The Basics:
At its core, interstellar navigation and brachistochrone transfers are a fundamentally simple concept. Provided you have enough dV (change in velocity), you point and shoot, nothing more. In other words, you accelerate to percents the speed of light (we will be using simply “c” to refer to the speed of light in this guide) at your origin and decelerate the same amount at your destination. However, the mechanics of such a transfer is not always that simple. This guide will set the reader on a path to interstellar navigation mastery and outline the general process of designing an XML edited starship based on “conventional” propulsion and navigating to another star. While written for SR2, the fundamentals of interstellar navigation outlined in this guide are also applicable for KSP.
Bear with me, this will be a very long, but comprehensive and clear guide.
Philosophy for Designing the Interstellar Vehicle (ISV):
For our ship, we will be using ion engines as the main method of propulsion. This is because in SR2, ion engines are the only form of propulsion able to burn through time acceleration. This is critical since our ship will be burning for hundreds of days to initiate the transfer and decelerate at our destination. This is due to the sheer magnitude of velocity we will be accelerating and decelerating to, it is not within the ten thousands or even in the hundreds thousands of m/s, but in the tens of millions m/s.
To begin our build, we must set a target. A good amount of total dV to aim for is 1/3 c (100,000,000 m/s). This will get you to a destination 15 light years away within 167 years at 8.3% c, which is a reasonable amount of time to wait while at the 5,000,000x time acceleration cap. With a little over 1/3 c of dV, this will get you a round trip to a destination 15 light years away. This was determined by taking our total dV and dividing it in quarters. You spend the first quarter of your dV on the initial acceleration, second quarter on the initial deceleration, third quarter on the acceleration to return, and final quarter decelerating to return. If you don’t intend a return you can go faster, you simply take that same value and divide it half. You will then have more dV allotted for your initial acceleration. Take the second half and use it to decelerate. Always make sure to save some dV for course corrections. This is not optional.
Interstellar travel is assumed to be within reach with propulsion within the specific impulse of 1,000,000 seconds or more. Due to the mechanics of ISV design, any less than that is simply not worth making the journey. This is because we will not have the dV to form a trajectory correctly shaped to intercept our target star. Transfers less than 3% or 5% c will miss their target without radial in or out burns. Such course corrections are difficult due to the fact that your trajectory becomes invisible when travelling at percentages of c. It must be noted Hohman transfers are NOT possible in interstellar travel, we are in a different realm of astrodynamics here. At this magnitude of dV expenditure, trajectories take the form of straight lines instead of circles, parabola, or hyperbola. You will be traveling in a straight line directly to your target. A curve of minimal time, low efficiency. All full brachistochrone assumes that you burn 100% of the time throughout the transfer. Accelerating up to the halfway point, and deccerating at the halfway point. This is not possible to do for interstellar transfers. No theoretical propulsion with specific impulse high enough to burn this long exists. However, full brachistochrones are indeed possible with interplanetary travel assuming far future propulsion systems with ISPs ranging from 300,000 to 700,000 seconds exist. Shown in the below image is a typical interstellar transfer, what we will be doing, or also called a semi-brachistochrone.
Additionally, such a propulsion efficiency is only within the realm of theoretical fusion, antimatter-augmented fusion, and antimatter annihilation propulsion, in order of efficiency. Nothing we have in real life or SR2 comes close to this level of fuel efficiency. As such, we will be using ion engines to design our vehicle due to its ability to burn while in time acceleration.
Designing the ISV:
Before beginning your ISV build, I highly recommend the mod “Overload”. It provides an easy to use XML editing interface. Ok, let’s start with an ion engine and xenon tank. Set the ion engine’s fuel consumption to maximum in the part properties window.
Add your batteries. 170,000 GJ is a good target for reusable vehicles.
Now open the XML editing interface by selecting the ion engine and clicking “edit XML” at the very top of the properties window. Navigate to the “engine” tab.
Let's adjust our thrust first. Aim for between 0.3 and 0.7 TWR. We have selected 4*10^8, or 1,000,000 kN. Notice the magnitude of our craft mass. We are at 211,000 metric tons. This is a normal order of magnitude for an ISV. It comes from our mass of batteries, we need a lot of power to sustain the engine for hundreds of days. Our batteries will act as a limited sort of pseudo-propellant as it is not possible to regenerate this order of magnitude of energy with pure solar panels.
We are currently at 1,179,662,000 for dV or almost 4 times the speed of light. Complete your build here, or reduce the value for gameplay balance. I recommend reducing it, limiting you dV to “realistic” values offers gameplay challenge. Let’s increase the fuel consumption from 0.00022 to 0.025. We are now at 105,000,000 m/s, which is a little over our target, 1/3 c.
Done! That is our propulsion bus complete. Now design the rest of your ship and your surface to orbit vehicles and put them all into Droo orbit. You can get your ISV in orbit by saving a launch location there. You can pretend it was built in orbit, as such ISVs are never launched fully built from the ground.
The Interstellar Transfer:
For our transfer we will be using a typical scout ship propelled by our inertial confinement propulsion bus with VTOL spaceplanes Aurora and Eventide drone docked.
We will be transferring from Sirius D II, the planet Sirius D’s second moon. A link to the propulsion bus will be included at the end of this guide. We will not be transferring from Sol to another star, but the mechanics of the transfer are the same and will suffice for this guide.
First, select your target star in map view and set attitude to target and time accelerate 10x. Your attitude should automatically align to your target now. Use this trick to turn your ISV around. It will be very heavy and not able to be turned by typical RCS thrusters.
Let's first begin by budgeting our fuel. I currently have 59,300,000 m/s of dV. This ship's maximum dV is around 110 to 120 million m/s, or a little over 1/3 c. Half of our dV was used on the initial transfer and what we currently have is the remainder. Let's start our calculation by deciding how much dV to save for our course corrections once we reach Sol SOI (sphere of influence). A good number is 6 to 8 million m/s. We’ll use 8 million m/s to be safe. Subtract 8 million m/s from your current dV, that is 59.3 million m/s. So, we get 51.3 million m/s. Then take that number, 51.3 million m/s, and divide it by two. What you get is the amount of velocity we will spend on our acceleration burn toward Sol. Next, take the acceleration burn dV and subtract it from your total dV, 59.3 million m/s in this case. This number is the value that we will burn our current dV down to. A great degree of accuracy is not required in these calculations. What is critical is that you save a reasonable margin of dV for course corrections. Here are the operations we used for this calculation if it was not clear.
Ok, now we have the information we need for the transfer. Time to do our acceleration burn. Quicksave and wait until your target marker on the navball is aligned with prograde on your orbit. We want to start our burn on the prograde side of the planet so that we do not deorbit ourselves. Your screen and navball should now look something like this.
Begin your burn towards your target and timewarp until your burn is complete. We will be burning our current total dV down to 33.7 million m/s. The burn should take something around between 70 to 300 days at most. Our transfer is at 8.55% c. It should take a bit over 101 years to travel the 8.61 light years back home. Ok, we are now going 25.5 million m/s, 8.5% c exactly. To calculate how fast you are going in percentages of c, take your current velocity and divide it by the speed of light, 300 million m/s. At this point, let’s turn retrograde. You should still be in timewarp, select retrograde in attitude selection and your ship should automatically align retrograde. Now that we are going percents the speed of light, our player marker and trajectory has disappeared. Don’t worry, the game is not broken, we are still travelling a straight line towards Sol. To see where you are in space, simply notice where the center of the screen is in relation to the game world or mouse over your trajectory. Also, zoom in really close to your player marker to see the orientation arrow again. It is still there, just invisible. Now we wait until it is time to do our braking burn. Your screen should look like this at this point. At the center of the screen is Sirius A and B and off to the top right is Sol. We are bound to intersect it in around 87 years.
It is difficult to time deceleration burns in SR2 and we are not given any tools to do it. What we can do is notice our stage burn time. That is around the time you should start your deceleration burn
We currently have 89 days worth of burn time left. Time accelerate until you reach your Sol or target star SOI. You will know when you’ve reached it when your time acceleration resets back to 1x time acceleration. At this point, notice your trajectory and closest approach arrow. You are likely off course at this point. That is ok, adjust your trajectory by doing a radial in or out burn until your closest approach arrow lands you inside the orbits of the planets in the star system. However, your trajectory could be warped at this point and radial in or out could be up or down relative to the game world, instead of right and left. This is ok, do a normal or anti-normal burn until your normal marker is north or anti-normal marker south. Then you can do your radial in or out burn to bring you closer to the star. Make sure that you do your correction burns ASAP after encountering your target star SOI. Every hour you wait increases the amount of dV you need to spend to adjust course. At this point, it is almost time to do our braking burn. Our remaining burn time will give us a good approximation for when to start our burn. Let’s time accelerate until we are 89 days until our closest approach. Give yourself a 3 or 5 day buffer to start the burn. It is better to burn too early then to do a boost back burn back to your target star. The latter wastes dV that you will likely not have if you have a tight fuel budget. When you start your burn, burn full throttle until things look under control. Gradually decelerate in full throttle bursts until you're traveling about 300,000 m/s to 200,000 m/s just when you’re entering the system. Do not decellerate to fast. We need to conserve velocity to minimize time spent in our transfer. Braking burns are difficult to do and require mistakes to understand when to time. Practice and your understanding will improve.
At this point, we should be on the edge of the star system. We are not done yet, as we still need to get back to our home planet via brachistochrone. To brachistochrone to your target planet, some more course corrections are needed. First make sure your target is set to the target planet and your inclination matches the plane of your target by doing normal or anti-normal burns. Next you can achieve your intercept. Do radial in or out burns until you achieve your intercept.
We’re passing awfully close to Sol, but it’s ok. Our ship is built to the finest quality.
Continue to decelerate in bursts along your path but make sure not to decelerate too much otherwise you will lose your encounter. After each braking burst, continue to adjust as needed. When you are a couple days until your target, make sure that you have decelerated down to about between 60,000 or 20,000 m/s to ensure that you have enough time to do your deceleration burn after encountering your target planet. We decelerated too late this time, luckily a boost burn does not cost too much when this close to your target.
We will simply do a boost back by burning toward the target until we get an intercept.
Once you encounter the planet on your hyperbolic trajectory, do a final braking burn to put yourself in orbit around the planet. Make sure that your trajectory always lies on the side of the planet that will land you into a prograde orbit after you decelerate and start falling towards the planet. This can only be learned through experience.
Well, now we have arrived. Do some final orbital adjustments and then send your surface to orbit vehicles down to explore. Congrats!
This general procedure should net you success, but if you struggle, practice, practice, practice. It is all about experience! These principles are applicable to interplanetary brachistochrones.
Linked is an interstellar propulsion bus for you to modify and use:
https://www.simplerockets.com/c/2xZgvt/Inertial-Confinement-Interstellar-Propulsion-Bus
it took 11 centuries to get to the comments lol