3000Km Downrange Milestone - KSP-RO/RP-1 GitHub Wiki

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Note: The following tutorial uses ZTheme, a mod that replaces KSP's GUI with a dark theme. It is NOT included with RP-1 Express Install by default and you will have to install it by yourself if you want it. It is NOT required.

Introduction

The 3000km downrange contract will be your space program's first foray into orbital flight, and is a major milestone in your career. Designing this rocket will require knowledge of basic multi-staged rocket design and is a first step towards designing orbital rockets of your own! The rocket, tech-wise, is slightly more technologically advanced than the previous chapter's sounding rocket, but it's mainly a design challenge. Hopefully you have completed some contracts, passed some time, and gained resources while you're waiting for technology to research; it's never good to have too much idle time. You should almost always be building a rocket or completing a contract or at least gaining science; staff aren't there to twiddle their thumbs, after all! And if you didn't, that's okay too; playing optimally is nice, but having fun is more important.

Contents

1: Contract Requirements

First let's take a look at what the contract requires. After all, you can't design a rocket without knowing what the rocket will do.

Luckily, the requirements aren't too complex. It's mostly the same as previous downrange contracts, requiring full control for the first 50 seconds. The only exception is that we now have to travel a whopping 3,000 kilometers downrange, which is much more than any rocket we have designed thus far can achieve. To achieve the stated 6,000 m/s requirement, we will need some unique designs and better technology, so let's get to it!

However, before we start building, we need to grab one more tech node; Early Rocketry. This node unlocks both the AJ10-27 config on the Aerobee engine, as well as the RD-101 config of the RD-100 engine. You may have more engine choices depending on what speculation level you chose, but this is assuming you chose "Operational," the lowest spec level. The following picture is an example of what your tech tree could possibly look like by now, but if you're behind, it's okay, as long as you have the required tech.

2: Creating The Upper Stage

Let's head into the VAB. The basis of our design will effectively be an upgraded Aerobee upper stage stacked onto a V-2 rocket; a conglomeration of two of our previous rockets. We can start by opening up your previous sounding rocket and removing the clamp and solid rocket booster (keep the decoupler though!) and then going from there.

However, we also need to strip the rocket of unnecessary equipment such as parachutes, sounding payload, and even the fins to reduce mass. We can remove the fins because we'll light the stage quite high into the atmosphere, where air doesn't have too much effect. Again, keep the decoupler. Afterwards, your rocket should look quite bare; something like this.

3: Creating The Lower Stage

We'll come back to the upper stage later, but for now, we need to design the lower stage. While I did say we are effectively stacking an Aerobee onto a V-2, we need to convert it into the form of a lower stage. If we look at our old V-2, the whole upper nose were avionics. This is quite inefficient for space, and from now on, avionics will likely be placed at the widest part so that they add the minimum height. Grab two conventional tanks and a procedural avionics part, placing them under the decoupler in the order of tank, avionics, then another tank.

You can make it look how you like, but I have colored it in accordance with the tutorial color scheme. To save some time, rather than remaking the boattail and fins of our V-2, we can simply merge our current rocket with the V-2 and then grab the parts we want. Save your current rocket and then select your V-2 from your saved crafts. Rather than pressing open, press merge. This will let you grab your entire rocket and place it somewhere onto your current rocket. Attatch it anywhere (probably radially). Make sure you don't have symmetry turned on to avoid lag.

My V-2 is a bit different from the tutorial one but that won't matter too much. Go to the bottom and grab the fairings, fairing base, fins, and engine. Then you can delete the rest of the merged V-2 rocket, and reattach the parts you took from the V-2, in the same order. For now just place the fins anywhere on the tank; we'll fix them later.

Looks a bit ridiculous, doesn't it? Let's fix that. Change the upper tank to smooth cone and change the top to 0.3m, matching the decoupler, and the bottom to 1.5m to match the fairing base. Then do the same for the avionics and lower tank, but keeping them as 1.5m wide cylinders. We will configure the avionics last. Make sure you remember to change your tank technology to Al-Fuselage on both tanks (remember, the RD-100 is not pressure fed).

Now select the engine and configure it to our newly unlocked RD-101. Taking a look at it, the improvements are obvious; higher ISP, higher burn time, and higher thrust. However, it burns Ethanol90 instead of Ethanol75, so we'll need to modify our LC later. Go to both tanks and fill them with the RD-101's fuel. Stretch the first tank out to 4.5m, the exact length of our previous V-2's nose, then stretch the lower tank out to 7.7m, which will give us almost exactly 90 seconds of burn time. Since the rated burn time of the RD-101 is 85 seconds, this is perfect! Now you can fix up the fins and place them back onto the fairing, as well as placing a clamp beneath the engine.

The final part is to configure the avionics. Our rocket weighs about 20 tons, so go to the avionics and select something along the lines of 22 tons and 500 EC to be safe, then press "Apply (resize to fit)" to minimize the size of the avionics. Note that this will sometimes overshoot the EC; in my case it gave me 716. We can fix this by simply typing in 500 again and the selecting "Apply (preserve dimensions)."

4: Modifying The Upper Stage

Our upper stage won't have control because of the use of Science-Core avionics; it'll be like a sounding rocket launched from midair. This is largely due to the weight of early game controllable avionics, which are still extremely crude and heavy. However, because we are tilting the rocket over, we must find a way to stabilize the upper stage without control, as it would simply tip over and fall if we did not. To do this, we will utilize spin-stabilization, which, as you can guess, uses rotation to stabilize an unguided stage. What this means, however, is that we must make the upper stage much more squat, and less thin. Why? Well, simply put, it's because long, thin objects have a much harder time maintaining stability than wide, short ones. You can read more about this phenomenon here.

So, to maintain stability, we need to shorten our Aerobee stage. Change the width to 0.8m and length to 1.15m. This will give the engine 58 seconds of burn time, slightly above the rated 52 seconds (57 seconds really, because of the 5 free seconds). Expand the avionics' width to 0.8m as well, but we don't need to extend it; high up in the atmosphere, there isn't too much aerodynamic effect.

Now we need a way to actually get the stage spinning. Luckily, RP-1 has specific SRB's used to spin up stages; in our instance, we'll need to use the smallest ones. Grab the Spin Motor (Small) and attach them in 4-way symmetry. Then grab the rotate tool and rotate them one "snap" by turning on snap mode. Anything more and the stage will spin too violently to actually be stable.

And we're done! Checking our DeltaV stats, we have about 6,300 m/s of DeltaV, which, in theory, should be just enough to complete the contract. Forget theory though; let's run a sim!

5: Running A Sim

Before running a sim, we need to make sure our staging is correct. Since the RD-101 needs time to spool up, stage it before the clamp. In addition, we will be hot staging the Aerobee stage, meaning we will light the Aerobee engine a few seconds before the RD-101 has fully burned out, so that we can use its remaining thrust to provide ullage. So we need to place the Aerobee engine after the RD-101 engine, and then the last stage will be the decoupler and spin motors.

We're all good to go! Let's simulate. We will keep using MechJeb Ascent Guidance, but there will be a little more complexity this time around. Since I'm using the dev build of MechJeb, the GUI looks a bit different, so don't worry if your screen looks a little different. All the important stuff is still in the same place. We can keep using the same ascent path as we used on our other downrange rockets. However, under the settings, make sure "Support Hotstaging" under Autostaging is checked. A lead time of 1 second is a good balance between providing ullage and not burning up the tank, so set it to that (lead time refers to how much burn time the first stage has remaining before lighting the second stage, similar to drop solids early).

Check your settings. When your ready, engage autopilot, press spacebar, and watch MechJeb do its magic.

Climbing...

Sucessful hotstage and spin-up...

No ullage issues and a perfect path to space!

Open up MechJeb's Ascent Stats to watch your downrange distance. If all went well, your spacecraft should have exceeded 1,000km in altitude and almost 3,500km in downrange distance! This is more than enough to complete the contract. The spacecraft burns up in the atmosphere, but that's okay; it was never meant to survive.

You may have noticed your spacecraft oscillating slightly during the unguided phase, with the oscillation getting worse as the engine burned. This is inevitable, due to the variance in SRB's, but as long as it isn't too extreme everything will be fine.

6: Completing The Contract

All that's left now is to actually build the rocket! Since our new engine burns a new fuel, Ethanol90, as well as exceeding our pad weight limit of 15 tons, we'll need to modify our LC. The modification could take some time depending on how you modified your LC before now, but it shouldn't take more than a month to complete; mine took 20 days.

You can change some properties around to get what you want. For example, I have modified the maximum tonnage to 24 tons instead of 22 tons. This is because the lower stage weighs about 19 tons by itself, meaning any V-2 with the RD-101 will weigh a minimum of 19 tons. Setting the maximum tonnage to 24 tons makes the minimum weight 18 tons, so it gives us a little more wiggle room while also guaranteeing that we can always build a rocket without needing to modify the LC (because the V-2 will always weigh at least 19 tons). Remember to assign staff after the LC finishes construction! Tooling the rocket will cost quite a few funds, 13,000, but hopefully you have accumulated some unlock credit to get it for free. After you've built the rocket and warped some 70 days, simply roll it out and let MechJeb take over!

Success! We completed 3,000km downrange with some altitude to spare. The rocket maxed out at 3,200km downrange, and with some optimal flying you could maybe push it to 3,500km. Unfortunately that isn't enough to complete the next contract and program capstone, the incredibly original 5,000km downrange milestone. However, this is still a great achievement; you have designed your first two-staged rocket and have stepped in the direction of orbital flight!

7: The Next Step

The next contracts after this are the 5,000km Downrange Milestone and the First Advanced Biological Suborbital Experiment contract. The first one is just 3,000km with upgraded engines and a little sounding payload, while the latter is even easier; just go really fast with an advanced biological capsule and some sounding payload. These are fairly easy compared to designing your first proper two-staged rocket, so we won't be going over those; just apply the same concepts!

So, congrats! You have just made your first step toward orbital rocketry. This is one of the first difficult contracts you will complete, and has hopefully prepared you for actual orbital rockets. The next chapter will cover how to build a proper orbital rocket and launch your first artificial satellite!

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