Almost all large-scale rockets today are chemical reaction-based, and so their total delta-v is limited primarily by their maximum exhaust velocity. However, if future rocket technologies can deliver a sufficiently high exhaust velocity, the total energy or total mass expended becomes the limiting factor. In this paper, we consider the optimal path of such a rocket through the mass-energy parameter space, and calculate the final delta-v as a function of the endpoints along such a path. We also consider Robert Zubrin’s nuclear salt water rocket design as a case study in theoretical limits on engine performance.
Chemical-based rockets, which, until now, have been used for all Earth-based launches and practically all space travel, are limited in their capabilities primarily by how high an exhaust velocity an engine can achieve, which is in turn limited by the reaction energy of the chemicals powering the engine. As a simple example, consider the Saturn V rocket; the Saturn V’s three stages contained a total of kg of kerosene/LOX propellant and kg of LH2/LOX propellant (Wade 2008). These fuels, if burned at the efficiency of the Saturn V’s engines, contain J of energy. The Apollo spacecraft itself masses only kg, and was accelerated to a velocity of m/s, for a total energy of J, or an efficiency of 22.8%.
The optimal value for can then be solved for analytically; this turns out to be, for any value of , and the total energy required is equal to, or 54.4% more than the kinetic energy of the payload. It is clear, then, that the exhaust velocity produced by chemical rockets is significantly lower than that the optimal velocity for minimal-energy space missions even within our own solar system; the optimal delta-v for the Space Shuttle main engines, the highest exhaust velocity chemical engines currently in use, is only 6,570 m/s, lower than that required for LEO. This simple fact means that virtually all current rockets are optimized to give the highest possible exhaust velocity, rather than optimizing for energy or mass usage.
We now consider the behavior of a hypothetical rocket in which the rate of reaction mass expenditure and the rate of energy expenditure are completely independent; that is, the source of reaction mass and the source of energy are separate, and arbitrarily high or low amounts of energy can be applied to each unit of reaction mass. Consider the two-dimensional parameter space , where is the internal (potential) energy of the rocket, and is the total mass of the rocket.
Before any thrust is applied, any given rocket will be at some point in this parameter space. After all the available energy has been expended, the rocket moves to the point , where it is assumed that and . The path from to can be expressed as the function , where f is continuous and monotonically increasing. The exhaust velocity of the rocket for any value of is, then, simply:
where is the specific energy of the reaction mass. The conventional assumption that is constant is, therefore, equivalent to assuming that is a constant, or that the path along from to is a straight line.
At a given point , define to be the (infinitesimal) amount of energy expended at that point, to be the amount of reaction mass expended, and to be the amount of velocity gained. We know from classical mechanics that:
and so it is easy to show that:
Since Q has no explicit dependence on f(m), this is equivalent to saying that:
Taking the derivative, this reduces to the simple first-order ordinary differential equation:
whose solution is:
from which we can derive the function describing the optimal potential energy at any point:
This equation, then, represents the end state of a rocket simultaneously optimized for both energy and mass; it describes the maximum velocity attainable by any engine given those initial parameters. Note that, if mass is removed as a constraint by taking the limit as , reduces to , which is just the velocity of a body with mass and kinetic energy . Hence, if reaction mass is removed as a constraint, the rocket becomes, not just optimally efficient, but perfectly efficient at converting potential energy into kinetic energy, although this is only a theoretical maximum.
As a case study, we can examine the proposed nuclear salt water rocket of Dr. Robert Zubrin. The nuclear salt water rocket (NSWR) works by using a solution of a soluble uranium compound in ordinary or heavy water to form a nuclear critical mass. This flashes the water into high-temperature steam, which is then expelled out of the rocket at extremely high velocity. Such a rocket could have an extremely high specific impulse, roughly in the range of seconds.
Suppose that such a rocket, with a mass ratio, were to be launched towards the outer solar system; using Tsilokovsky’s equation, we can calculate a theoretical final velocity of roughly m/s. However, if we allow the exhaust velocity to vary- using a lower concentration of uranium at the beginning, and a higher concentration at the end- we get a theoretical final velocity of m/s, a improvement over the original.