History of reusable rockets
In the early 19th century, the US government rushed to complete the Transcontinental Railroad (TCRR) to connect the established rail systems in the East with the resource-rich, developing colonies in California. Both sides understood that fast, low-cost transportation was the key to a strong, united America. In 1869, the TCRR was completed replacing the wagons, ponies and carriages that eked they way across the plains and was considered the greatest engineering feat of the century.
Just one hundred years later, Neil Armstrong made history when he stepped onto the moon ushering in a new era of exploration. Sadly, the last steps off of our home planet also took place nearly 50 years ago, in 1972, and have become a part of history as well.
The big-budget rocket programs of the 20th century, fueled by the cold war, collapsed along with the Soviet Union. But there has been a revitalization recently, and a desire to reclaim space and unite our solar system by returning to the Moon and putting human footprints on Mars. To do this, the cost to leave Earth’s orbit must continue to fall.
Reusable rockets have become the Holy Grail for many rocket launch companies today. Until recently, all rockets have been single-use delivery systems that cost millions or even billions of dollars to run for just a few minutes. But, if reusable rockets are so simple, why haven’t we been using them all along? The truth is, that they are actually very difficult to build.
Some math of the fuel cost
The most difficult aspect to understand when dealing with a rocket is the fuel costs. This is because the fuel is both the cheapest and most expensive part of every rocket. It is the cheapest part of the rocket because, in dollars, only 1% of the cost of the rocket is in the actual fuel. For example, a typical rocket might cost $100 million dollars. If it were fueled with liquid hydrogen and oxygen, the cost might be just $1 million.
On the other hand, the weight of the rocket might be 110,000 kg. Of that, 100,000 kg will be the fuel, another 8000 kg the structure, fuels tanks, and engines, while the remaining 2000kg is for the capsule being delivered as the payload.
As you can see, the cost of the fuel in dollars is only 1%, but the cost of the fuel in kg, is nearly 90%, meaning that the final payload is only 2% of the total weight of the fully fueled rocket.
It goes without saying that during launch, there are massive strains put on the rocket. On a small percentage of the rocket is actually for the structure and rockets themselves. In order to make a rocket reusable, each component needs to be made stronger and more durable than for a single-use rocket. But, if you increase the weight of the structure, fuel tanks, and engines by 25% to make them reusable, your 8000kg of structure grows to 10,000kg, and your payload drops to 0kg! This is why for so long, engineers and professionals felt that reusable rockets were infeasible.
One part of the solution has been found in new materials and improved engineering techniques to keep the weight down while increasing structural stability. Another part is scaling up the size of the rocket. There is a reason that orbital rockets can’t be build too small. The ratio of volume to surface area is not linear. If you increase a single dimension of a 3D object, the surface area of the object increases by the square and the volume increases by the cube. By building a rocket larger, the ratio of fuel to weight increases, making orbital velocities possible with fuels that have lower specific impulses.
This was the idea behind the Sea Dragon, a 150m tall rocket that was conceived in the 1960’s but never built. The idea was to take advantage of a much larger rocket to lower costs per kg. It was designed to lift 550 tonnes into low Earth orbit (LEO), ten times today’s most powerful launchers. Unfortunately, the project was cancelled due to budget cuts and the Sea Dragon was never built.
The reality today is that a single vehicle utilizing a pure rocket engine to get to space is still out of reach. In order to get to space, rockets currently use multi-stage rockets that shed structure and fuel tanks as they ascend and the fuel is used up.
One of the most famous space vehicles today is still NASA’s space shuttle program. It was originally conceived as a fully reusable launch system, but this reality never materialized. In the end, the program either opted for single-use components, including the centre tank, or the cost of refurbishing the parts they did reuse became more than it would have been to construct them from scratch. According to their own records, the shuttle was one of the most expensive launch systems ever devised with each launch costing an estimated $1.5 billion. Given the 30,000kg payload, this means that the shuttle costs a whopping $50,000 per kg to LEO.
Today, companies such as SpaceX are paving the way to reusable rocket designs. But, even the Falcon 9 only reuses the first stage as the second stage would have to survive reentry. SpaceX claims that the cost per kg for a Falcon 9 rocket is just $11,000. If you take into account that much of the launch system is reusable, they report that the cost drops to just $5700 per kg to LEO.
It is not likely that this amount will drop much lower using today’s rocket fuels since we are limited by Tsiolkovsky rocket equation.
Simply put, this equation tells us what the exit velocity of a rocket fuel must be to reach orbital velocities on Earth. Most rocket fuels today have exit velocities in the range of 3,000 – 4500 m/s.
If you took our typical 110,000 kg rocket that we mentioned earlier using PR-1 for fuel, it would have a specific impulse of 370 seconds and carry a payload of just 2,000 kg into LEO, or 2% of the overall weight of the rocket.
But, if we could find a new rocket fuel that would double the specific impulse to 740 seconds, it would not double the payload. Instead, the payload would be increased to nearly 25% of the weight of the rocket, or 25,000kg for the same cost!
Then what’s the solution?
Finding a rocket fuel that could do this has proven to be a difficult task. But, this year, researchers Dr Ranga Dias and Professor Isaac Silvera at Harvard University claim that they have been able to produce the first samples of atomic metallic hydrogen. If their research is confirmed and this material is able to be produced as a rocket fuel, it could produce a specific impulse of 1700 seconds, making it the most powerful rocket fuel by a factor of 5.
It is predicted that atomic metallic hydrogen is naturally occurring at the extreme conditions in the core of Jupiter. Getting access to this new fuel on Earth would revolutionize rocketry. Being able to produce metallic hydrogen on Earth would allow more robust, single stage rockets to enter orbit directly, or chemical rockets to more easily explore the solar system.
The impact of making safe travel to LEO affordable to the average person can not be overstated. Not only would this allow for new industries, such as space tourism and orbital construction, it would also drive down the cost of everything associated with space exploration. Research that can only be conducted in micro-gravity would suddenly become feasible for every research institution, leading to breakthroughs in medicine, biology, materials, and the feasibility of prolonged space flights.
Just as the TCRR united California with the Union and lead to an era of unparalleled prosperity and growth, reusable rockets and the development of space has the potential to unite the Earth and connect us all with the celestial bodies that surround our pale blue dot. Every generation has a defining achievement. It is time for us to stand on the shoulders of the giants that have gone before us and reach for the stars once again.
See also: The Next 10 Years of Rocket Technology