Halfway to Anywhere
Adam Kall, Director of Science
5.5 minute read
Robert Heinlein was a famous science fiction writer in the 20th century, most known for his book Starship Troopers. As an orbital dynamicist, I know him from his 1974 quote, “If you can get your ship into orbit, you’re halfway to anywhere.” What Robert is describing is the absolute absurdity of trying to get off our planet and into space, compared with the relative simplicity of getting to other planets once you’re in space. As a brief review, to get off of the planet a rocket must overcome gravity, push through an atmosphere that weighs 14.7 pounds per square inch at sea level, and gather enough horizontal velocity to achieve orbit and not fall back to Earth, all while carrying the payload and fuel that will be needed once in space. There is a reason why the Saturn V rocket weighed 6.2 million pounds at take-off, but was only able to deliver 311 thousand pounds to Low Earth Orbit (LEO). It means that, for the Saturn V at least, you need 20 pounds of launch material for each pound of payload to orbit. To add to this insanity, the nearly 6 million pounds of mostly fuel and some rocket were expended in the first 8 minutes of flight, as those 8 minutes represent a frantic race to avoid smashing back into terra firma.
However, the situation is entirely different once that sprint is completed and the spacecraft is in orbit. Now the spacecraft can stay in orbit forever and any tiny force applied will change its velocity. This means instead of the massive engines of the first stage, designed to hurl a torrent of fire at the ground to push the rocket away, the in-space portion can use a highly efficient engine that gets more overall change in velocity out of each pound of fuel, with the easy trade-off of it taking more time. This means the dreams of outer space airliners, taking off from JFK airport and flying directly to the planets of the solar system, are impractical due to the almost conflicting needs of the first few minutes of the flight and the remaining days of the mission. So instead, forget having a direct flight. Have a flight to LEO that is designed and optimized to get the payload and passengers to orbit safely and quickly, then transfer them to the interplanetary vehicle that continues the journey in space. Once this travel paradigm is achieved, there are some fascinating scheduling choices that can be made.
For any spacecraft flying to another gravitational body, whether a moon, planet, or asteroid, there are two main ways the craft can “arrive” at its destination. Either the ship arrives on a path that is catching up to the body, which will cause the body’s gravity to pull it along and give it a free speed boost (okay not completely free, it will slow down the body a bit, but the difference between the weight of a spacecraft and the weight of a planet can only be described as astronomically comical).
This maneuver is called a gravity assist and is the reason why the Voyager spacecraft went on “The Grand Tour” before leaving the solar system so that each gas giant could give the spacecraft a big boost on its journey. But if our spacecraft is trying to deliver something to the planet by orbiting it (remember the journey to and from the surface will be handled by a different vehicle) then it should instead aim to arrive ahead of the gravity body. This will mean the relative velocity of the spacecraft to the planetary body has it moving away, but gravity is pulling it back, slowing it down, and ideally capturing it into an orbit. The larger the gravity of the body the easier it is to be captured into its orbit, hence why Jupiter, the most massive planet in our solar system, has 92 moons.
Visualization of gravity assist with the object passing in front of the body to slow down and behind to speed up.
Credit: https://wiki.kerbalspaceprogram.com/wiki/File:Gravity_Assist.svg
So the in-space schedule is revealed. From an orbit around one body, accelerate onto a path that takes the spacecraft in front of the target body so that gravity pulls it into an orbit with little additional fuel required. Then, once the cargo is offloaded and a new shipment of goods and passengers are on board, boost back to the parent body and get captured by its gravity. Rinse and repeat. Except, the oddities of space present one key issue to this. The most optimal maneuver to boost an orbit means the highest point of the orbit is opposite the lowest point. If the route is an Earth to Mars route, this means the optimal arrival point at Mars will be on the opposite side of the sun from where Earth is when it departs, and the journey would take 11 months! Not great for the passengers on board to have to spend almost a year in a tin can flying through the vacuum of space. So what about not being optimal? If a larger change in velocity is applied to the spacecraft, along with more fuel spent at the destination to actually achieve orbit and not just sail past the target body, then more direct flights are absolutely possible, down to a matter of weeks in the most extreme cases. To get this larger change in velocity the spacecraft either needs more fuel, which is a losing battle due to the tyranny of the rocket equation, or less mass, which can be achieved by only bringing the human passengers and their essentials, leaving the bulkier cargo for the more efficient flights.
So now we split our paradigm of two spacecraft, rockets to LEO then space-optimized spacecraft from there, into three spacecraft. Still the same rockets, but now with a slower, more efficient cargo spacecraft and faster passenger express spacecraft. With this established, let's end with imagining the design of these spacecraft. Starting with the cargo spacecraft, they are constructed in the vacuum of space so they don’t need to be aerodynamic. They will still want to be roughly symmetrical so the thrust of the engines doesn’t cause it to spin, but this symmetry is across all axes. What makes the most sense is then a central core with what limited crew accommodations are necessary, in addition to power, engine, and fuel reserves. Then expanding outwards cargo can be latched to the spacecraft to protect the critical crew portions. Classes of a cargo ship would be measured by the length of the core tower, with the surrounding volume available for cargo only limited by how much mass the engine and fuel can deliver to the destination. The express ships will experience much more acceleration over their operational lifetime as they accelerate and decelerate more aggressively. While not strictly “aerodynamic,” the shapes that resist this thrust best do have a pyramid-like architecture to them, with a wider base narrowing to a point at the front. The ships will have large engines but an overall small size as the goal is minimizing mass.
Example of shape for express ship, similar to Cheops Class Starship from Stargate. Credit: Patricia van den Ekart on stargate.fandom.com
So why dream of this? Besides a fun exploration of the physical limitations of interplanetary travel and the paradigm choices that result, this column was also meant to highlight why KMI believes humanity’s future in space needs in-space assembly. We need to be able to construct these efficient space vehicles in orbit, where their design can be optimally suited for their environment. In order to do that, we will need a lot of material in space. For better or worse, there are already over 20 million pounds of material in orbit, just in the form of uncontrolled orbital debris. KMI’s mission is not just about cleaning up this debris to keep space clear for all, but also utilizing this debris to help construct the next important step in humanity’s expansion into space.
Recommended column to read next: Space Agriculture