should be on an impact course with the moon, but your trajectory might not be exactly what you planned it to be. Maybe it's not even impacting the moon!

This calls for RCS correction! Do the following:

1. Delete the maneuver you just burned.
2. Click "Rebase". This resets the flight plan to your vessel's current state.
3. Click "Add maneuver".
4. Set "t initial" to about an hour from now.
5. Click "Active RCS".
6. Set "Maneuvering frame selection" to ECI.
7. Adjust the Δv sliders until you get an impact point where you want it. This shouldn't take more than a few Δv to accomplish.

8. In the MechJeb Maneuver Planner window, toggle on "RCS Burn" and click "Execute next node".

After this, your trajectory should be what you want.

Once you are certain about your trajectory and impact point, right click on your Near-Earth avionics core (NOT THE SCIENCE CORE) and click "Disable avionics permanently". Once you do this, you won't be able to ever control the vessel again! As a reminder, we're doing this in order to cut down on power draw; the avionics core is one of our biggest power draws, and we won't have enough charge when we get to the moon if we don't do this!

At this point, we can just sit back and watch the impactor tumble towards the moon. We'll go from Low Space Earth to High Space Earth, then High Space Moon, then finally Low Space Moon; each of these are different experiment situations and will generate science. If our antenna is working right, this should be transmitted directly home and not be stored in the impactor's hard drive (if it has one).

At this point, both of the contracts should be marked as complete!

[!IMPORTANT] There's a known Sol bug that causes the First Lunar Impactor contract to not complete. If you're confident you should've completed the contract, but it didn't get marked as completed, then press Alt+F12, go into Contracts > Active, and cheat-complete the contract. Don't feel bad, you earned it.

You may notice that the impactor still had a lot of energy left in its battery. In my case, I had about 18.5 MJ of energy when it impacted the moon, and this was in a flight that kept the Near-Earth avionics active for over two hours; depending on how long you stay in orbit, you could be using even less energy. We overestimated our energy need by an order of magnitude!

Let's go back into the VAB and reduce the battery to just 2 MJ. This gives us a big boost in Δv, and when we shrink down the tank to compensate, we're under 2t! We can set the Near-Earth avionics controllable mass down from 2.5t to 2t, giving us a decent reduction in cost.