What does a reusable launch vehicle get us?
Imagine what life would be like if, after each trip to Grandma’s, we had to throw away the car. Even with the benefits of mass production, the cost to an individual would be prohibitive, especially if there exist reasonable alternatives like horses or walking. Such automobiles could be employed only by governments, extremely wealthy enthusiasts or perhaps by a few skilled specialists who lived for the challenge.
This is pretty much the situation with current spacecraft technology. Not even the Space Shuttle program achieved the lofty goal of reliable reusability, although it tried very hard. The shuttle was such a complicated system that every time it returned to Earth, intense maintenance had to be performed and systems rebuilt or overhauled, making it three times as expensive as that of an expendable rocket. For example, a shuttle launch cost $450 million to $1.5 billion, compared with $110 million for a Russian Proton rocket with about the same lift capacity.
Truly reusable launch vehicles would significantly reduce the cost of getting material and people into orbit and enable new uses of space with far-reaching socioeconomic consequences that will ultimately reduce our impact on Earth’s environment, such as space-based energy collection, mining and manufacturing.
In order to get an idea of the savings, the retail price of a Falcon 9 rocket is around $60 million to build and launch (including fuel). Given its total lift capacity of 13,150 kilograms to low-Earth orbit, this translates into a price tag of about $400,000 to ferry a 90-kilogram (198-pound) person into space. But if you had to pay only for fuel, about $300,000 a launch, the price tag drops drastically to just $2,000 for the same person. That’s not far from the cost of flying from New York to Sydney, which makes a future family vacation to a Bigelow B330 Space Habitat a viable alternative to Disney World.
At the pace things are going, we project that within 10 years the space industry will achieve the goal of a fully reusable launch vehicle. Companies and municipalities, small and big, are all starting to look into ways of taking advantage of this complete disruption in, or better yet creation of, the commercial space market.
So our next question is this: what do we need to make the Millennium Falcon – that is, a single-stage-to-orbit completely reusable spacecraft – a reality?
A little physics can help us see exactly what needs to happen and exactly how far we are from this goal.
Rocket Science 101
Space travel is all about speed. The old adage, “What goes up must come down,” is true only to a point. If you throw something up fast enough, it won’t come back down; it will have escaped Earth’s gravity. The question is, exactly how fast is fast enough?
A simple application of Newtonian gravity theory tells us that if we achieve a speed of 11 kilometers per second – the equivalent of a plane flying 25,000 mph straight up – we are not going to fall back to Earth. Scientists and engineers refer to this speed, which depends on the physical properties of the Earth, as our planet’s escape velocity.
A rocket tries to achieve that speed by taking mass and throwing it out the back as fast as possible. Thanks to Newton’s third law – which states that for every action there is an equal and opposite reaction – this propels the rocket forward.
The ratio between the change in velocity needed to escape the Earth’s pull (known as delta-v) and the speed at which the rocket sends stuff out the back (exhaust velocity) is the most important number in rocket science. It determines how much mass needs to be expelled and how much energy is necessary to get to space. The smaller we can make the ratio, the better.
In addition, the propellants and fuels are themselves massive, and the rocket needs to carry these things with itself, making it heaver and harder to accelerate.
So we need propellants and fuels with a high energy content and low mass.
Now we can begin to appreciate the enormous feat of engineering that private companies and governments have achieved by not only launching a rocket but learning to land it as well.
The maximum achievable exhaust velocities for the rockets we’ve been using since the dawn of space travel are much less than the Earth’s escape velocity (about 4 km/s or 9,000 mph), forcing us to come up with ingenious and costly multistage launch techniques to get even a modest payload into space.
In summary, in order to leave the surface of the Earth with the grace and apparent effortlessness of the Millennium Falcon, we need to achieve speeds in excess of the escape velocity, 11 km/s. In order to do that without carrying a fuel tank that far exceeds the size of our ship, we need to achieve exhaust speeds significantly higher than the escape velocity, something not possible with the chemical fuels we use.
So where do we go from here?
So in order to make the Millennium Falcon a reality, we need a new type of fuel, as chemical-based engines are severely limiting.
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