A Propulsion System for Interstellar Travel — Overview
Achieving interstellar velocities by decoupling the element of propulsion from the space vehicle
Introduction
This overview will describe a propulsion system for interstellar travel. This is a conceptual overview only. It presents the basic idea and main elements of the system and explains its viability. A more detailed description including suggested design specifications will follow shortly.
Limitations of Current Space Propulsion Systems
Humanity’s ability to travel to other star systems is an achievement that will mark a new era for science, technology and society in general. Currently, space travel is based on technology that is several decades old and is limited to earth orbit and sending small probes to the moon, Mars and other bodies in our solar system. The main hurdle to interstellar travel is the current state of development of the propulsion systems that power our space vehicles. A new mechanism of propulsion is needed if we are to traverse the vast distances of interstellar travel.
With current chemical rocket systems a one-way trip to our closest stellar neighbor would take tens of thousands of years rendering it impractical. Advanced forms of propulsion such as ion thrusters and solar sails have not provided a viable design for interstellar travel despite much effort. Alternative concepts such as the Alcubierre drive and the solar rocket, are based on unknown technology and seem very far from any practical implementation. Of note is the ground based laser system proposed by the Breakthrough Starshot project. It seems to have a sound basis but has yet to show that a material exists from which the proposed light sail can be manufactured and that a laser with the necessary power can be built. It will also be limited to very small payloads of a few grams only.
The task at hand is to find a way to use current available technology and show how it can be used to propel an object from the earth (or its immediate vicinity) to stellar distances within a reasonable time frame.
The Tyranny of the Rocket Equation
Chemical rocket systems are constrained by their need to carry large amounts of fuel. A large amount of thrust is expended accelerating the mass of the onboard rocket fuel. So the rocket is essentially using most of its fuel to push forward the remaining fuel it carries. The total amount of fuel that can be carried on a rocket is limited by structural constraints and/or the ability to refuel the rocket in earth orbit. And rockets can only be scaled up to a certain size. The result of all this, when compiled into an equation, is that the amount of payload mass hardly ever exceeds five percent of that of the entire rocket. To state it simply: most of the rocket is fuel, you can’t make a rocket big enough to carry enough fuel to get to another star and you can’t refuel on the way. These limitations make rockets an impractical mechanism for interstellar travel.
Decoupling the means of propulsion
The way to allow a space vehicle to accelerate to interstellar speeds using current technology (no warp fields or antimatter drives) is to decouple the payload from the propellant. If no propellant is carried by the vehicle and can be supplied to it en route, the equation of motion changes dramatically. The basic concept I am suggesting is to use pre-launched metallic pallets as propellant for the vehicle. Velocity will be achieved by means of imparting electromagnetic force on those pallets during flight.
Interstellar Drive Mechanism
The drive mechanism onboard the vehicle will consist of a power source, a stack of electromagnetic rings, and a large supercapacitor that will enable the drive to generate a very large magnetic field in a very short time interval as the pallet passes through the ring stack. The vehicle will travel towards its destination and engulf the pallets that were launched before it. When a pallet is inside the funnel the electromagnetic field generated by the rings will impart momentum on it thus propelling the vehicle forward in a manner similar to a reverse railgun or particle accelerator. The vehicle will be shaped like a tube that will hold the electromagnetic rings with an enlarged opening in the front to receive incoming pallets and direct them into the funnel. Power will be supplied by a nuclear reactor. The electromagnetic ring assembly will be based on superconducting elements to increase their yield. The low temperature in space will help make this design easier to implement. The ship will be traveling along the path of pallets and gaining velocity with each pallet encounter. The amount of energy imparted on the pallet will be released in shorter intervals as the ship accelerates.
Pallet Launch Assembly
Pallets will be pre-launched using a lunch mechanism similar to a rail gun. The launcher will be based in Earth orbit, on the Moon or on another suitable location that is devoid of any atmosphere which can impede the pallets. The launcher will launch batches of metallic pallets at set intervals on a pre-calculated trajectory. The pallets will form a trek on which the vehicle will later travel and which it will use to accelerate.
Velocity considerations
The main factor influencing the acceleration and maximal velocity of the vehicle is the amount of momentum imparted on each pallet during flight. This is determined by the total field energy of the funnel assembly and thus the force it will activate on the pallet, the mass of the vehicle and the pallet, and the time the pallet spends inside the funnel. While most of these remain constant the last factor is reduced as the vehicle travels at higher velocities. The velocity of the pallets can be varied by the launcher to accommodate the vehicle while it is still moving relatively slowly but as it accelerates it will move far faster than the incoming pallets.
The crucial factors are therefore electric generation capacity onboard the vehicle, the timing and magnitude of the EM bursts the rings can generate and the length of the funnel.
Pallet sequence considerations
The timing intervals at which pallets are shot and the amount of time between them and the launch of the vehicle are of great importance. A large number of pallets will have to be shot in order to have enough pallet-ship encounters to allow acceleration to sufficient speeds. This means that a portion of the way to the destination will need to be “paved” with pallets before launching the vehicle. The pallet firing sequence will start months or even years before the ship is launched. The gaps between one pallet and the next will be determined by the capacity of the pallet launcher but must also be spaced so that the supercapacitors on board have enough time to recharge and give the vehicle time to make subtle course corrections.
Course corrections
A collision with a pallet at high velocities will be catastrophic. Any deviation that will cause the pallet to enter the funnel at an incorrect angle or impact the hull will mean the immediate end of the mission. A system will be put in place to constantly monitor the location and motion vector of incoming pallets and perform course corrections when necessary. The system will use radar to detect the metallic pallets and simple gas thrusters to reorient the ship as needed. Only minute corrections will be needed if the trajectories are correctly calculated and thus the amount of thruster gas the vehicle will need to carry should not be an issue.
Multi-part missions
A single vehicle may prove to be insufficient to accelerate to the necessary speed for interstellar travel. A multi-part mission may be needed. In such a mission a vehicle will, at some point, become a launch platform for pallets mid-way. It will leverage the velocity it has gained in order to launch pallets at a much higher velocity than is possible from the base launcher. The next vehicle will continue the journey using the pallets launched mid-way. There may be more than two vehicles needed. They may be launched together or at timed intervals.
References:
- https://www.nasa.gov/mission_pages/station/expeditions/expedition30/tryanny.html
- Ion thrusters for electric propulsion: Scientific issues developing a niche technology into a game changer: https://aip.scitation.org/doi/10.1063/5.0010134
- Marine nuclear reactor: https://en.wikipedia.org/wiki/A1B_reactor
- Li, Z., Gadipelli, S., Li, H. et al. Tuning the interlayer spacing of graphene laminate films for efficient pore utilization towards compact capacitive energy storage. Nat Energy 5, 160–168 (2020).
- Rashleigh, C. S. & Marshall, R. A. (April 1978). “Electromagnetic Acceleration of Macroparticles to High Velocities”. J. Appl. Phys. 49 (4): 2540. Bibcode:1978JAP….49.2540R. doi:10.1063/1.325107.
- Agnew, J. (16 August 2019). “An Examination of Warp Theory and Technology to Determine the State of the Art and Feasibility”. AIAA Propulsion and Energy Forum and Exposition. doi:10.2514/6.2019–4288. ISBN 978–1–62410–590–6.
- Breakthrough Initiatives: https://breakthroughinitiatives.org/
