Kerbal Space Program isn’t for everyone. The video game, which lets you design, build, and fly rockets and spacecraft across the solar system, truly belongs to space nerds, engineers frustrated at the lack of real-life progress in our space program, and people with a penchant for frustrating themselves with the particulars of orbital mechanics and Hohmann transfer calculations. The learning curve is steep, perhaps as steep as the path of a rocket’s ascent out of our atmosphere, yet the pay-off can be as rewarding as a “giant leap for mankind.” There are few other video games out there that double as honest and true-to-life simulations of the trials, travails, and triumphs that come with the exploration of the “final frontier.”
To demonstrate just how challenging, how realistic, and how thrilling Kerbal Space Program (KSP) can be, I’ve documented one of my recently-completed missions to the Moon. I’ve done so before, over 2 years ago. Yet much has changed since then.
For starters, the excellent and game-changing Realism Overhaul and Real Solar System mods were released. Un-modded KSP, once you get the hang of it, is a relatively straightforward game: many rocket systems are streamlined or simplified and the “Kerbal” solar system, a fictionalized version of our own, is small, making it far more conducive for space travel. Yet Realism Overhaul (RO) and Real Solar System (RSS) fundamentally change those equations. With these mods, like in real life, rocket engines take different versions of fuel and can hardly be throttled, cryogenics require pressurization and insulation, ullage is necessary for starting and restarting stages, unmanned spacecraft require communications relays and are subject to command delays because of distance, and aerodynamic and gravitational forces dramatically influence flight profiles. The Real Solar System mod transforms the in-game solar system into a true-to-scale version of our own. In doing so it drastically increases the size of the game, making it far more difficult, as is the case in real life, to calculate and account for planetary transfers, orbital maneuvers, and descents and landings. Quite unlike the un-modded game, every second of fuel counts, and rockets must be designed to squeeze out the highest “delta-v” possible.
The aforementioned space nerds and frustrated engineers likely understand what all of these terms mean. For those who don’t, no worries… let’s jump into the action and you’ll see soon enough. Though, as an interesting comparison of the game between its modded and un-modded state, compare this report with the past one linked above. Also note, each image corresponds to the text description right above it.
(Click the images for their full-size versions. They make for some good wallpapers!)
A Mission to the Moon!
The rocket stands proudly at the launchpad. Quite the beauty, isn’t she! I have a real appreciation for the aesthetic qualities of rocketry. I don’t think its a phallic thing, though there seems to be a real fixation in the industry. Anyway…
The rocket, filled to the brim with fuel and with the lander payload on top, weighs about 708 tons. It stands nearly 64 meters tall and is 10 meters wide. In this regard, its roughly similar to (albeit a bit smaller than) the real-life Delta IV Heavy rocket.
The rocket features 4 stages (including the lander as the final stage). The first two stages will propel the rocket out of the atmosphere and high above the Earth, the third stage will inject it on a trajectory toward the Moon, and the lander stage will conduct the actual landing.
The first stage is powered by a fuel mixture of Aerozene and NTO, a fuel mixture used since the early era of rocketry. Though highly toxic, this fuel mixture doesn’t “boil away” while sitting in the fuel tank, so I can keep the rocket out on the pad until the moment is perfect for launch. The second and third stages are powered by the more conventional liquid hydrogen/liquid oxygen mixture, which provides a good balance of efficiency and effectiveness. As these are cryogenic (super cooled) fuels, and “boil off” due to evaporation, the upper stage fuel tanks are insulated to keep heat out. All in all, this is a highly efficient rocket, capable of delivering large payloads (including manned craft) to low Earth orbit, medium sized payloads to Geosynchronous orbit, and medium to small sized payloads to Lunar orbit and onto the Lunar surface. I haven’t tried any interplanetary missions using it as the launch vehicle, yet. Perhaps next time.
The lander is relatively simple. It features 4 communications antennas and 4 solar arrays along with a nuclear RTG (reactor) to provide it power. At a weight of nearly 8 tons, it is almost 8 times heavier than the early Surveyor spacecraft that NASA landed on the Moon in the 1960s. Most of this weight comes from the fuel stored inside the lander, however, which will do the bulk of the work involved in decelerating the craft and setting it down gently on the Lunar surface.
This image shows the rocket’s various statistics right before launch. More significantly, there is a small “X” situated in the middle of the navigation ball on the bottom of the screen (you’ll need to click on the picture to see it clearly). This means the Moon is directly on the opposite side of the Earth from the launch pad. I’m at an optimal timing for a “direct ascent” launch toward the Moon. More on that later…
You can’t just launch a rocket to the Moon whenever you want, as the Moon is slightly inclined in its orbit in relation to the Earth. This means you need to wait for a “launch window” when the inclination of the Moon matches perfectly with a directly East-bound ascent from the Earth. Otherwise, the rocket will waste precious fuel translating North or South in orbit in order to match its orbital plane with that of the Moon. As every second of fuel counts in a mission to the Moon, this would be fuel dangerously wasted.
Liftoff! The rocket rises into the sky as dusk gradually settles in over the horizon.
59 seconds after takeoff, while traveling at a speed of 223 miles per hour, the rocket begins its roll program.
Rockets don’t just shoot straight up into the sky. Rather, in order to reach a circular orbit around the Earth, they follow an arcing trajectory, often headed due East. Doing so uses the rotation of the Earth to help provide extra momentum to the rocket moving forward. When it comes to escaping the bond of gravity, any additional force possible can help.
The roll program also uses the Earth’s gravity to slowly tilt the rocket downward in such a way that it is moving forward in addition to upwards, so that it will reach orbit instead of simply flying up and then falling back to Earth. This has an advantage: by using gravity to do the turn, valuable fuel is saved that would otherwise be used to power the thrust needed for such a maneuver.
2 minutes and 50 seconds into flight, the rocket is traveling downrange at over 7 times the speed of sound. Cape Canaveral, Kennedy Space Center, and Florida begin to recede in the distance. At a height of 85 kilometers, the rocket is in the far upper reaches of the atmosphere, but has not yet entered outer space. Still, the bluish “limb” of the Earth’s atmosphere, a sight often associated with the iconic imagery of orbital spaceflight, can be made out over the horizon.
3 minutes and 10 seconds into flight, the rocket’s first stage runs out of fuel and cuts off. The rocket is now traveling nearly 7,800 miles per hour and is 90 kilometers above the Earth. Seconds before the first stage engines shut down, the second stage engine ignites. This is a crucial maneuver which provides the second stage engine ullage. In a low-gravity environment such as that 90 kilometers above Earth, the rocket fuel inside the fuel tank begins to float away from the engine intake valves. If and when this happens, the rocket engine won’t be fed enough fuel and therefore won’t start. To counter this, thrust is needed which pushes the fuel to the bottom of the fuel tank so that the engine can be fed. Some rockets, such as the Saturn V which took astronauts to the Moon, used small boosters between stages to provide ullage. Other rockets, such as mine, ignite the second stage while thrust from the first stage engines keeps the fuel in place.
As the second stage engine roars to life, the fairings surrounding it are blasted away with small rocket motors and the coupling linkage between the first and second stages is detached. Slowly, the first stage falls away from the rocket and back toward the Earth. It will likely either burn up in the atmosphere or impact the waters off the coast of Florida.
The second stage burns for another 3 minutes and 3 seconds before it is shut down. The rocket is now in space, headed toward an apoapsis of 226 kilometers above the Earth. This is the highest point in its trajectory, the point where the rocket will stop moving upwards and start falling back to Earth.
This image shows KSP’s “map screen,” which provides a picture of the rocket’s trajectory and which serves as a tool for calculating orbits, maneuvers, and transfers. The various windows in the image provide all the crucial orbital, fuel, and vehicle information needed to calculate these maneuvers. As every second of fuel counts using the Realism Overhaul mod, I’m often staring at this map screen trying to plan my trajectories during the mission. This particular image was taken about a minute after the second stage ignited.
4 minutes and 26 seconds after second stage shutdown, a total of ten minutes and 42 seconds into flight, the second stage reignites. Moments before, a brief, 3 second burst of a RCS (reaction control system) maneuvering engine located near the second stage engine provides the necessary ullage. Few rocket engines are capable of restarting in-flight, although many of those designed for upper stages can perform one or two restarts. The one I’m using is only capable of a single restart, so the timing on it needs to be perfect to inject the rocket into the proper trajectory.
By now, the rocket is midway across the Atlantic, traveling nearly 15 times the speed of sound, and is headed toward the darkness of the night-side of the Earth.
14 minutes and 15 seconds into flight, the second stage runs out of fuel and shuts down. Usually, the full burn of the first and second stage on this rocket is capable of lifting a payload into low Earth orbit about 250 kilometers above the Earth. However, such isn’t the case here. Instead, the second stage has lobbed the rocket on a trajectory with an apoapsis of 350 kilometers above the Earth. The third stage will do the rest of the work from this point.
There are two ways of “transferring” to another world. One involves first getting into orbit around Earth. After a few laps around Earth, when the moment is just right, the rocket ignites its engine and sends the payload on its way. This “parking orbit,” which is what the astronauts of the Apollo program experienced during their missions to the Moon, is often used to perform final checks on equipment before it is too late to turn back. The other way is a “direct ascent,” in which the rocket continues to burn its engines from launch all the way to the point where it is injected on a trajectory to the other world. By doing so, it won’t complete a single lap around the Earth before heading out to the depths of space.
For this Moon mission, I’ve opted to perform a direct ascent instead of placing myself in parking orbit. The decision to do so comes down to budgeting for fuel. Rocket engines come in two flavors: either they are very high thrust and low efficiency, or low thrust and high efficiency (ISP vs Delta-V). Some engines can shoot out huge amounts of thrust in a short span of time, but can’t travel very far on the fuel allocated to do so. Other engines take a long time to burn, but will go far further with the same amount to fuel. The third stage of my rocket uses a high efficiency engine, as I need to squeeze as much distance as possible out of the fuel I have in order to reach the Moon. This means, however, that my third stage engine burns very slowly. It burns so slowly, in fact, that if I were in low orbit around the Earth, I’d have a very hard time calculating and performing the burn needed to get to the Moon. The velocity of my orbit would take me around to the other side of the Earth in the time it would take my engine to deplete its fuel… there’s no reaching the Moon that way! Instead, by having the second stage lob me into a high altitude trajectory above the Earth, I can have the third stage fire its engines well before reaching apoapsis. This will give me all the time I need to extend my trajectory outward in the direction of the Moon before my rocket starts moving away from the optimal point of injection and, worse, falling back to Earth.
The map screen shows my planned trajectory (the dotted orange line headed away from the Earth). I’ll need to begin my third stage burn shortly after the second stage shuts off and well before the rocket reaches the high point in its trajectory. As the third stage engine burns for 10 minutes before it depletes its fuel (compare that to the ~3 minutes each the first and second stages took to deplete theirs), there will likely be a number of course corrections necessary to keep my upward velocity going.
Moments after the second stage shuts down, the coupling linkage holding it to the third stage detaches and the stage falls away. It will impact the waters off the West coast of Africa (or it may hit Africa itself… I didn’t exactly take into consideration where the debris would fall. This is a departure from real life, where mission planners are always very cognizant of where their used stages will land, lest they be held liable for any damages incurred.) The fairings covering our spacecraft also split open and fall away. They protected the vulnerable spacecraft well while it was still traveling through the atmosphere and subject to damaging aerodynamic forces, but in space this isn’t a concern. The third stage ignites, and sends the spacecraft on a trajectory toward the Moon.
The third stage, now high above Africa, shuts down 25 minutes and 13 seconds after launch. At this point, the spacecraft is traveling nearly 22,881 miles per hour. As can be seen by the map screen, the spacecraft is now on a trajectory that will, without any other course corrections, take it right past the Moon (the blue line), around it, and back toward Earth (the purple line). Unlike the first and second stages, which will fall back to Earth, the spent third stage will likely remain stuck in this orbit indefinitely. My contribution to space debris!
Basked in darkness high above Africa, the third stage gently decouples from the lander.
And slowly, the Earth recedes into the distance as the Moon gets ever closer.
As mentioned earlier, my trajectory will take me into the Moon’s gravitational sphere of influence, around the Moon, and back toward Earth. This leaves with me a choice when it comes to landing on the Moon. I can decide to place myself into orbit around the Moon prior to landing. This will make it easier for me to choose an optimal landing location, but will also waste precious fuel that could otherwise be used for the landing itself. I could also opt to make a direct landing on the Moon without first going around it, decelerating while on an impact trajectory instead. This is how NASA’s early Surveyor landers were put onto the Moon.
In order to save fuel, I decide to go the direct landing course. 27 minutes and 11 seconds after launch, while still close to the Earth, the lander re-positions itself and ignites its engine for an 8 second burn. As the map screen shows, this will leave me plummeting directly into the Moon.
And now we wait. It takes a number of days for the spacecraft to transit between the Earth and the Moon. As the Moon grows closer, the spacecraft positions itself facing away from it. The landing legs extend. We’re now on our final landing phase of the mission!
Over the course of 25 minutes, as the spacecraft begins its descent, it ignites, shuts off, and reignites its engine to kill velocity. Most of the landing burn will occur closer to the Moon’s surface, but these high-altitude engine bursts not only keep downward velocity steady, they also help perfect and fine-tune the targeting of our landing location.
The engine on the lander, as seen by the info tab in this picture, is highly, highly capable. It runs on hypergolic fuels, which ignite on contact with each other, meaning that the engine is pressure-fed and capable of multiple restarts (up to 15!). Because of this, I can perform multiple short-burst burns without fear of exhausting my engine’s restart capabilities and I don’t need to concern myself too heavily with providing ullage before each restart. This allows me to focus squarely on my landing velocity and trajectory, and vital time isn’t wasted with the ullage process (which would often take a few seconds before each engine restart). Hypergolic fuels are also extraordinarily efficient. Though the lander engine takes a while to burn through its fuel supply (which isn’t an issue when I’m keeping downward velocity steady), it is capable of performing a lot of work. This is necessary for killing the downward velocity of our direct impact trajectory.
We’re now extraordinarily close to the Moon. The Earth disappears over the Lunar horizon, and our shadow begins larger and clearer on the Lunar surface. Out of direct contact with the Earth, my spacecraft is now relying on an orbiter in place around the Moon for relaying commands. Far more than an in-game abstraction, this has tangible effect on our landing: commands to the spacecraft now take up to 5 seconds to register in-game. Every maneuver and every change of thrust needs to be perfectly planned, or else the lander will come crashing to the Moon’s surface. We’ve gone this far, we can’t let that happen now!
In the final seconds before touchdown, the lander ignites its engine one last time at max-thrust. This short, 5-second burn bleeds off the remaining 150 meters/second downward velocity. For a brief moment after the engine shuts off, the spacecraft is hovering in place above the Moon.
Of course, gravity begins to gently drag it down to the Moon’s surface. As it does, the spacecraft utilizes its RCS engines to provide upward lift. These engines, which are typically used to maneuver and rotate a spacecraft in place during its flight, hardly provide any substantial thrust. Indeed, in these final seconds, our downward velocity is actually increasing. Yet they do minimize this increase in downward velocity, so that the lander will touch down on the Moon at a (relatively) slow and safe speed of 4 meters/second.
4 days, 15 hours, and 29 minutes after launch, and our lander has set down on the Moon! It is a resounding success by any standard. Interestingly enough (and to my surprise!), the lander still has 3 engine restarts and nearly a minute of fuel left. I probably was too conservative in my estimates and budgeting; we likely could’ve pulled off an orbit around the Moon before our landing. Oh well, no matter!