09ICES-0072 Starship Life Support Copy

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09ICES-0072

Starship Life SupportHarry JonesNASA Ames Research CenterCopyright 2009 SAE International

ABSTRACTThe design and mass cost of a starship and its life support system are investigated. The mission plan for a multigenerational interstellar voyage to colonize a new planet is used to describe the starship design, including the crew habitat, accommodations, and life support. Only current technology is assumed. Highly reliable life support systems can be provided with reasonably small additional mass, suggesting that they can support long duration missions. Bioregenerative life support, growing crop plants that provide food, water, and oxygen, has been thought to need less mass than providing stored food for long duration missions. The large initial mass of hydroponics systems is paid for over time by saving the mass of stored food. However, the yearly logistics mass required to support a bioregenerative system exceeds the mass of food solids it produces, so that supplying stored dehydrated food always requires less mass than bioregenerative food production. A mixed system that grows about half the food and supplies the other half dehydrated has advantages that allow it to breakeven with stored dehydrated food in about 66 years. However, moderate increases in the hydroponics system mass to achieve high reliability, such as adding spares that double the system mass and replacing the initial system every 100 years, increase the mass cost of bioregenerative life support. In this case, the high reliability half food growing, half food supplying system does not breakeven for 389 years. An even higher reliability half and half system, with three times original system mass and replacing the system every 50 years, never breaks even. Growing food for starship life support requires more mass than providing dehydrated food, even for multigeneration voyages of hundreds of years. The benefits of growing some food may justify the added mass cost. Much more efficient recycling food production

is wanted but may not be possible. A single multigenerational interstellar voyage to colonize a new planet would have cost similar to that of the Apollo program. Cost is reduced if a small crew travels slowly and lands with minimal equipment. We can go to the stars!

INTRODUCTIONA preliminary analysis of the life support system for a multigeneration interstellar starship is developed. It includes mission plan analysis needed to define the life support requirements. For humans to go to stars while constrained by current physics, the travel speed must be significantly less than the speed of light. The travel time necessarily extends to many decades or centuries. Multiple generations of humans will make the voyage. Two concepts indicate the value of investigating life support for a multigeneration starship. It is often assumed that growing the large quantity of food needed will be necessary on a multigeneration starship voyage. Bioregenerative life support, using crop plants in a closed hydroponics system to provide food, oxygen, and water, appears cost effective on decades-long missions. (Jones, 2006-01-2082) Recently it has been found that it is possible to develop ultrareliable physicochemical life support systems to recycle spacecraft oxygen and water with a relatively small cost in spacecraft mass. Ultrareliable bioregenerative support equipment can be similarly designed. We can build life support systems to take us to the stars. (Jones, 2008-01- 2160) Consideration of starship life support clarifies the effect of reliability mass provision on the attractiveness of bioregenerative life support. This paper first provides an overview of interstellar human colonizing missions that includes new planets

and the travel time, propulsion systems, and a possible mission scenario. This is followed by a discussion of the appropriate crew size. Then reliability and Equivalent System Mass (ESM) considerations are summarize. The mass cost of physicochemical and bioregenerative life support systems is investigated and their behavior with increasing mission duration described. To assess the context and reasonableness of the life support conclusions, the crew habitat and interstellar spacecraft mass requirements are investigated and an overall description of the starship given. Finally the results and heir implications are discussed.

POSSIBLE NEW PLANETS FOR HUMANSStars that are not part of a multiple star system are expected to have planets, which conserve the angular momentum of the originating gas and dust cloud. In recent years hundreds of extrasolar planets have been observed. Most are large gas giants close to their primaries, but Earth-like planets may soon be found. NEW PLANETS TRAVEL DISTANCE, SPEED, AND time - The nearest star system that might have habitable planets is Alpha Centauri at 4.3 light years distance, although it is a triple star. There are only ten stars within 9 light years, and only three of these are single stars. However, there are 55 stars within 16 light years, including 31 single stars. (Kraus, p. 2-5, 6) The earliest human interstellar colonization voyages will probably traverse from 4.3 to 16 light years distance, with 10 light years a reasonable estimate.

Traveling at or near c, the speed of light, appears impossible according to current physics. If a multigenerational starship achieved 0.10 c, a 10 light year trip would take 100 years, and at 0.01 c, it would take 1,000 years. A voyage at 0.001 c would take 10,000 years and appears practically impossible. Travel speeds of 0.01 to 0.10 c seem needed for interstellar colonization.

where E is the rocket exhaust speed, ln is the natural logarithm, Mt is the total initial mass of the rocket propulsion fuel plus Mv, the vehicle mass after all the fuel is burned. The natural logarithm, ln, is a slowly increasing function of its argument (Mt/Mv). Therefore, achieving a rocket final velocity, V, that is much larger than the rocket exhaust speed, E, requires a propulsion mass (Mt Mv) much larger than the vehicle mass (Mv). See Figure 1.

ROCKET PROPULSION AND TRAVEL SPEEDV/E=ln(Mt/Mv) 4.0 3.5 3.0 2.5 2.0 V/E 1.5 1.0 0.5 0.0 0 5 10 15 20 Mt/Mv 25 30 35 40

Chemical rockets cannot take us to the stars, but nuclear rockets using currently understood technology could. A rocket is thrust forward by expelling mass backward at high speed. The rockets final velocity is determined by the amount of mass expelled and its exhaust speed. The rocket equation for final velocity, V, is V = E ln (Mt/Mv)

Doubling V/E from 1.5 to 3.0 requires a little more than four times the mass, since Mt/Mv increases from 4.5 to 20. The inverse function is Mt/Mv = exp (V/E) See Figure 2.

Figure 1. Rocket final velocity/rocket exhaust speed (V/E) versus total mass/vehicle mass (Mt/Mv).

Mt/Mv = exp (V/E) 40 35 30 25 20 Mt/Mv 15 10 5 0 0.0

0.5

1.0

1.5

2.0 V/E

2.5

3.0

3.5

4.0

Figure 2. Rocket total mass/vehicle mass (Mt/Mv) versus rocket final velocity/rocket exhaust speed (V/E).

It is clear that increasing V/E beyond 2 or 3 incurs an increasingly large mass cost. To achieve the required rocket speed V of 0.01 to 0.1 c, E must be only a factor of 2 or 3 less. CHEMICAL ROCKETS - Current chemical rockets cannot achieve the required final velocity. E is not more than 4,500 m/s. (HSF, p. 768) For Mt/Mv = 20, V = 3 E, or 13,500 m/s. The speed of light, c, is 3 * 108 m/s, so V = 4.5 * 10-5 c. The highest practical velocity for a chemical rocket is less than one ten-thousandth of the speed of light, so that crossing one light year takes more than ten thousand years. At V = 4.5 * 10-5c, the time to Alpha Centauri, 4.3 light years away, is 95,000 years. There is essentially no chance of successfully completing an interstellar mission using chemical rockets. Much higher velocities are needed and they can be provided by nuclear propulsion. NUCLEAR PROPULSION - Nuclear rocket propulsion has been studied but not developed, for example in Project Orion and the nuclear salt-water rocket. Project Orion was conducted around 1960 and developed the concept of nuclear pulse propulsion. Small nuclear bombs are exploded behind the spacecraft, accelerating inert mass placed between the bomb and the spacecraft into a spring mounted pusher plate on the rear of the spacecraft. Project Orions pulsed nuclear power can

provide higher exhaust speed than chemical and most other nuclear propulsion systems and can be constructed entirely with current technology. The plasma debris exhaust speed can range up to 3 * 107 m/s = 0.10 c. Orion nuclear pulse propulsion using atomic fission explosions can achieve a rocket speed of 0.03 to 0.05 c. A thermonuclear fusion Orion starship might reach 0.08 to 0.1 c. (Wikipedia, 7/25/08, http://en.wikipedia.org/wiki/Project_Orion_ %28nuclear_propulsion%29) (Czysz and Bruo, 2006) A more conservative estimate of Orion top speed is 0.01 c. (Dyson, in Hart and Zuckerman, p. 41) The maximum spacecraft velocity depends on the propellant mass, as defined by the rocket equation and Figures 1 and 2. A nuclear salt-water rocket was proposed by Robert Zubrin. The exhaust speed can reach 4.7 * 106 m/s = 0.016 c, and the rocket speed can reach 0.036 c. A nuclear salt-water rocket is somewhat similar to the Orion propulsion system, except that it generates continuous rather than pulsed thrust. It may be much smaller than the smallest Orion designs. Project Orion nuclear pulse propulsion systems may mass hundreds of tons due to the mass of the shock-absorber system and the minimum size of efficient nuclear explosives. (Wikipedia, 7/25/08,

http://en.wikipedia.org/wiki/Nuclear_salt-water_rocket) (Zubrin, 1991.) Nuclear propulsion systems seem capable of exhaust speeds of 0.01 to 0.1 c, and final rocket velocities several tim