Proven reusability
“For a long time people were using the space shuttle as an example of why reusability is dumb. You can’t take a single case example and make an entire theory out of it.”
Elon Musk
Musk has frequently made the case that discarding a rocket is like flying a jumbo jet across the Atlantic and then crashing it when you get there. Flying would be prohibitively expensive.
The challenges of getting spacecraft back on the ground are greater than landing a jet, but Musk’s company SpaceX began with a determination to overcome these challenges. Now that SpaceX has been successful, they can offer access to space at a much lower cost. Musk himself has claimed that it will reduce the cost by a factor of 100 or more. Landing at a predetermined location also makes logistics for reflight easier and less expensive.
Early in SpaceX’s rocket evolution, the company decided to make their first stage rocket land on its tail. This allows the rocket to reuse its system of propulsion to perform the landing, augmented with components that don’t drastically increase weight (grid fins, and landing legs). The primary weight penalty comes in the form of fuel. To make rockets reusable, their payload capacity is reduced in favour of reserving fuel for landing.
Scaling up
Musk has said that the cost of fuel for a Falcon 9 flight is on the order of $200,000, a tiny fraction of the cost of a mission.
So if reusability requires extra fuel, but fuel is cheap, what is SpaceX to do? The short answer is to make its rockets bigger. Enter the BFR, the Big Falcon Rocket. This lets SpaceX add the extra weight for systems that support reusability and the extra requisite fuel but still put useful amounts of payload into space.
To scale up quickly without exorbitant costs, the BFR will use the the same proven engines from the latest Falcon and Falcon heavy, albeit more of them. And the second stage will also be recovered. Instead of mounting the payload on top of the second stage within a disposable fairing, the payload will be kept inside the second stage. This is something the shuttle got right.
Recovering the second stage will be the greatest engineering challenge, as it will, by definition, need to slow from orbital speeds. This will call for a large reserve of fuel and substantial heat shielding. Good thing the BFR is so falcon big!
The Principles
Principle 1 – Segmentation
At first glance SpaceX’s decision to use 31 engines on the first stage of the BFR is baffling. Surely that is less reliable than a smaller number of engines. Surely a handful of larger engines would be more efficient.
Saturn V used five huge engines to produce similar thrust to the BFR, but the larger the engine the less stable its combustion, making them prone to failure. Also, the Saturn V engines could not be throttled. They were either on or off.
SpaceX using multiple existing rockets means they can take advantage of their proven design, and having more engines actually improves reliability, since a couple of engines failing has a manageable impact on total thrust. Further, the ability to shut down some engines and use fewer for maneuvering, particularly during the tricky landing, is an advantage.
Score one for Segmentation.
Principle 35 – Parameter Change
Parameter change asks, what if we made it something-er? And SpaceX has made BFR much bigger than the Falcon 9 – even bigger than Falcon Heavy. This allows it to deliver useful payloads to orbit while building in the systems and additional fuel reserves that lets both stages return to earth for reuse.
Principle 6 – Universality
SpaceX vehicles return to earth on their tails, using their primary propulsion rockets.
