In the year 1865 and 1901 Jules Verne and H.G.Wells published their now famous novels “From the Earth to the Moon” and “The First Men in the Moon” respectively. Their ideas were not completely crazy, although the physics needed some work. In Verne’s novel the crew would be propelled to the Moon using a projectile mechanism, the same way a bullet is fired from a gun. That the acceleration would have killed the crew is a minor detail. In Well’s novel, he decided to embark on what today we call a breakthrough propulsion physics device, painting the vehicle with a special kind of paint called Cavorite, which had the unusual property of effecting anti-gravitational lift.
We got to the Moon eventually, but the Saturn V was not quite the way these authors imagined it. At some point the ideas went from imagination to reality, as the examination of the physics and engineering technical issues became more credible. A technological transition occurred when the real lunar ship emerged into our world, and crystallised from a thing of thought, to a thing of matter and energy. Today, we imagine what a future starship may look like. And we are fortunate to live in an age where the idea is crystallising out of the imagination as many credible methods for how we accomplish this seemingly impossible mission begin to materialise.
Essentially, we are constrained by the ideal rocket equation, which expresses the additive velocity increments from successive engine stages, as a function of the vehicle exhaust velocity and mass ratio – the mass ratio being the ratio of initial wet mass (fuel + structure) divided by final mass (payload + structure) which arrives at the target. The objective is to get a high mass ratio, which means that the amount of mass remaining at the end of the mission should be a low percentage of the total start mass, although there is a trade-off because you want your payload fraction to be a minimum size. Alternatively, one can have a high exhaust velocity, but this means choosing fuels which are energetic and release large amounts of energy/gram of substance. Typically, these go alone the lines of chemical, nuclear fission, nuclear fusion, antimatter. All Starship designs which carry propellant rely on the ideal rocket equation in this way.
Some people like to search for “loop-holes” to this equation however, in the form of propellantless solutions. This includes using solar energy or stay at home laser propulsion, so that only the payload itself is accelerated. These are known as solar sail and laser sail type systems. You can combine rocket and non-rocket solutions in the form of the Bussard interstellar ramjet which utilizes the interstellar hydrogen of space, funnelled in through a large magnetic scoop, as it accelerates up to the speed of light. Another hybrid is the Medusa sail, which combines elements of Orion nuclear pulse propulsion with solar sail technology.
When you look at Starship design this way, through the lens of the ideal rocket equation, it’s almost as if nature has given us a toy to play with: “here! Take this rocket equation, study it, bend it, exploit, it, perturb it, break it…find a way”. The stars call us ever closer by their glistening shines of light across the Cosmos, beckoning us ever closer to discover the riches that orbit them on their myriad of worlds. But meanwhile, the ideal rocket equation acts as our constraint. The stars make us want to go, and the rocket equation tethers us to a limitation on our desires.
There is a way around this problem of course, which is to go around the ideal rocket equation altogether and even the need for stay at home propulsion drivers. Welcome to the exotic science of breakthrough propulsion physics where anything appears possible if you can throw enough variables together. This is in the form of space drives, warp drives and exotic constructions like worm holes. But we must be careful as we play with these toys that nature has given us; these “gedanken experiments”. Because, as we move away from understood physics (which is validated by experiment) there is always the danger that our speculations will become wide of the mark. But without any constraint to pin us to reality, there is no way to tell for sure. So whilst we marvel at these new creations from our minds of intellectual thought, we should also be aware that this does not guarantee that it is represented by reality.
One of the tools of theoretical thought is so called “back of the envelope” calculations. This helps us to develop an instinct for the ball-parkness of an estimate to satisfy ourselves that the full calculations (e.g. computer codes) are giving us the right answers. But sometimes, these estimates can become so vague that physicists (and astronomers) often refer to it as “hand waiving”. I am okay with hand waiving, within reason, but I do worry that when our physics speculations become so vague that this is equivalent to disco dancing in the dark. The public do not always know the difference between what is known science and what scientific speculation is. I feel that is our duty as scientists to communicate to them the distinction clearly. We do so by carefully caveating our statements and declaring our assumptions with honesty and transparency and avoiding unnecessary hype. But sometimes, our caveats, assumptions and uncertainties can be so large, that our speculations diverge wildly from how we know nature really works.
In science what grounds us is experiment and observations. That is how we test our ideas and refine our theories. When we move into territory where we cannot test those ideas, we move away from the scientific method and you are in danger of no longer practising science but something else; a form of pseudo-science. This is a cautionary note, a heads up to all those would be Starship designers – to be aware of losing your way.
"In physical science the first essential step in the direction of learning any subject is to find principles of numerical reckoning and practicable methods for measuring some quality connected with it. I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely in your thoughts advanced to the state of Science, whatever the matter may be." [PLA, vol. 1, "Electrical Units of Measurement", 1883-05-03, Lord Kelvin]