The Fundamentals of space Travel
The Rocket Problem
Space is difficult and expensive to explore in any kind of large way. The reason is we lie at the bottom a very deep gravity well. Consequently, for conventional rockets only around 10% of the mass we launch into space reaches low earth orbit (LEO). In automobiles a car gets a fixed mileage per gallon. Thus the relationship between fuel expended and distance is linear, while in the case of rocket the greater the change in orbital velocity a rocket must make (delta v) the more fuel it will take if it takes the rocket per delta v it travels. That is the further the rocket travels the worse “gas mileage” it gets.
The reason a rocket gets less gas mileage the further it travels is a rocket must take all the fuel it needs with it. It is like trying to travel across Canada without stopping at any gas stations. How might one do this. You could use a little car and hull a very big trailer of gas. The further the car must travel without refueling the bigger the trailer of gas will need to be relative to the size of the car. Thus we see that without gas stations cars would face similar problems as rockets.
Combating The Rocket Problem
Most near term opportunities of manned exploitation and commercialization lie in the region between Earth and the Moon. For conventional rocket only 10% of the mass launched reaches LEO, 20% of that mass lifted to LEO reaches Mars. Thus the majority of cost for a mission to Mars is the cost of launching the stuff to Earth orbit. Moreover, more near term solutions exist for improving the gas mileage between Earth and Mars than exist for lowering the cost to orbit (cheap access to space).
Near term Solutions For earth Mars Travel
Some near term solutions to reducing the cost of traveling between earth and mars include: solid core nuclear thermal propulsion, nuclear electric, solar electric and aero capture. These technologies are essentially here. Nuclear thermal is basically 1960’s technology. It will basically double the percentage of mass that reaches mars that is launched to LEO. Aero capture could perhaps more then double the percentage of mass that reaches LEO that make it to mars. These two technologies combined help to significantly reduce the cost of traveling between earth to mars in comparison to the cost of launching mass to LEO. Also both of these technologies are fast enough for people.
The electric propulsion may seem more near term then nuclear thermal since there have been probes that successfully used this technology. However, the ion engines to date, don’t produce significant amount of thrust. The first ion probe deep space one only produced about as much thrust as a piece of paper on earth exerts on a table. Also current ion engines use rare expensive fuels. Other electric forms of propulsion include: arc jets, which has problems with corroding electrodes and a limited improvement of performance and plasma electric (VASIMR) which is not as near term and requires large amount of power.
This problem of power is not only a problem for VASIMR but a fundamental problem of any form of electric propulsions. Electric propulsion trades energy efficiency for fuel efficiency. A more fundamental problem then producing a better electric engine is producing a better power source to power that engine. We will call this the electric propulsion problem.
Current methods to reach LEO
There are many vehicles in use that deliver people or goods to LEO, including: Progress, Soyuz, Sea Launch and the space shuttle. These vehicles fill a range of roles from, delivering space probes and military spy satellites to earth orbit as well as delivering people to the International Space Station (ISS) and supplies to the international space station. However, most of these vehicles are small or medium lift and do not deliver sufficient mass to LEO to be a practical vehicle to use for a Mars mission.
To allow NASA to once again move beyond LEO as they were able to do when the Saturn V took them to the Moon, options were investigated that would allow a greater amount of mass to be delivered to LEO (Heavy Lift). The three approaches investigated where the shuttle derived vehicle, the EELV and the clean slate approach.
The shuttle derived approach was chosen because: it would be the fastest approach to develop, it would result in the least layoffs and it can deliver the most mass to LEO. It is the quickest and easiest solution but not necessarily the best solution.
EELV stand for evolved expendable launch vehicle. When the military realized the space shuttle was not going to meet expectations it chose a different method for delivering military satellites into space. The two lines are the Delta by Boeing and the Atlas by Lockheed Martin. Boeing has showing that the Delta V can be modified to deliver 40-50 MT into LEO without pad modifications and 80MT into space with pad modifications. EELV has the advantage over the shuttle that it uses a higher degree of automation in the factors. This could mean a cheaper more reliable product.
The Clean Slate Approach
The other options explored was the clean slate approach. As the name implies this approach would start a new rocket line. It would be the most expensive option but should yield the best product.
[Griffin vs Rumsfeld] A good news article on the debate between Shuttle derived and EELV in terms of how the decision is good and bad for the milllitary.
Near Team Methods of Reaching LEO
The prospect of reaching LEO cheaply (cheap access to space) in the near future is atleast 20 years away. The most near term solutions include fly back boosters, air launched rockets and Elon Musk’s Falcon rocket.
The cheapest of these in terms of capital investment is probably the Falcon Rocket, the rocket has not yet flown. It is unproven and it cannot deliver large payloads to low earth orbit (LEO). Moreover it will only be a limited improvements in the cost to LEO.
Fly Back Boosters
Fly back boosters will help to reduce the recovery and turn around time of solid rocket boosters. On the space shuttle the shells of the solid rocket boosters must be recovered from the ocean and completely rebuilt every flight. This is a considerably labor intensive process.
The air launch option could allow for a completely reusable space plane with weekly turn around times. Out of the above options this option, air launch, would reduce launch costs the most but it will come at a large capital cost, and have minimal payload. Also in order for it to be economical it must be able to meat a weekly flight rate, both technically and in terms of economic demand. This economic and technical challenge is a consequence of the time value of money.
Medium term (20 years in the future or greater)
The two technologies that are most likely to reduce the cost to LEO are the scram jet and the space elevator. The scram jet beats the rocket equation because the vehicle only has to carry the fuel and not the oxidizer. The fuel only weights 1/16 th as much as the oxidizer. The vehicle would build up speed at the edge of the atmosphere breathing in air until it has reached orbital velocity. Prototypes of the scram jet produced by NASA have reached mach 9 which is a record for air breathing rockets. To reach orbit the scram jet must reach mach 40. Although the scram jet has a ways to go to be able to reach LEO it has tremendous military applications. The mach 9 tests were the last tests while the project was under NASA. Now the project is under the united states air force, code named (project falcon). The falcon bomber is a near future military vehicle which will be able to strike anywhere in the world in 40 minutes.
The other near team technology that will lower the cost to earth orbit is the space elevator. The space elevator is essentially a thin cable made out of a super strong material (carbon nano tube) and a method of climbing that cable. The cable is held up by a counter weight which extends past geosycronus orbit. The space elevator faces similar problems as air launch in that it requires a large capital investment. There are synthetic carbon fiber materials today that exist today that are strong enough to build a space elevator on earth but will probably result in a space elevator that is too expensive. However present day materials may be viable for a lunar space elevator. A space elevator is too expensive if the output of the space elevator isn’t large enough relative to the cost of building it to make up for the time value of money.
ISRU stand for In-Situ Resource utilization (living off the land). It is a back door method to problem of cheap access to space. Basically if we can produce what we need in space we don’t need a method for getting the goods to space in the first place. It will probably take considerable time before all the necessary resources are in space to produce everything we need thus in the short term ISRU is only part of the solution. Moreover, both the technologies scram jet and space elevator could make a large amount of commercial activity viable in space with limited ISRU. The space elevator is particularly attractive becase if the elevator can be made cheaply enough then the resources lifted to orbit by the space elevator can be used to produce space elevators even cheaper in the future. This decline in cost to LEO would could continue until the majority of the cost of getting people and goods to orbit is the cost of electricity to lift them to orbit and not the capital cost.
Although project Orion was a concept researched relatively early in space exploration It faces more challenges than some of the other methods and has questionable economics. However, for launching huge amounts of material into space in one short 1000 MT, it seems like the most viable option should the need arise. It also is competitive with VASIMR, gas core reactors, And salt water nuclear reactors as methods for traveling to the outer planets for vehicles larger than a given size.