On December 14, 2021, Tim Dodd had the opportunity to interview Rocket Lab founder, CEO, and CTO Peter Beck about their upcoming medium-lift launch vehicle, Neutron. Peter Beck describes the Neutron process as taking all of the lessons learned from Electron and incorporating those into something that he believes is going to be very important for the future of rocketry.
Beck describes a rocket as “one giant engineering compromise,” adding that every rocket design is going to have downsides, echoing Musk’s “all designs are wrong, it’s just a matter of how wrong” attitude. He points out that this is where materials play such a massive role: having an ultra-light carbon structure allows for Rocket Lab to deal with dynamic heating later in flight and add more structure to the vehicle, both of which are advantageous for a reusable launch vehicle. “It all comes down to mass at the end of the day, it is the biggest challenge you need to solve.”
It was noted by Beck that on a lot of larger launch vehicles one of the primary battles is fighting tank buckling. On larger rockets, buckling is generally prevented with either stringers (longitudinal support structures) in the stage or an isogrid structure. However, with carbon fiber this is not needed since it is so stiff and light: for the same relative mass, you can have a significantly thicker, and therefore stronger, structure with carbon than with steel.
Rocket Lab started off initial design requirement was that Neutron must be able to be reused in 24 hours, despite Beck stating that he doesn’t think that Rocket Lab will ever reuse a Neutron booster in 24 hours. This decision drove a large amount of the engineering compromises, such as the fairing design.
Beck opened up discussion about the Archimedes engine by stating that because the structure of Neutron is so light, they do not need to push the boundaries of propulsion. Because of this, they decided to pursue an open cycle gas generator engine that runs on CH4 (liquid methane) and LOx (liquid oxygen), which due to the simple cycle would reduce development time. This is similar to the engine that Tom Mueller wanted to build for Starship, which was essentially just a CH4 powered version of the Merlin engine that is used to power the Falcon 9 rocket.
He noted they have already decided on an injector design, stating that there is only one clear choice for a gas-liquid interaction; the CH4 will be a gas by the time it enters the combustion chamber as it boils off in the regenerative cooling channels and the liquid oxygen will remain in liquid form. This likely means that Rocket Lab has chosen to use a coaxial injector of some sort, but this has not been confirmed by Beck or Rocket Lab.
Beck also noted that there would be a large amount of 3D printed parts on the engine, which is an interesting juxtaposition to both the Raptor engine and Astra’s rocket 3, where they are trying to remove as much 3D printing as possible to reduce costs.
According to Beck, the primary reason why Rocket Lab chose CH4 and LOx for Neutron was reuse. Since CH4 offers high thrust with good ISP, while creating no soot, the engine can be rapidly reused without needing to be cleaned.
Neutron Second Stage:
In the same vane of the vehicle being “one large engineering compromise,” the second stage is “hung from” the first stage. Beck noted that as they were going through the Neutron design, they started looking into a reusable upper stage, but because the first stage is the majority of costs of the rocket it made little sense. Additionally, since the second stage is so insanely light, allowing it to be high performance, it is very cheap to manufacture. Because of this, they decided that an expendable upper stage would serve the market the best.
The second stage will have the same engine as the first stage, albeit a vacuum optimized version, that also runs on CH4 and LOx. However, due to the small mass of the second stage and the high thrust of the engine (~1 MN), the second stage’s engine will have to be able to throttle low for the end of the second stage’s burn. Beck pointed out that the throttle requirements for the second stage engine is very similar to the throttle requirements for the landing engine on the first stage.
There is still a trade going on between having four petals and two petals for the fairing, and they hope to decide on this by the end of the year. “The best solution for reusing fairings is to just hold them on.” However, this requirement drove a lot of design decisions, such as having the first stage attached to the fairings.
Dodd asked if it would be possible to re-open the fairing on descent to act as drag brakes and use them for control authority. Beck stated that this was initially the plan, and is the reason why Neutron’s fairing currently is in 4 pieces; however the loads on the fairing during descent are massive, making the idea not feasible. Instead, they will opt for fins on the second stage. Beck noted that the fins are likely going to be electrohydraulically driven, which is something that they have experience with from Electron.
For a human rated version of Neutron, Rocket Lab would remove the fairing and have the capsule act as one, like SpaceX does with the Falcon 9 and Dragon.
Similar to Musk, Beck said that during initial Neutron development meetings the “legs were driving me insane,” noting that creating a light, reliable mechanism for landing legs is very difficult. (SpaceX has had similar problems with landing legs on Starship and Super Heavy, which has led to them removing legs from the vehicle and trying to catch it with ground equipment). He noted that while the design isn’t final, the leg design is likely going to be composite with a hot skin, similar to the upcoming recovery version of their Electron rocket.
Beck noted that it is much much more expensive to fly a launch vehicle than to build one, adding that this is why Neutron doesn’t have a strongback and that the pad is so clean. This is the same reason why Neutron is going to remain vertical for its entire life, so a structure to rotate the rocket (generally called a transport erector) is not needed; the rocket will be built at the launch site, which Beck implied may no longer be Wallops.
He also noted that cost is the reason why Neutron is only going to return to launch site, stating that for just having a small ship in a port is around $60,000 each day; he also added that the helicopter that they will be using, a modified Sikorsky S-92 “Superhawk”, for Electron recovery is $5,000 per hour, while the helicopter they’ve been using so far, a Bell 429 “GlobalRanger”, is $3,000 per hour. Additionally, Beck noted that Neutron’s payload hit from RTLS is smaller than other vehicles as it has more cross range capability and that because Neutron’s structure is so light, less dV is needed to return to the launch site. Furthermore, Neutron will not conduct a reentry burn, saving even more propellant.
Beck said that the timeline that they have previously announced–first engine firing and structures in 2022 with a launch in 2024–is “as reasonable as any other rocket timeline.” He added that “thus far it is way easier to develop a larger launch vehicle than a small one,” as the mass constraints are not there: while on Electron they worry about each and every gram, on a larger rocket they don’t.