Like the ‘golden age’ of sea exploration in the middle ages, the history of space travel is often romanticised as a noble scientific endeavour for the betterment of man. In reality it was a competitive struggle between competing world powers, enabled by their need to deliver large, destructive weapons over long distances. The scientific achievements and discoveries were a later by-product of this race.
This point is worth considering, because the space programs of USA and Soviet Union were made possible only by the development of military rockets (the R7 rocket that launched Sputnik was a modified ICBM, as was the Redstone record that launch Alan Sheppard in Mercury 3). And whilst rockets are complex machines, the greatest influence on their performance is the fuel used. So the development and advances in rocket fuel has been the driver of all space exploration. Ironically, the great facilitator of space travel now stands out as the greatest limitation for expansion of space exploration, with current fuelling mechanisms approaching their upper performance limits. Whilst Pioneer and Voyager have escaped the solar system, and scientific missions can be performed at great distances, the goal of manned interplanetary travel presents many challenges that today’s fuel technology are not able to address. This report will look at the background to space travel, past manned space flight, and the fuel and rocket technologies that have allowed them. Having followed the development of space fuel to the present point, the report will look at future fuelling technologies that will allow the current limitations of manned interplanetary travel to be overcome. A manned mission to Mars will be the scenario used to discuss these future directions.
Why manned interplanetary travel?
Any programs involving manned travel are several orders of magnitude more expensive than unmanned missions. Estimates of a manned Mars mission are believed to be 100 times greater than similar unmanned missions. To justify such additional expenditure, additional benefits must be derived[i]. Some benefits include:
- Subjective human decision making will greatly increase the variety and detail of experiments that can be performed.
- The quantity of information that can be gained from manned missions is far greater than that for unmanned missions – most of the knowledge of the moon we have today is derived from the six US lunar landings[ii]. Validation of experiment data is also more thorough.
- More complex missions will involve more complex equipment that may require human operation or maintenance
- Control and response to mission situations may only be achievable by humans (particularly at long distances where real time communication is not possible)
- The establishment of planetary settlements, for industrial and scientific purposes (and possibly supporting other space exploration initiatives)
As there are no currently active manned interplanetary missions, the discussion on the present state of affairs will be necessarily limited.
Requirements of manned flight
The fuelling considerations for manned flight may differ greatly from those for unmanned flights. Some key requirements of any manned flight are:
- It must return. This means that any interplanetary mission must have enough fuel for the return journey[iii]. This was key in Kennedy’s challenge “of landing a man on the Moon and returning him safely to the Earth[iv]”
- Greater system redundancy and fault tolerance.
- Support systems for occupants, including nutrition, atmosphere and waste management.
- Greater size to allow movement of occupants
- Greater protection system, from cosmic rays, micrometeorites, etc
- Physical limitations on environmental conditions that humans can tolerate, specifically:
- Composition and pressure of the atmosphere
- Gravitational forces (acceleration)
- Temperature
- Radiation
- Additional factors such as noise and vibration (particularly during takeoff and landing phases), toxic substances and by-products of chemical reactions in other mission functions, and general physical and psychological wellbeing[v]
- Possible finite limits on the time occupants can spend in space or a low gravity environment
Getting anything into space
Before man can embark on an interplanetary mission, they must first leave earth, and more specifically, its gravitational force. This was the first hurdle in space exploration, and represents the starting point for discussions of propulsion and fuel. To escape the gravitational attraction of the earth, a spacecraft needs velocity of 11.2 km/sec is need (8.2 km/sec for a low earth orbit). At this speed the upward force on the rocket exceeds the downward pressure of gravity[vi]. Note: this speed is the same regardless of the mass of the object. However, the greater the mass, the greater the energy required to achieve this speed. Newton’s 2nd law, stating force = mass x acceleration (f = ma), means that an increase in mass will require an increase in force to achieve the same acceleration (to reach a speed of 11.2 km/sec). Rockets are necessary for two reasons. Firstly, they are capable of generating the forces required to reach such high speeds. More importantly, because of the exceedingly low density of space, there is no matter for a spacecraft’s propulsion mechanism to interact with (such as the interaction between a car tyre and a road, or a plane’s propeller and the atmosphere). So the propulsion mechanism for rocket relies entirely on internally generated thrust. The simplest rocket that can be made is probably a party balloon. Blow it up, then let go of the end, and the balloon will make a short flight. The flight is driven by the compressed air escaping the balloon. According to Newton’s 3rd law of motion, for every action there is an equal and opposite action. The air leaving the balloon exerts a balancing force against the balloon, thus achieving motion. Whilst far more complex, this is the same mechanism by which rockets operate. Now, a party balloon is not going to get you into orbit. However, if we wanted to make our balloon go faster, there are two possible approaches. Firstly, we could put more air in the balloon, thereby increasing the volume of air escaping (mass), providing more force. Secondly, we could increase the speed at which the air escapes providing greater velocity. This can be achieved by either placing pressure on the balloon or reducing the size of the opening. Via f = ma, both of these will increase the force of the balloon. So to produce a rocket, we simply need to create compressed gas, and provide an opening for it to escape. The fuel of a rocket is what produces the compressed gas. Rockets work on the basis of chemical reactions between substances producing compressed gas. Different chemicals react differently, and the aim of researchers in the field of rocket fuel is to find chemical combinations that produce the greatest reaction. However, the chemicals and their reactions needed to have many other properties before they could be considered suitable for use in rockets – such as the fuel must not react with the structural material, and the combustion being controllable. When determining a rocket’s suitability to a task (getting a specific weighted payload a specific distance into space), two key measures are required. The first is thrust, which is a term for force in a particular direction, and is measured in Newtons (N) or pounds. Thrust measures the upwards pressure that a rocket can apply to itself and the payload. The second factor is known as specific impulse, and is related to the thrust produced by the fuel. Although measured in force per second per unit of mass, specific impulse (Isp) is usually referred to in seconds. The specific impulse is the period in seconds for which 1 pound mass of propellant will produce a thrust of 1 pound force. Whilst thrust is a measure of force, we must apply this force to the load it must propel. For the most powerful rocket in the world, there will still be some mass that the rocket is unable to lift (the downward gravitational pressure exceeds the upward thrust pressure). Therefore, the power to weight ratio (the amount of thrust for each unit of weight) is an important concept. To achieve orbit, it should be at least 1.3-1.5 (30% - 50% more upwards pressure than gravitational pressure[vii]).
Fuelling rockets
To achieve the compressed gas required for rocket flight, two elements are required:
- Fuel – the generic term for any combustible matter.
- Oxidiser - for fuel to burn, oxygen or an equivalent is required. This substance is referred to as an oxidiser. One of the keys reasons an oxidiser is required is that there is no oxygen in space, and fuels would not be able to combust without its presence.
These two components are collectively known as propellant. The development of rocket fuels took two paths, each having advantages and disadvantages. Both have been used in space programs, often in tandem.
Solid Propellants
The first rocket to be developed were solid fuel rockets, of which early Chinese fireworks were an example, and are the simplest rocket designs, involving far fewer parts than liquid rockets (one of their key benefits). This helps to reduce their cost (another major benefit). A mixture of solid compounds (fuel and oxidizer) are housed in a metal structure. The mixture burns at a rapid rate, expelling hot gases through a nozzle to produce thrust. The solid fuel mass is referred to as the ‘grain’. Figure 1 - Solid Fuel Rocket Design[viii] Because they are solid, the fuel can only burn where the surface area is ignited. The shaping of this surface area allows some control over the rate and pattern of burning. Most commonly, the solid fuel has a hollow centre (called the ‘perforation’), which is ignited, and burns through to the outer edges. However, because the grain is a single structure, once it is ignited, it cannot be shut down until all the propellant is consumed. This represents a significant disadvantage. Also, because the fuels is already in place, prior to launch, there are dangers of accidental ignition. Two general types of solid propellants are in use:
- double-base propellants – these do not contain a separate fuel and oxidiser, but rather are a mixture of nitrocellulose and nitroglycerene. They rely on the unstable nature of the molecules, which break up upon ignition, giving off large amounts of heat. This fuelling method is primarily used in smaller rockets
- composite propellants – distinct fuel and oxidization chemicals are combined during production of the solid grain. Common oxidizers include ammonium nitrate, potassium chlorate, and ammonium chlorate, constituting up to 80% of the propellant. Hydrocarbons are used as fuels, and include asphaltic-type compounds, or plastics (see appendix A for detailed list of fuels). As well as providing the combustible fuel, the fuel must supply form and rigidity to the grain, as the oxidiser (which makes up the majority of the propellant) is not suited to this function.
Liquid propellants
Unlike solid rockets, liquid rockets utilise a fuel and propellant that are in liquid format.. These are kept separate prior to combustion, and their combination and ignition are managed by mechanics within the rocket. The vessel in which they are combined is called the ‘combustion chamber’. Typical fuels include kerosene, alcohol, hydrazine and its derivatives, and liquid hydrogen, although others have been tested and used. Oxidizers include nitric acid, nitrogen tetroxide, liquid oxygen, and liquid fluorine. Liquefied gases, such as oxygen and fluorine, perform exceptionally well as oxidisers. However, because they normally exist in gaseous format, they must be stored at very low temperatures to achieve fluid status. This cooling is one of the key challenges to incorporating them into rocket design. By comparison, most fuels are liquids at ordinary temperatures, with the key exception of hydrogen.. When combined, certain fuel and oxidiser combinations ignite automatically – a property referred to as ‘hypergolic’. Others require the burning process to by triggered by an external igniter. Because of the high throughput of fuel required to achieve necessary thrust levels, the fuel must be forced into the combustion chamber. Two mechanisms are available to perform this. Mechanical pumps may be employed, or a pressurised gas may be introduced to the fuel and oxidiser containers to force them into the combustion chamber. Whilst the pumping mechanism adds the weight and complexity to the rocket, a pressure driven system requires the a large volume of compressed gas to be carried, the weight of which may exceed that of pumping equipment (the compressed gas levels are proportional the fuel levels, where as pumping equipment scales more readily). The choice is therefore driven by rocket size, and the importance of reliability. As a rule, pumping is favoured for space rockets. Figure 2. Liquid Fuel Rocket Designs[ix] The chief advantage of liquid rockets are the higher specific impulses they can achieve over solid rockets. Given the criticality of fuel weight to rocket performance, this can make major differences to the capabilities of a rocket. Also, because the fuel and oxidiser are mixed, this process can be controlled, allowing the rocket engines to be switched off, or the thrust levels to be altered (particularly important for in-space maneuvering and control). With this additional performance and control comes the price of complexity. This is particularly true in systems involving liquefied gases. Cooling(cryogenic), pumping and control mechanisms not only add to the cost and complexity of these systems, but also to the weight. Like double-base solid fuels, certain liquid chemicals can decompose without an oxidant, producing thrust. Called ‘monopropellants’, they may break down when passed through a catalyst. Hydrogen peroxide passed through a platinum mesh, decomposes into hot steam and oxygen, which are ejected to develop thrust. Whilst simpler than bi-propellants, they do not generate high specific thrust levels. Appendix A details a number of liquid fuels, and the specific impulses that have been achieved.
Hybrid Propellants
It is not necessary for both oxidiser and fuel to be solid or liquid. In some rocket designs the fuel may be solid, while the oxidizer is a liquid (the reverse rarely applies). The liquid is injected into the fuel reservoirs of the solid, which serves as a combustion chamber. These engines can achieve high performance, but with control over the combustion. However, this design is not effective for achieving high thrust levels, limiting their use.
Other rocket components
Whilst fuel is the key element to achieving space flight, it is not the only component of a rocket. The other components are:
Payload
The ultimate aim of rocket is not to reach a space orbit, but to put something into that orbit. The payload is the cargo that a rocket carries, such as a satellite, a Space Shuttle or a manned interplanetary vehicle. Any discussion on fuel should not lose sight that it is only a means, not an end, and that the successful delivery of the payload is the top priority. This has many implications for fuelling manned interplanetary travel.
Rocket engine
The mechanisms for enabling the combustion and emission of the fuel. Includes the pumping mechanisms, fuel cooling systems and combustion chamber (for liquid rockets), as well as the nozzle and any thrust directional guidance systems
Air frame
The propellant tanks, as well as the structures which carry the structural loads and deal with stresses imposed during launch and ground handling. Because the material in the airframe does not contribute directly to the production of thrust, nor is it a component of the final payload, the airframe is considered ‘dead weight’, imposing limits on maximum velocity, before even considering the size of the payload.
Staged rockets
Because achievable speed is related to thrust and weight, a greater height may be achieved by reducing the weight of the payload. However, because the weight of the empty rocket (dead weight) is a large proportion of the unfuelled weight, such a reduction will not achieve a much greater height. However, if a big rocket is used to launch a smaller rocket, and then detaches when the fuel is exhausted, then the smaller rocket will already be moving quickly, and will have a much lower weight, and be further from earth’s gravitational pull. This will allow a much greater height to be reached.[x] This technique is known as staging, and is used by most major space missions.
Rockets in action
In the previous section, we have covered the underlying technology needed to achieve space flight. This next section will look at how that technology has been applied to achieve manned space flight. The first payload placed into space was the Soviet Sputnik satellite in 1957. The satellite was launched from an R-7 rocket, using a liquid fuel - kerosene – with a liquid oxygen (LOX) oxidiser[xi]. The R-7 was originally designed as a ballistic missile, and had its upper stage modified slightly to hold the Sputnik payload. It had two stages with four strap-on booster rockets for the first stage. The primary stage used RD-107 engines, which provided approximately 100,000 kg of thrust. Whilst this was a monumental achievement (both technically and politically), the first Sputnik payload only weighed 83.6 kg[xii]. To achieve the goal of manned space flight, much greater payloads would need to be launched. The first manned space flight was by Soviet cosmonaut Yuri Gagarin (April 12, 1961). The 4,725 kg[xiii] payload was also launched from an R7 rockets, using a three-stage A-1 booster, also with a Kerosene/LOX propellant. The two early American manned space programs were the Mercury and Gemini programs. The manned Mercury program utilised the Atlas rocket, which was a two stage rockets, with the following fuel and thrust characteristics:
| Stage | Stage name | Engine(s) | Propellant(s) | Thrust |
|
0 |
Atlas MA-2 |
2 Rocketdyne XLR-89-5 (boosters) |
LOX, RP-1 |
367,000 lbs. |
|
1 |
Atlas D |
1 Rocketdyne XLR-105-5 (sustainer) + 2 LR-101-NA7 verniers |
LOX, RP-1 |
59,000 lbs. |
Stage |
Stage name | Engine(s) | Propellant(s) | Thrust |
|
1 |
Titan 2-1 |
2 Aerojet LR-87-7 |
Nitrogen tetroxide / Aerozine 50 |
430,000 lbs. |
|
2 |
Titan 2-2 |
1 Aerojet LR-91-7 |
Nitrogen tetroxide / Aerozine 50 |
100,000 lbs. |
Manned Interplanetary Travel
The first and only program of manned interplanetary flights was the US Apollo program of the 60’s and 70’s. In all six lunar landings were performed. As this is the current high point of interplanetary travel, and represented a significant step beyond low earth orbit manned missions, its fuelling requirements will be discussed in more depth. The Apollo mission also represents a benchmark for which the requirements of more ambitious manned interplanetary travel can be compared. The payload for the Apollo lunar landing missions (Apollo 11,12,14,15,16 & 17) consisted of the Command Module (CM), Service Module (SM) and Lunar Module (LM). These three modules had a combined weight of 50+ tons[xvi] - some ten times the weight of the craft that Gagarin was first launched into space in. The fuelling challenges of the Apollo mission were several. Firstly, the craft had to leave earth. It then needed to have sufficient fuel reserves to reach the moon, some 380,000 kms away, and insert itself into a lunar orbit. Then the landing module needed to detach and descend, using fuel to slow and control this descent (there is no atmosphere on the moon the allow friction braking). The lunar module then needed to relaunch itself from the moon, and reattach to the command and service module. Finally, the craft needed enough fuel to make the return journey! These challenges led to the largest and most powerful U.S. expendable launch vehicle ever built – the Saturn V rocket. The Saturn V was a three-stage rocket. The first stage burned five F-1 engines for 2.5 minutes from lift-off, producing nearly 7.7 million pounds of thrust. The second stage contained five J-2 engines which burnt for approximately 6 minutes after the first stage was discarded. This stage took the vehicle and payload to 184 kms above the earth. This second stage was also discarded. The third stage contained one J-2 engine. This engine burnt for 2.75 minutes boosting the spacecraft to orbital velocity of about 17,500 mph. The third stage was shut down with fuel remaining until the maneuvering into a trans-lunar trajectory was initiated, when it was is reignited to accelerate the spacecraft to 29,400 kph, before also being discarded.[xvii] The lunar module contained 40 tons of descent propellent, a hydrazine fuel and nitrogen tetroxide oxidiser. A different fuel source was used for the ascent[xviii], although the same fuel and oxidiser combination were used, and at lift-off, about 11 tons of fuel (about half the craft’s weight).Properties of missions
The most significant distinction of the Apollo program was the fuelling requirements beyond reaching orbit. For other low-earth orbit mission (essentially all missions outside the Apollo program), the fuelling requirements focused almost solely on achieving orbit, with modest requirements for initiating descent and some orbital maneuvering. Also, the large payload (specifically landing modules and return fuel) was significantly heavier than other payloads launched. The Apollo missions only lasted a 8-12 days each, shorter than the manned duration of many other previous missions, and whilst the equipment requirements were greater, these were not significantly higher than other missions. The distance traveled for the missions was roughly 800,000 kms.The current situation
Budget cuts and the achievement of initial mission objects (essentially to beat the Soviets to the moon) meant that the final Apollo landing (Apollo 17) was in 1972. Since that time, man has not even looked like undertaking another manned interplanetary mission. Instead the focus was been on near earth orbit scientific work in manned space stations, as well as military and commercial satellite operations. The major contributions in this time to the cause of manned interplanetary travel have been experiments into physiological and psychological effects on humans in space, and the refinement of fuel technologies. Currently, the largest payloads being launched are the various US Space Shuttles, which fully laden represent a payload of about 100 tons. The space shuttle uses two solid fuel boosters, configured as follows: “The propellant mixture in each SRB motor consists of an ammonium perchlorate (oxidizer, 69.6 percent by weight), aluminum (fuel, 16 percent), iron oxide (a catalyst, 0.4 percent), a polymer (a binder that holds the mixture together, 12.04 percent), and an epoxy curing agent (1.96 percent). The propellant is an 11-point star- shaped perforation in the forward motor segment and a double- truncated- cone perforation in each of the aft segments and aft closure. This configuration provides high thrust at ignition and then reduces the thrust by approximately a third 50 seconds after lift-off to prevent overstressing the vehicle during maximum dynamic pressure.”[xix] The main rocket engine uses liquid hydrogen fuel and a liquid oxygen oxidiser, with a specific impulse of 450 seconds. By comparison, the Apollo Saturn V rocket generated 250 seconds. The space shuttle accommodates a crew of 8 people, with missions normally 5-16 days. Whilst not suited to interplanetary travel (primarily because it was not designed to be), the Space Shuttle represents the most advanced spacecraft developed. Combining this technology with the achievements of the Apollo program, we can now look at the future on manned interplanetary flight.The future
Whilst there are a number of viable scientific and commercial reasons to revisit the moon, the future of direction of manned interplanetary travel is travel to another planet, or one the large satellites of the Jovian planets. The first candidate in any such discussion is Mars, which offers the following attractions:- large solid surface
- gravity within human tolerances (40% of earths)
- presence of water
- tolerable temperature range (-140° – 20° C) - this factor rules out Venus and Mercury
- presence of atmosphere
- lower solar radiation due to distance from the sun
- relative proximity (compared to Jovian satellites, the other possible candidate)
The three big problems
What are the top three obstacles standing in the way of a manned Mars mission? Distance, distance and distance. Or more specifically, distance, time and speed. The Apollo project has shown that a crew and support equipment can be launched into space, and landed successfully on another body. Given advances in light-weight materials and computing, a Mars spacecraft should not need to be much heavier than the Apollo spacecraft. However, whilst the moon is 385,000 kms away, Mars is 80,000,000 kms away – some 200 times further. Compounding this is the fact that a journey to Mars will not be in straight line, but will need to be of a trajectory that incorporates the solar orbits of both Earth and Mars. Because the two planets orbit the sun at different speeds, they do not orbit in a synchronous manner. Additionally, as highlighted, any manned mission must not only leave the earth, but must also return. Further, any manned interplanetary mission with a landing component must land on the surface of the destination planet. The key challenge here is to remove the huge amounts of potential energy that a space craft possesses (as a combination of its high speed and large mass – energy being proportional to the square of velocity[xx]). When a space craft enters an atmosphere (or indeed strikes any matter), friction from the atmosphere converts this energy to heat. Reentry techniques[xxi] and materials used (which impacts rocket weight) can offset this heat production. However, present techniques are designed for objects with a velocity needed to maintain an earth orbit An object traveling through interplanetary space will ideally be traveling much faster. This means that its potential energy is much greater (squared). Before landing on a planet, the craft must slow down, which requires more fuel! Therefore, if a space craft targets a certain speed, it must ensure that it has sufficient fuel to decelerate from that speed, to a speed suitable for reentry. [xxii] When a spacecraft leaves earth, it is still orbiting the sun at the same speed as the earth, in the same orbit. A journey to Mars will involve moving the spacecraft into a different orbit, one that has the same semi-major axis as Mars. This alteration has to be timed so that when this larger orbit is reached, mars will be in that location. Given the speed of a vessel, there is a limited time period that the two planets are in the correct orbital positions. – this time period occurs every 25 months, although especially favourable windows occur every 15-17 years (based on the eccentricities of the orbits of the two planets). This ideal trajectory is known as a Hohmann Transfer Orbit[xxiii]. Using current fuel technology, and taking a Hohmann trajectory, such a trip would take roughly 10 months, as well as a 10 month return journey. However, because planetary alignments are a key requirement, Mars departure would need to wait until alignments are appropriate – calculated at some 300-550 days, giving a best case mission length of 900 days – nearly 3 years! A crew in an Apollo sized module for 600 days, and living in a lunar module sized craft for 300-550 days suddenly seems very unrealistic. Besides the psychological effects, the food, on-board energy and basic human requirements for a trip of such duration would make a craft such as Apollo totally inadequate. To make such a mission realistic, two possible approaches could be taken. Firstly the craft size could be increased, to provide more crew space and room for physical requirements. Something at least the size of the space shuttle would be envisaged. Secondly, the ship could travel faster, reducing the mission length. A third possibility is using a technique know as gravitational swing. This process sees a craft fly close to a planet, and use the gravitational influence of that planet to achieve a sling-shot effect, increasing speed, and reducing the mission’s fuel requirements[xxiv]. However, the only potential source of gravitational is Venus, which introduces the dangers of increased solar radiation to the crew (or requiring greater shielding). At best, such a technique would save 12 months, still giving a two year mission length. If we are to increase the ship size, or attempt a much faster journey, then much greater fuel will be needed. And this fuel will need to be transported into space. As the weight of fuel increases, a lesser proportion will be available for achieving escape volume (more will be required for the journey and return). The 450 seconds of specific impulse achieved by the H2/LOX engines of the Space Shuttle represent an approach to the upper limit chemical fuel efficiency. A new method of fuelling this travel needs to be investigated. The research into alternate fuelling technologies has not emerged recently. Most alternatives to chemical fuels were originally proposed in the 50’s and 60’s, and some exciting proposals and research has been presented. However, their applicability to manned travel varies, as we shall see.Jettisoning chemical fuel?
Firstly, it must be identified that it is unlikely that chemical rockets will be abandoned in the near future, as they are the only technology that delivers high enough thrust (and thrust to weight ratio) to achieve orbit. Any alternate solutions will need to be used in conjunction with chemical rockets.Ion drives
This technology is discussed first because it is currently being utilised in the NASA Deep Space 1 (DS1) probe[xxv]. Like chemical rockets, it utilises accelerated gases escaping through a nozzle to achieve propulsion. However, the gas acceleration is achieved not by chemical combustion, but by stripping electrons from a gas (Xenon is currently preferred), and accelerating that gas via electrical forces allowing very high velocities to be achieved (30km/sec)[xxvi]. The ejected high speed ions are recombined with electrons through a targeted beam to prevent the craft building up an electrical charge (which would attract the emitted ions back to the craft, negating any thrust gained). A schematic of such an engines is shown below[xxvii] The chief benefit of this system is that it has a very high specific impulse (1900-3200 seconds[xxviii]). However, the thrust levels generated are quite low. On smaller systems, this is compensated for by running the engine for extended periods (achieving high speed by long, slow acceleration). To achieve the magnetic force necessary for acceleration of ions requires a sufficiently powerful electricity source. DS-1 utilises solar panels, which are acceptable for its low mass (486 kg, of which 81 kg is xenon gas fuel). However, generating sufficient thrust to propel a large manned craft would require much large quantities of gas, and more importantly, a much stronger power source. Potential methods of supply these power levels are discussed in the section under nuclear fuels.Solar sails
Utilising the force of sunlight, solar sails function by reflecting light off a mirror. Although the amount of pressure generated is very low (and reduces with distance from the sun), the amount of available space for the reflective sail is very large, and capture areas of several square kilometres are possible. Also, as there is no fuel required, vehicle weight would be greatly reduced. Once again, however, the thrust generated is far too low to propel a craft large enough to support a Mars mission, and the speeds achieved would be much smaller than those obtainable by chemical rockets (increasing the distance/time/speed issues for a Mars mission). Proposals to utilise ground or space based lasers as the source of light (much more concentrated than sunlight) have been raised, but whilst they offer improvements for long-term, very long distance missions, they would not assist a Martian mission, as the distance is not far enough to achieve the cumulative acceleration these systems offer.Fission/Fusion
The amount of energy held within atoms is enormous, as evidenced by nuclear weapon detonations. Harnessing this energy could offer enormous increases in power efficiency. However, whilst offering greater energy levels than chemical rockets, nuclear reactions don’t act directly as a propellant. Rather they are a source of energy for heating gas for propulsion. Space rockets require not just energy, but also mass to be ejected. Therefore, the nuclear reactor we are familiar with on earth cannot easily substitute into a rocket design. Figure 4 Nuclear Rocket Propulsion Reactor[xxix] One of the main limiting factors in the operation of rockets is not a shortage of energy but the high temperature at which they operate. Rocket engines (particularly the rocket nozzle) already operate at high temperature – additional energy would only raise the temperature further, potentially compromising the spacecraft. However, if the ejection mass is composed of lighter molecules than are created via normal combustion reactions (water vapour in that case of a H2/LOX), higher temperatures can be achieved and faster molecular speeds utilised for greater propulsion. Hydrogen is the most appealing candidate in this area, having one third the weight of water. This technique was utilised by NERVA rocket, in the 1960’s[xxx]. However, the reactor needed to be operated at a temperature that was less than the melting point of the structural materials (about 3000K), limiting its specific impulse to about 900 seconds. Alternately, fissionable material in a reactor could be used to provide power to an electrical propulsion mechanism, such as the ion drive mentioned described. The large amounts of electrical energy capable of being generated could power a much larger ion drive, achieving greater levels of thrust.Nuclear Propulsion
A much more ambitious use of nuclear fuel has been proposed. Seeking to overcome the physical limitations of needing to house a reactor in a rocket, and the limitations on energy that could realised, a proposal to harness the power of atomic weapons was proposed. The ship would have at its base a massive plate (metal or plastic), through which nuclear weapons would be intermittently ejected and detonated (some distance from the craft). The weapon would be wrapped in plastic casing, rich in hydrogen, which would be converted to a hot gas upon detonation. This gas would be blown into the surrounding space, striking the plate, and providing thrust to the rocket. Initial designs limited the size of the explosion, and triggered them in an internal combustion chamber, using injected water as a propellant, and were predicted to achieve a specific impulse of about 1150 seconds. By moving the explosions outside the craft, and incorporating the propellant and the fuel into the one devise, it possible simultaneously produce high thrust with high exhaust velocity. This lead to a theoretical Isp as high as 10,000 to one million seconds. However, such force would have created acceleration greater than that a manned vehicle could operate in. By placing a shock absorber between the plate and the vehicle, however, the impulse energy delivered to the plate was stored in the shock absorbers and released gradually to the vehicle.[xxxi] Whilst in the post-Cold War era, such a device sounds ludicrous, estimates placed potential payloads at 10,000 tons. To place this into perspective, the weight of the International Space Station is predicted to be 453 tons, and will require 44 launches to put into orbit.[xxxii] A nuclear rocket could put 20 such space stations into orbit in one mission! From an economical viewpoint, it is estimated to cost $500/kg to place an object into orbit, against $12,000/kg for present rocket technologies[xxxiii] In the context of interplanetary travel, there are two key benefits. Firstly, the limits on the size of the craft are reduced as a mission limitation (a 10,000 ton space ship should be more than ample for any realistic mission). Secondly, with virtually unlimited thrust very high speeds can be achieved, reducing flight times to weeks, and reducing reliability on favourable orbits. The obvious downside is the radiation emissions given off by the nuclear explosions. Whilst designs indicated that this was not a significant risk to crew, it would present a general environmental hazard. The International Test Ban treaty of 1963 effectively outlawed this design, and the program was closed down. Public opinion about such a proposal would likely be a much greater hurdle to its recommencement than any technical challenges Whilst fusion, rather than fission, is presented as a cleaner energy source, the problem of achieving controlled fusion reactions has not been solved, and remains an ideal scenario.Conclusion
This review of current and emerging fuel technologies, in the context of manned interplanetary travel, indicates that significant technical and social breakthrough would be required before such a mission could be realistically under taken (even proponents of a manned Mars mission identify a raft of new technologies that would need to be invented[xxxiv]). However, a number of developments do offer future potential to address this area of space exploration. In the short term, any solution is likely to be a hybrid solution, with chemical rockets still appearing the only viable solution to achieving initial orbit. Once in orbit nuclear powered craft could then be employed to travel the long distance to Mars and back. However, a craft of sufficient size to accommodate anything more than a ‘flag placing’ exercise would still be likely to weigh more than existing chemical rockets can launch, calling for a space construction approach, similar to the International Space Station, with costs also at similar levels. However, with advances in nuclear technology, and efforts invested in nuclear powered spacecraft design, Earth based Martian missions could become a reality. Once this hurdle is crossed, the way is open for another ‘golden age’ of exploration.Appendix A
Specific impulse of some typical chemical propellants [xxxv]| Propellant combinations: | Isp Range(sec) |
| Monopropellants (liquid): | |
| Low-energy monopropellants | 160 to 190 |
| Hydrazine | |
| Ethylene oxide | |
| Hydrogen peroxide | |
| High-energy monopropellants: | |
| Nitromethane | 190 to 230 |
| Bipropellants (liquid): | |
| Low-energy bipropellants | 200 to 230 |
| Perchloryl fluoride | |
| Analine-Acid | |
| JP-4-Acid | |
| Hydrogen peroxide-JP-4 | |
| Medium-energy bipropellants | 230 to 260 |
| Hydrazine-Acid | |
| Ammonia-Nitrogen tetroxide | |
| High-energy bipropellants | 250 to 270 |
| Liquid oxygen-JP-4 | |
| Liquid oxygen-Alcohol | |
| Hydrazine-Chlorine trifluoride | |
| Very high-energy bipropellants - 270 to 330 | |
| Liquid oxygen and fluorine-JP-4 | |
| Liquid oxygen and ozone-JP-4 | |
| Liquid oxygen-Hydrazine | |
| Super high-energy bipropellants | 300 to 385 |
| Fluorine-Hydrogen | |
| Fluorine-Ammonia | |
| Ozone-Hydrogen | |
| Fluorine-Diborane | |
| Oxidizer-binder combinations (solid): | |
| Potassium perchlorate: | |
| Thiokol or asphalt | 170 to 210 |
| Ammonium perchlorate: | 170 to 210 |
| Thiokol | 170 to 210 |
| Rubber Polyurethane | 210 to 250 |
| Nitropolymer | 210 to 250 |
| Ammonium nitrate: | |
| Polyester | 170 to 210 |
| Rubber | 170 to 210 |
| Nitropolymer | 210 to 250 |
| Double base: | 170 to 250. |
| Boron metal components and oxidant | 200 to 250 |
| Lithium metal components and oxidant | 200 to 250 |
| Aluminum metal components and oxidant | 200 to 250 |
| Magnesium metal components and oxidant | 200 to250 |
| Perfluoro-type propellants | 250 and above |
[i] Whilst some reasons for manned interplanetary travel are given, this report will not debate the philosophical arguments for and against such programs, near get into cost justification exercises. [ii] http://www.nasm.edu/apollo/apollotop10.htm [iii]Space colonisation does not have this requirement, but is beyond the scope of this report [iv] President John F. Kennedy's Special Message to the Congress on Urgent National Needs May 25, 1961: http://www.luminet.net/~tgort/moon.htm [v] McCormack, J, “Astronautics And Its Applications: Submission to the Select Committee on Astronautics and Space Exploration”, p.106 December 29, 1958 http://www.hq.nasa.gov/office/pao/History/conghand/mannedev.htm [vi] This notion of escape velocity assumes a crising speed (ie. no additional propulsion is being provided). Technically, you could enter space travelling at 10 km/hr, providing this speed is maintained for the duration of the launch. [vii] http://www.hq.nasa.gov/office/pao/History/conghand/vehicles.htm [viii] http://www.hq.nasa.gov/office/pao/History/conghand/propulsn.htm [ix] http://www.hq.nasa.gov/office/pao/History/conghand/propulsn.htm [x] http://www-istp.gsfc.nasa.gov/stargaze/Srockhis.htm [xi] http://www.russianspaceweb.com/r7.html [xii] http://nssdc.gsfc.nasa.gov/nmc/tmp/1957-001B.html [xiii] http://www.nauts.com/vehicles/60s/vostok.html [xiv] http://www.robsv.com/cape/c14lv.html [xv] http://www.robsv.com/cape/c19lv2.html [xvi] The weight varied for each mission, so exact figures are not given. [xvii] http://www.nasm.edu/apollo/saturnV.htm [xviii] To prevent vital ascent fuel being lost during descent, two separate fuel systems were built [xix] http://spaceflight.nasa.gov/shuttle/reference/shutref/srb/srb.html [xx] http://www-istp.gsfc.nasa.gov/stargaze/Spaccrft.htm [xxi] The space shuttle actually enters the atmosphere with its tail lower than its nose, to create a blunt point of contact, building a shockwave to disappate some energy. [xxii] http://www-istp.gsfc.nasa.gov/stargaze/Spaccrft.htm [xxiii] Miles, F. & Booth, N., “Race to Mars”, Roxby Press, 1988 [xxiv] This technique has been used with great success by various space probes, such a Pioneer and Voyager [xxv] http://nmp.jpl.nasa.gov/ds1/index.html [xxvi] http://nmp.jpl.nasa.gov/ds1/tech/ionpropfaq.html [xxvii] http://www.grc.nasa.gov/WWW/PAO/html/ipsworks.htm [xxviii] http://www.grc.nasa.gov/WWW/PAO/html/ipsworks.htm [xxix] http://internet.cybermesa.com/~mrpbar/scntr.html [xxx] http://www.fas.org/nuke/hew/News/Flora [xxxi] http://www.fas.org/nuke/hew/News/Flora [xxxii] http://spaceflight.nasa.gov/station/reference/faq/index.html#1 [xxxiii] http://www.fas.org/nuke/hew/News/Flora [xxxiv] http://www.sciam.com/2000/0300issue/0300zorpette.html [xxxv] http://www.hq.nasa.gov/office/pao/History/conghand/propelnt.htm
