Written for RP-1 v4.3.0.0

[!IMPORTANT] This guide will use two mods not included with RP-1 Express Install: Principia, and Target Intercept Planner (TIP). The usage of these mods will be explained later in this guide, but if you do not have these mods installed, some of the steps in this guide will be significantly different.

This is not meant to encourage you to use these mods. Principia in particular drastically alters gameplay by adding n-body gravity, so updating to it in the middle of a save is inadvisable. You can replace Lunar Transfer Planner (LTP) with TIP without worry, however.

Introduction

After completing the "First Scientific Satellite" contract in either of the "Early Satellites" programs, the "Early Lunar Probes" program will become available. The first two contracts are "First Lunar Impactor" and "Lunar Flyby (Uncrewed)".

You can easily complete both of these contracts in the same flight, which is what this guide will do. To do this, we need to meet the following criteria:

• Impact the Moon (obviously)
• Have electric charge at the time of impact
• Have a antenna connection to ground control in High Moon Space

Setting Up

Before we start designing our impactor, we need to do some work.

First of all, we MUST get "Lunar-Range Communications" if we want any hope of sending science back home from the moon. That's this node:

We must manually buy "Comms Tech Level 1", which we will be using.

In real-life, the first lunar impactor (Luna 2) came about in 1959, which may or may not be a little earlier than when you would take this program in your playthrough. However, this guide will use 1959 tech, to provide a reasonable lowest-common-denominator as well as to maintain historical accuracy.

Once all of that is done, let's move onto the KSC. If you haven't done so already, upgrade both the Mission Control and Tracking Station buildings. We need to do this if we want to be able to perform orbital maneuvers properly. The Tracking Station upgrade also lets us use the "Comms Tech Level 1" upgrade we purchased, and it gives us a nice boost to our communications range too.

You can simulate your craft before these finish constructing, but you'll have to timewarp on the pad to after they finish constructing. This guide will assume they've already been constructed.

Once all of these have been completed, we can now design our lunar impactor.

Designing the Impactor

We'll start by designing the impactor portion, then later add the rocket that will get the impactor to LEO.

First, place an avionics core. We'll make this a Near-Earth core. We'll adjust the battery capacity and controllable mass later.

Now, let's add a second avionic core on top of this. Make this a Science core. We can get rid of the battery on it. Let's also make it as small as possible. This will make more sense later.

On top, let's add a "Communotron 16" antenna. This will be the main antenna we will use.

Configuring the Antenna

RP-1 uses the RealAntennas mod, which makes antennas and communications significantly more complex. This section will serve as a practical example of how to use antennas in a situation where tinkering with antenna settings is mandatory.

First, let's start by explaining each of the settings in the antenna's PAW.

• Transmit Power (dBm): This is the main setting you'll adjust. It controls how much power the antenna uses when transmitting at full bandwidth. Increasing this will increase the antenna's range, but also increases the antenna's active power consumption. We want to set this as low as possible while maintaining an acceptable bandwidth.
• Tech Level: This should be 1, and the text should be white and not orange. If the text is orange at 1, then that means you didn't purchase the upgrade in R&D.
• RF Band: This controls the radio frequency the antenna uses. At low frequency (VHF) we get worse bandwidth, and DSN stations will have reduced range, but smaller ground stations and inter-vessel communications will have better range. At higher frequencies (UHF) we get better bandwidth and longer DSN range, but worse connections with smaller ground stations and inter-vessel communications. We are going to use this antenna to transmit science from the moon, so we want to use UHF.
• Active Transmission Time: This simulates the percentage of the antenna's max bandwidth you're actually using. For example, at 50%, things like the Kerbalism window's electric charge duration estimate will use only half of the antenna's active power. This has no effect outside the VAB.

Now, let's open the Antenna Planning window. This is how we'll figure out what our antenna's bandwidth is, and it will be a vital tool for optimizing our antenna. It should look like this:

We want to make sure the following settings are set:

• "Ground Station (Planning) TechLevel" should be set to 1. If it isn't, adjust the slider until it is then click "Apply".
• "Ground Station Sort Order" should be "RxGain" and "Desc.".
• Click the Communotron 16 entry in the left list.
• Click one of the "UHF 50 dBm" stations on the right (it doesn't matter which).
• Click "Moon" in the "Remote Body Presets" section to set the Moon's apogee and perigee.
  • Principia may mangle the preset a little, so you probably want to manually set the "Distance Max" to 405.5 Mm and "Distance Min" to 363.3 Mm, which line up with the Moon's actual apogee and perigee.

After this, the window should look like this:

Note what it says on the bottom: our transmission rates (Tx) are 0 bps and thus we don't have a connection!

[!NOTE] Both the Tx and Rx must be non-zero for there to be a connection. However, the actual value of the Rx rate does not matter as long as it isn't 0, so we only need to focus on the Tx rate.

If we increase the dBm slider, we'll eventually see the Tx rate increase to 1 bps, and the "no connection" blurb disappears. For me, that happens at 33 dBm.

Now we know how to find our antenna's bandwidth, but what bandwidth should we actually have?

Experiments

Each transmittable science experiment we bring has a "Data rate". This is the rate at which the experiment produces data, and is also how much bandwidth we need to transmit that experiment.

As for which experiments we want to bring, consider the following: a lunar impactor will typically spend a few hours in High Space around the Moon, but only a few minutes in Low Space (within 250 km) before impacting. Thus, we should only bring experiments that can reasonably complete in these time periods.

At our current tech level, we have the following experiments unlocked:

• Telemetry Analysis: Comes with the avionics cores, so we don't need to select it. It only takes 5 minutes, so we should run it.
• Barometer: Only takes 10 minutes, so we should bring it.
• Thermometer: Also takes 10 minutes, so we should bring it.
• Radiation Detector 1: Takes 91 days, which is way too long for the mission, so let's skip it.
• Magnetometer 1: Takes 30 days, which is still way too long, so let's skip it.
• Mass Spectrometer 1: Takes 2 hours. This is long enough to get good High Space science, but too short for Low Space. For now, we'll bring it.
• Micrometeorite Detector: Takes 91 days, so let's skip it.
• Early TV Camera: Takes 40 minutes, which is decently long, but we'll take it for now.

That leaves us with the following experiments:

...and their data rates:

Experiment	Data Rate
Telemetry Analysis	1.5 bps
Barometer	1.0 bps
Thermometer	1.0 bps
Mass Spectrometer 1	1.1 bps
Early TV Camera	1.1 Bps (8.8 bps)
TOTAL	13.4 bps

This means that we need at least 13.4 bps of antenna bandwidth to transmit the data faster than it is being generated.

With RealAntennas, an antenna's bandwidth decreases in power-of-2 increments (rate-halvings), and TL1 UHF starts at 32 bps bandwidth, which means the lowest bandwidth greater than 13.4 bps is 16 bps; there's no bandwidth in-between the two.

[!NOTE] The actual max bandwidth of UHF is 31.5 bps, not 32 bps. The antenna debugger window shows the extra decimal point, while the antenna planner doesn't. I'll use 1-2-4-8-16-32 bps for this guide, but know that the actual bandwidths are very slightly less than these.

So, let's set the antenna's bandwidth to 16 bps.

This works, but look at the "Power (Active)" on the antenna. It's all the way up to 411.52 watts! This is a lot of power for just an antenna. The silver lining for this is that the antenna only uses this much power if it's using all 16 bps of its bandwidth, but we're only using 13.4 bps at peak, and a lot of our experiments only run for a few minutes so in reality it'll be a bit lower than that.

The biggest culprit for this is the Early TV Camera, which is using up more than half of our bandwidth. Doing some math, we can find that a full transmission of the Early TV Camera uses 550 kJ of energy (411.52 W * 40 min * [8.8 bit/s / 15.8 bit/s]), which is way too much for what we'll be doing.

If we drop the Early TV Camera, we get the following data rates:

Experiment	Data Rate
Telemetry Analysis	1.5 bps
Barometer	1.0 bps
Thermometer	1.0 bps
Mass Spectrometer 1	1.1 bps
TOTAL	4.6 bps

Don't forget to start each of these experiments!

This is a pretty awkward bandwidth, because rate halving means our only antenna options are 4 bps (slightly too slow) or 8 bps (decently too fast). Let's compare the two:

-	4 bps	8 bps
Power (dBm)	39 dBm	42 dBm
Power (active, W)	103.59 W	206.40 W
Active Transmission Time	117.95% (100%)	58.228%
Peak Power Draw after ATT (W)	103.59 W	120.18 W

If we go with 4 bps, we'll transmit about 15% less data from Low Space around the moon, but the antenna will use about 14% less power. Whether this tradeoff is worth it is up to you, but I'll go with the 8 bps antenna.

Finally, we should disable the antennas in the avionics cores. Every avionics core has a built-in antenna that's a bit worse than the Communotron 16 we're using, and if we don't disable them they'll eat up mass and cost. We can remove the antennas by going into each avionics core, then under the "Communications" section click the button labeled "Antenna" so that the text next to it should say "Disabled".

The Engine

Now, let's add two fuel tanks. For the first, I'll put down a procedural tank because small procedural tanks can be made smaller for the same wet/dry mass compared to a modular tank, and I'll also make this tank isogrid because isogrid tanks are somewhat overpowered compared to conventional and balloon tanks in the current version of RP-1. This tank will also be high-pressure. This tank will store the RCS fuel.

For the second tank, I'll use a modular tank because I think it looks better, and I'll also make it isogrid. We'll store the engine fuel in this tank.

Let's set the diameter of both tanks to match the probe core, but don't touch the tank's length just yet.

Next, let's add our engine. At our current tech level, the best engine for this is the RD-0105. It has a better vacuum Isp than anything else at this tech level. If you're only using US engines, then the Agena engine (XLR81-BA-1 config is my recommendation) should also work, but it is slightly worse than the RD-0105.

[!TIP] There's an attachment node on the back of the "dinner plate" that you can use to attach the engine flush to the fuel tank.

Let's fill the non-HP tank with RD-0105 fuel.

RCS

We'll need RCS blocks to orient the craft in orbit. For this, you'll want to use the HTP fuel type on the blocks, and these blocks to be the smallest ones available (28/45 N class).

There's multiple ways to place the RCS blocks, but I'll elect to have two on the bottom and two on the top, with the top ones rotated 90 degrees from the bottom ones, while all of them are above the gaps in the RD-0105's vernier arms. I'll also make these the five-horn variant.

Fill the HP tank with the RCS HTP fuel, and make the tank as small as possible. Even 10 mm of length is enough.

Fuel Tank

Now, let's finally adjust the length of the main fuel tank.

Accounting for lunar precession and fuel boiloff (RD-0105 uses cryogenic fuel), we need about 3300 m/s of Δv to do the Trans-Lunar Injection (TLI), which is the name of the burn that sends us to the Moon. Let's adjust the fuel tank's length until MechJeb's Δv readout says we have that much Δv.

Avionics

Let's finally adjust the avionics cores, specifically the Near-Earth core.

Right now, our impactor has a mass of 1.440 tons, so let's increase the "Controllable mass" of the avionics to 1.5 tons.

This works, but now our mass is 1.546 tons, and our Δv dropped to 2919 m/s! But instead of increasing the controllable mass or lengthening the fuel tank, let's first try reducing the battery capacity. The current battery size is way too heavy; from experience, we can probably get away with just 2 MJ of energy.

That alone drops us down to 1.377 t and our Δv is now all the way up to 3576 m/s! Now we should make the fuel tank shorter. When we do this, the mass of the impactor is now 1.246 t, and we can decrease it further (but do try to set it to an already-tooled mass).

At this point, the actual impactor part of the rocket is complete! Now let's test it to make sure it works.

Simulating the Impactor

[!IMPORTANT] If you are using LTP instead of TIP, then skip this section, go to the "Simulating from the Pad to the Moon" section, then afterwards come back here. Without TIP, we have to launch from the launchpad instead of simulating the impactor directly from orbit because we don't know the LAN to launch to otherwise.

Save your design, then exit out of the VAB into the KSC. We can technically do this in the VAB, but from experience it's pretty buggy unless you step out of the VAB first.

In the toolbar, click this icon:

This will open TIP. It should look like this:

Next to the "Next Window" button there should be a "+" button. You should see more readouts.

Finally, let's set the flight time to 3.5 days. Anywhere from 3 to 4 days should be good. The longer the flight, the less Δv you need.

There's three readouts here that we care about:

• The inclination, under the "In." button next to "Next Window". If the button says "Az." instead of "In.", press it.
• The LAN.
• The time to next window.

Once we have these noted, let's go into the VAB and simulate the flight. Make sure "Start in orbit" is enabled, "Circular" is enabled, set the "Orbit Altitude (km)" to 200, set the inclination to what you found, the LAN to what you found, and the "Time" to the time you found. It should look something like this:

Then click simulate! If everything works, you should be in LEO on a roughly similar orbital plane as the Moon.

Performing the TLI

Once we are in orbit, we can use the following process to get an impact trajectory.

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 RSS and not Sol, so MEO will be used.

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!

13. Open the MechJeb "Maneuver Planner" window.
14. Make sure that "RCS Burn" is disabled.
15. Press the "Execute next node" and let MechJeb do its thing.

After this, you should be on an impact trajectory with the Moon. However, there's a good chance that your trajectory could be off by a bit, or not even be impacting the moon anymore! If this happens, just wait until you have a connection and use RCS to adjust your trajectory until it is acceptable.

Once we are done adjusting our trajectory, we should now disable our Near-Earth avionics. We can do this by right clicking on it, then under "Avionics" press the "Disable Avionics permanently" button. This will kill the avionics core, making us lose control of the impactor. We do this because it saves A LOT of power. This is also why we have the science core; if we only had the Near-Earth avionics, then our antennas would shut off because the impactor would have no live avionics cores! The science core stops this and lets us keep running the experiments and transmit them back home.

Now, all we need to do is wait for the impactor to reach the moon and impact. While this happens, keep an eye on the battery level and make sure it doesn't get too low.

Build the Rest of the Rocket

From here, what you should build for your rocket doesn't really matter; it just needs to get the ~1.25t impactor to LEO. In this case, this Luna-1 inspired rocket design can bring the impactor from above to a 200x200 km Earth orbit, from which the impactor can do the rest of the job. For example, the rocket design from the Early Light Orbital rocket page could be modified to have a payload capacity of 1.25t, which would let it work with our impactor design.

There are some general tips for building this type of rocket:

• Use as few engines and avionics cores as possible. This is because these parts have disproportionately high effective costs and will make your rocket more expensive to build. Generally, the rocket's weight doesn't have a major impact on its cost.
• Remove the antenna in the rocket's avionics core. You don't need a connection for MechJeb ascent guidance to work.
• Reuse tooling whenever possible. If nothing else, always make sure to use the same tank diameter as an existing tooling for that tank type, since that's the bulk of the tooling cost.
  • With this specific rocket design, the boosters and the core fuel tanks are just different enough that tooling one does not grant diameter tooling for the other, but if you place a dummy tank with a diameter in the middle and tool that, we get diameter tooling for both tank types.
• You don't need to doll up your rocket like I did here, but there's no reason not to!

Simulating from the Pad to the Moon

In LTP and TIP, there is a readout for the launch window for the next lunar transfer window. The most important thing to know is that, regardless of if you're using LTP or TIP, you must begin the launch as close to the lunar launch window as possible. It isn't when you need to be in orbit, it's when you need to launch the rocket.

With my rocket, the engines take about 3 seconds before liftoff happens, so I would want to ignite the engines 3 seconds before the launch window, but this varies based on your rocket design.

Make sure to use MechJeb's ascent guidance instead of trying to fly manually! It might take some tweaking, but it'll give you a more efficient launch profile than anything you'd be able to do manually.

Once you're in orbit, you should be in the exact same situation as the in the section "Simulating the Impactor" and the steps to do the impact should be the same as from there.

Next Steps

The logical next step from this point is creating a lunar orbiter. Obviously this guide isn't going into details on how to do a lunar orbiter, but the general premise is very similar. The major differences are:

• You're obviously not hitting the moon. Instead of aiming for impact, aim for a close flyby, i.e. 100 km perilune.
• You need to remain guided at the moon (unguided lunar orbiter is pain). This means that the double-avionics trick we used here wouldn't work, and that you should use a Deep-Space core instead of a Near-Earth core.
• You need to be able to perform a retrograde burn at the perilune, which means you need either a restartable engine or a second orbit-insertion engine. This can just be an SRB, a bee, or if you're feeling frisky a restartable Agena/AJ-10/RL-10.
• With more experiments, you'll need more antenna power and for much longer. This means that you'll either need to skimp on experiments, have big and costly solar panels, or what I'd consider the best option, research "Digital Communications". This unlocks TL2 antennas, which get 1000x higher bandwidth (yes you read that right) and lets you have a usable antenna with MUCH less power. The downside is that the idle power draw is a lot higher, but the active power is so much lower that it's pretty much always better at lunar range. I'd strongly recommend getting that tech before doing a lunar orbiter.
• Even with that antenna, you'll still want solar panels. Our impactor design uses a big battery and assumes it lasts long enough for the mission, but for an orbiter this is not feasible; you need power generation. You could technically use RTGs, but it's much cheaper to use solar panels. Make sure you're purchasing solar panel upgrades!