The Raptor, SpaceX’s rocket engine designed to send humans to Mars and beyond, is getting a sequel.
Last week, CEO Elon Musk wrote on Twitter that the company plans to build an engine known as “Raptor 2.” This would be a more powerful version of the Raptor engine, currently being used for the under-development Starship vehicle.
It’s a big boost of power for the engine behind Musk’s Mars-bound rocket. First unveiled in 2017 under the name “BFR,” the stainless steel Starship is a fully reusable rocket powered by liquid oxygen and methane. Unlike previous rockets like SpaceX’s Falcon 9, which uses rocket propellant as part of its fuel, the Starship’s new fuel is intended to enable astronauts to fly to Mars and refuel using the planet’s resources.
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SpaceX Starship: what is Raptor 2?
It’s a more powerful, tweaked version of the existing Raptor engine. This engine has been used for test flights with Starship prototype models, the most recent of which in May 2021 reached a height of 33,000 feet.
The Starship consists of two components:
- The ship itself, also called Starship. This measures 160 feet tall and includes the crew and cargo space.
- The Super Heavy booster. This measures 230 feet tall and pairs with the Starship to enable it to leave Earth.
In total, the construction is expected to measure around 400 feet tall.
These are both powered by the Raptor engine. While the ship features six engines, Musk explained in September 2019 that the booster could hold anywhere between 24 and 37 engines.
There are two variants of the Raptor engine:
- A sea-level variant. The Super Heavy booster will use this variant, and three of the six engines on the ship will be sea-level variants.
- A vacuum variant optimized for space. Three of the six engines on the ship will use this version.
In May 2020, Musk noted that the sea-level variant has a thrust of around 200 tons, but there were plans to increase this over time to 250 tons.
That appeared to be confirmed by Musk last week when he posted on Twitter:
“Current plan is to increase base Raptor thrust to ~230 tons or ~500 million lbs & increase booster engine count to 32 or 33. All Raptors on booster, whether fixed or gimbaling, would be the same. 33*230 gets ~7600 tons of thrust & T/W of ~1.5.”
It should be noted that 230 tons are equivalent to around 500,000 pounds, not 500 million.
Musk further confirmed:
“Center engines on ship will be same as booster engines. This is basically Raptor 2. Raptor Vacuum would be only variant. Tbd as to whether to commonize R-Vac with Raptor 2 (more thrust), keep same or tighten throat (more Isp). Adding 3 more R-Vac to ship with max Isp maybe …”
So, in effect, this sequel is an engine that packs a bit more thrust.
SpaceX Starship: what does Raptor 2 mean for Mars missions?
SpaceX’s website shows the previous plan was for the Super Heavy to offer thrust of 16 million pounds. Musk’s comments suggest this figure could reach 17 million pounds.
By comparison, the most powerful rocket to ever fly was the Saturn V. It last flew in 1973, and generated just 7.6 million pounds of thrust.
The firm has kept things slightly vaguer when it comes to payload capacity. Its website claims it can lift over 100 metric tons to low-Earth orbit. That may be an underestimate: a user guide in April 2020, analyzed by YouTuber "Everyday Astronaut," suggested the rocket could lift something closer to 156 metric tons to low-Earth orbit.
At the time, Musk confirmed this was indeed the long-term goal:
"Mass of initial SN ships will be a little high & Isp a little low, but, over time, it will be ~150t to LEO fully reusable.”
For Mars missions, the more important information could be that “T/W” of 1.5. The thrust-to-weight ratio shows how the thrust compares to the weight of the vehicle itself. Unlike an aircraft that takes off horizontally, a rocket that launches vertically needs its thrust to be higher than its weight. The higher thrust-to-weight, the greater acceleration.
In January 2021, Musk explained that a higher ratio is critical for a reusable ship like Starship:
“T/W will be ~1.5, so it will accelerate unusually fast. High T/W is important for reusable vehicles to make more efficient use of propellant, the primary cost. For expendable rockets, throwing away stages is the primary cost, so optimization is low T/W.”
Sending up more cargo at a time is cool, but for the reusable Starship, the ship needs to use propellant as efficiently as possible to return home from Mars.
Raptor 2 could help enable SpaceX’s crewed mission to Mars, expected sometime this decade.
THE STARSHIP’S JOURNEY, SUMMARIZED:
- November 2018 — BFR, first announced in September 2017, gets renamed to Starship.
- December 2018 — Musk confirms the new ship has switched to stainless steel.
- January 2019 — Shortened “Starhopper” prototype unveiled and Musk explains the switch to steel.
- February 2019 — Raptor engine beats a long-standing rocket record.
- April 2019 — Starhopper completes a tethered “hop.”
- July 2019 — Starhopper launches 20 meters (67 feet).
- August 2019 — Starhopper launches 150 meters (500 feet).
- September 2019 — Starship Mk.1 full-size prototype unveiled.
- May 2020 — Starship SN4 full-size prototype completes a static test fire.
- August 2020 — SN5 launches 150 meters (500 feet).
- October 2020 — SN8 completes the first triple-Raptor static fire.
- December 2020 — SN8 launches 12.5 kilometers (41,000 feet) and crashes into the ground.
- February 2021 — SN9 launches 10 kilometers (33,000 feet) and crashes into the ground.
- March 2021 — SN10 launches 10 kilometers (33,000 feet), lands, and explodes eight minutes later. That same month, SN11 launches 10 kilometers (33,000 feet) and hits the ground in several pieces.
- May 2021 — SN15 launches 10 kilometers (33,000 feet) and lands without a hitch, except for a small fire at the base.
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Update 07/08 11 a.m. Eastern time: Updated with clarification on Elon Musk’s figures.