Electric fuel pump
New Zealand’s company Rocket Lab has an Electron rocket that uses a similar layout to SpaceX’s Falcon rocket. Both have a nine engine first stage and a single engine second stage. Both use liquid oxygen and RP-1, a refined form of kerosene. The most notable difference is scale – the Falcon is much larger. Also, the SpaceX recovers its rocket’s first stage, while Rocket Lab does not.
One other aspect is of interest. The Electron is named for its unique approach to pumping fuel to its rocket engines. Almost all liquid-fuelled rockets use a turbo pump to supply fuel at the rate rocket engines demand. Turbo pumps are like jet engines that burn the same fuel as the rocket engines to produce power for a pump.
Electron dispenses with the turbo pump and uses an electrically powered pump instead. Principle 28 – Substituting of Mechanical Systems with Electromagnetic Systems
The advantage is easier rocket starting, no use of fuel, easy control over flow, simplicity and removing the weight of the turbo pump. The downside is the weight of the battery that runs the electric motor.
What surprises me is that somebody beat SpaceX to the use of electric fuel pumping. After all, SpaceX’s CEO is none other than Elon Musk, CEO of Tesla, the electric car and solar company.
Stage and fairing separation
Unlike the pyrotechnic systems almost all other launch vehicles use for stage and fairing separation, Falcon 9 uses actuators driven by high pressure helium to release latches and activate pneumatic pushers that provide a gentle push to ensure separation. Principle 29 – Pneumatics and Hydraulics
As opposed to one-use explosive bolts, this allows for testing on the ground and reduces the risk of damage upon separation. It is also reusable, in keeping with the focus SpaceX has on reducing discarded components.
I’ll still need to be convinced why electric motors couldn’t do the same job more reliably.
Mulling ullage
The trick with getting liquid fuel to run a rocket in space is the fact that the fuel could be floating anywhere in the tank, not sitting on the bottom where the fuel pickup is, which one can reliably expect on the ground or during liftoff.
To address this rocket designers use ullage motors. These motors are typically monopropellant or pressurized gas motors that can be activated in microgravity. SpaceX uses compressed nitrogen gas. These motors provide just enough impulse to nudge the rocket forward so that the contents of its tanks rest at the bottom.
Electric alternatives
I wonder if electricity could instead be deployed for ullage. Principle 28 – Substituting of Mechanical Systems with Electromagnetic Systems
- A positive ion spray at the top end of the tank and a negatively charge at the other end could coax the fuel to the right end of the tank. Falcon 9 tanks are made of aluminum-lithium alloy, so that would probably ground the ions prematurely. Carbon fibre is also a good conductor, so this line of thinking may not pan out.
- Scientists are experimenting with acoustic tractor beams to manipulate objects in the air with sound waves, so perhaps this technology could be used to coax the fuel toward the pickup.
- Another option is to have electric motors act on blender blades in the tanks. That would tend to push the fuel toward the perimeter of the bank due to centrifugal force and an auxiliary pickup point could be placed on the tank’s perimeter. The force applied to the fuel would have an opposite reaction causing the craft to rotate on its long axis, but there are two tanks with the same long axis, so one tank would be spun clockwise and the other counter-clockwise.
Accumulator alternative
Axiomatically, engines only need to be restarted in space after they’ve been run.
I’ve talked about water hammer in these pages before, and that it might be harnessed for benefit. Principle 29 – Pneumatics and Hydraulics & Principle 22 – Blessing in Disguise
Imagine the hydraulic shock from turning off the engines after a burn being used to direct fuel and oxygen into separate hydraulic accumulators. The accumulators’ contents could be used for main engine restart. Each engine stop would recharge the accumulators.
What’s the point?
Using cold reaction jets is a perfectly reliable method for maneuvering and ullage. The misgiving I have, particularly as space missions become longer and more complicated, is a SpaceX rocket could have lots of fuel available but have mission options hampered by a lack of cold reaction fuel. By contrast, in space, there is plenty of electric freely (and weightlessly) available thanks to solar panels.
Taking out cold reaction fuels and relying on electricity are examples of Principle 2 – Taking Out and Principle 6 – Universality.
