Relativistic rocket

A relativistic rocket is any spacecraft that is travelling at a velocity close enough to light speed for relativistic effects to become significant. What "significant" means is a matter of context, but generally speaking a velocity of at least 50% of the speed of light (0.5c) is required. The time dilation factor, mass factor, and length contraction factor (all these factors equal the Lorentz factor) are 1.15 at 0.5c. Above this speed Einsteinian physics are required to describe motion. Below this speed, motion is approximately described by Newtonian physics and the Tsiolkovsky rocket equation can be used.

We define a rocket as carrying all of its reaction mass, energy, and engines with it. Bussard ramjets, RAIRs, light sails, and maser or laser-electric vehicles are not rockets.

Achieving relativistic velocities is difficult, requiring advanced forms of spacecraft propulsion that have not yet been adequately developed. Nuclear pulse propulsion could theoretically achieve 0.1c using current known technologies, but would still require many engineering advances to achieve this. The relativistic gamma factor (γ) at 10% of light velocity is 1.005. The time dilation factor of 1.005 which occurs at 10% of light velocity is too small to be of major significance. A 0.10c velocity interstellar rocket is thus considered to be a non-relativistic rocket because its motion is quite accurately described by Newtonian physics alone.

Relativistic rockets are usually seen discussed in the context of interstellar travel, since most would require a great deal of space to accelerate up to those velocities. They are also found in some thought experiments such as the twin paradox.

Relativistic rocket equation

As with the classical rocket equation, one wants to calculate the velocity change Δv that a rocket can achieve depending on the specific impulse I_sp and the mass ratio, i. e. the ratio of starting mass m₀ and mass at the end of the acceleration phase (dry mass) m₁. Subsequently specific impulse means the momentum produced by the exhaust of a certain amount of rocket fuel divided by the mass of that amount of rocket fuel. Thus specific impulse is a velocity, as opposed to the common usage of the word as the ratio of momentum and weight (weight would not make much sense in this context).

Specific impulse

The specific impulse of relativistic rockets is the same as the effective exhaust velocity, despite the fact that the nonlinear relationship of velocity and momentum as well as the conversion of matter to energy have to be taken into account; the two effects cancel each other. I.e.

I_sp = v_e

Of course this is only valid if the rocket does not have an external energy source (e. g. a laser beam from a space station; in this case the momentum carried by the laser beam also has to be taken into account). If all the energy to accelerate the fuel comes from an external source (and there is no additional momentum transfer), then the relationship between effective exhaust velocity and specific impulse is as follows:

$I_{sp} = \frac {v_e}{\sqrt{1 - \frac{v_e^2}{c^2}}} = \gamma_e \ v_e,$

where γ is the Lorentz factor.

In the case of no external energy source, the relationship between I_sp and the fraction of the fuel mass η which is converted into energy might also be of interest; assuming no losses, is

$\eta = 1 - \sqrt{1 - \frac{I_{sp}^2}{c^2}} = 1 - \frac{1}{\gamma_{sp}}.$

The inverse relation is

$I_{sp} = c \cdot \sqrt{2 \eta - \eta^2}.$

Here are some examples of fuels, the energy conversion fractions and the corresponding specific impulses (assuming no losses if not specified otherwise):

Fuel	η	Isp / c
electron-positron annihilation	1	1
proton-antiproton annihilation, using only charged pions	0.56	0.60
electron-positron annihilation with simple hemispherical absorption of gamma rays	1	0.25
electron-positron annihilation with hemispherical Compton scattering	1	>0.25
nuclear fusion: H to He	0.00712	0.119
nuclear fission: 235U	0.001	0.04

Delta-v

In order to make the calculations simpler, we assume that the acceleration is constant (in the rocket's reference frame) during the acceleration phase; however, the result is nonetheless valid if the acceleration varies, as long as I_sp is constant.

In the nonrelativistic case, one knows from the (classical) Tsiolkovsky rocket equation that

$\Delta v = I_{sp} \ln \frac {m_0}{m_1}$

Assuming constant acceleration a, the time span t during which the acceleration takes place is

$t = \frac {I_{sp}}{a} \ln \frac {m_0}{m_1}$

In the relativistic case, the equation still valid if a is the acceleration in the rocket's reference frame and t is the rocket's proper time because at velocity 0 the relationship between force and acceleration is the same as in the classical case.

By applying the Lorentz transformation on the acceleration, one can calculate the end velocity Δv relative to the rest frame (i. e. the frame of the rocket before the acceleration phase) as a function of the rocket frame acceleration and the rest frame time t'; the result is

$\Delta v = \frac {a \cdot t'} {\sqrt{1 + \frac{(a \cdot t')^2}{c^2}}}$

The time in the rest frame relates to the proper time by the following equation:

$t' = \frac{c}{a} \sinh \left(\frac{a \cdot t}{c} \right)$

Substituting the proper time from the Tsiolkovsky equation and substituting the resulting rest frame time in the expression for Δv, one gets the desired formula:

$\Delta v = c \cdot \tanh \left(\frac {I_{sp}}{c} \ln \frac{m_0}{m_1} \right)$

The formula for the corresponding rapidity (the area hyperbolic tangent of the velocity divided by the speed of light) is simpler:

$\Delta r = \frac {I_{sp}}{c} \ln \frac{m_0}{m_1}$

Since rapidities, contrary to velocities, are additive, they are useful for computing the total Δv of a multistage rocket.

Matter-antimatter annihilation rockets

It is clear on the basis of the above calculations that a relativistic rocket would likely need to be a rocket that is fueled by antimatter. Other antimatter rockets in addition to the photon rocket that can provide a 0.6c specific impulse (studied for basic hydrogen-antihydrogen annihilation, no ionization, no recycling of the radiation) needed for interstellar space flight include the "beam core" pion rocket. In a pion rocket, antimatter is stored inside electromagnetic bottles in the form of frozen antihydrogen. Antihydrogen, like regular hydrogen, is diamagnetic which allows it to be electromagnetically levitated when refrigerated. Temperature control of the storage volume is used to determine the rate of vaporization of the frozen antihydrogen, up to a few grams per second (amounting to several petawatts of power when annihilated with equal amounts of matter). It is then ionized into antiprotons which can be electromagnetically accelerated into the reaction chamber. The positrons are usually discarded since their annihilation only produces harmful gamma rays with negligible effect on thrust. However, non-relativistic rockets may exclusively rely on these gamma rays for propulsion. This process is necessary because un-neutralized antiprotons repel one another, limiting the number that may be stored with current technology to less than a trillion .

Design notes on a pion rocket

The pion rocket has been studied independently by Robert Frisbee and Ulrich Walter, with similar results. Pions, short for pi-mesons, are produced by proton-antiproton annihilation. The antihydrogen or the antiprotons extracted from it will be mixed with a mass of regular protons pumped inside the magnetic confinement nozzle of a pion rocket engine, usually as part of hydrogen atoms. The resulting charged pions will have a velocity of 0.94c (i.e. β = 0.94), placing the theoretical upper limit on the speed of the rocket, and a Lorentz factor γ of 2.93 which extends their lifespan enough to travel 2.6 meters through the nozzle before decaying into muons. Sixty percent of the pions will have either a negative, or a positive electric charge. Forty percent of the pions will be neutral. The neutral pions will decay immediately into gamma rays. These can't be reflected by any known material at the energies involved, although they can undergo Compton scattering. They can be absorbed efficiently a shield of tungsten placed between the pion rocket engine reaction volume and the crew modules and various electromagnets to protect them from the gamma rays. The consequent heating of the shield will cause it to radiate visible light, which could then be collimated to increase the rocket's specific impulse. The remaining heat will also require the shield to be refrigerated. The charged pions would travel in helical spirals around the axial electromagnetic field lines inside the nozzle and in this way the charged pions could be collimated into an exhaust jet that is moving at 0.94c. In realistic matter/antimatter reactions, this jet only represents a fraction of the reaction's mass-energy : over 60% of it is lost as gamma-rays, collimation is not perfect, and some pions are not reflected backwards by the nozzle. Thus, the effective exhaust velocity for the entire reaction drops to just 0.58c. Alternative propulsion schemes include physical confinement of hydrogen atoms in an antiproton and pion-transparent beryllium reaction chamber with collimation of the reaction products achieved with a single external electromagnet; see Project Valkyrie.

Sources

1. The star flight handbook, Matloff & Mallove, 1989
2. Mirror matter: pioneering antimatter physics, Dr. Robert L Forward, 1986

External links

• Physics FAQs: The Relativistic Rocket
• Javascript that calculates the Relativistic Rocket Equation
• Spacetime Physics: Introduction to Special Relativity (1992). W. H. Freeman, ISBN 0-7167-2327-1