On September 27th, NASA announced a new round of Tipping Point funding worth a total of $43.2M that will be dispersed among 14 separate companies, all focused on advancing “important technologies necessary for sustained exploration of the Moon and Mars.”

Aside from Blue Origin and a dozen others, SpaceX received $3M to work with NASA’s Marshall Space Flight Center (MSFC) to build and test “cryogenic fluid couplers”, a type of nozzle Starship will need to refuel in orbit.

Noted multiple times over the years (and in recent days) by both SpaceX and CEO Elon Musk, an extremely robust and reliable method of orbital refueling is essential to the success of Starship’s current designed – perhaps more so than any other single aspect of the next-generation launch vehicle. Although Starship-Super Heavy will likely offer respectable performance in single-launch mode, the implicit need to recover and reuse both booster and spacecraft takes a big chunk out of the rocket’s potential capabilities.

Much like Falcon 9 and Heavy must sacrifice performance for booster landings, Super Heavy will also need to retain some amount of propellant for its boostback and landing burns. However, while just 1 kg of orbital payload is lost for every ~5-10 kg of extra hardware and propellant on the booster, things are far more consequential when discussing orbital stage reusability. Every single kilogram of hardware and propellant meant for Starship recovery and reuse will result in a 1:1 reduction in payload capacity. This becomes highly consequential when recovering the spacecraft involves the addition of something like 100-200 metric tons worth of wings, fins, heat shield tiles, batteries, propellant, and more.

As such, SpaceX is faced with a conundrum: to make spaceflight truly cheap, full reusability is a necessity, but full reusability almost fundamentally constrains the resultant rocket’s performance and utility. SpaceX’s solution: prolific orbital refueling on the order of anywhere from 1-10+ dedicated tanker launches for every Starship launch. By refueling in Earth orbit, be it low Earth orbit or something much more eccentric, Starship can be extremely reusable while still offering performance that even a similarly-sized, fully-expendable rocket couldn’t begin to compete with.

Although the theory behind SpaceX’s strategy is undeniably sound, the fact remains that orbital refueling has never been tested anywhere close to the scale, speed, or reliability Starship will need for numerous in-orbit refuelings to be a practical solution. Assuming SpaceX uses every ounce of Starship and Super Heavy’s performance for each tanker launch to low Earth orbit (LEO), Starship will have to be able to repeatedly and rapidly transfer at least 150 metric tons (330,000 lb) of liquid oxygen and methane in microgravity conditions.

Without Earth’s gravity and the force of multiple Raptor engines to help things along, it could be quite a challenge to transfer hundreds of tons of propellant in a reasonable timeframe while in orbit. In fact, SpaceX already has Space Act Agreements in place with NASA centers to mature orbital propellant transfer technology. The September 27th award simply continues that relationship, although this particular case involves the first direct funding from NASA – meant to help SpaceX over the “tipping point”.

This time around, SpaceX will work directly with NASA MSFC to build and test actual prototypes of the nozzles (“cryogenic fluid couplers”) Starship will use to mate and refuel in orbit.

“SpaceX will collaborate with NASA’s Marshall Space Flight Center in Huntsville, Alabama, to develop and test coupler prototypes – or nozzles – for refueling spacecraft such as the company’s Starship vehicle. A cryogenic fluid coupler for large-scale in-space propellant transfer is an important technology to aid sustained exploration efforts on the Moon and Mars.”

NASA, 09/27/19

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