Robert Zubrin talks about reducing the mass of SpaceX Starship by a factor of 5 for a Starboat mini-Starship. The linear dimensions would scale by the cube root of 1/5 ≈ 0.58. This gives a diameter of 9 × 0.58 ≈ 5.2 meters. The mass would then be 1320 / 5 = 264 tons (assuming proportional scaling of dry mass and propellant). For a thrust-to-weight ratio of 0.91, the required thrust is 0.91 × 264 ≈ 240 tons. One Raptor (200 tons thrust) is slightly insufficient, while two Raptors (400 tons) provide excess thrust, yielding a ratio of 400 / 264 ≈ 1.52, which is reasonable for an upper stage. There are more advanced Raptors where one engine could get the thrust.
Falcon Heavy rocket with a mini-starship perched atop its central booster. The mini Starship would be fatter than the 3.7 meter wide Falcon 9 and Falcon Heavy central booster. The mini-starship, 5 meters wide and 25 meters tall, mirrors the Starship’s design with a pointed nose, cylindrical body, and fins at the base. It sits on a subtle adapter connecting it to the 3.7-meter-wide central booster. Flanking the central core are two side boosters, forming a total width of 11.6 meters at the base. The assembly stands 95 meters tall on a launch pad.
Reusability, especially for landing, favors redundancy. The current Starship uses three sea-level Raptors to ensure engine-out capability—if one fails, it can still land with two. For a smaller, reusable mini Starship, using two Raptors provides similar reliability, allowing it to land safely even if one engine fails. One Raptor, while sufficient for thrust in some scenarios, lacks this redundancy, increasing mission risk. Two Raptor engines—likely sea-level variants for landing, possibly supplemented by vacuum versions if space efficiency is needed—are the most practical choice.
The mini Starship would use two Raptor engines.
Fitting onto Boosters
Now, let’s assess how this mini Starship, with an estimated diameter of about 5 meters (based on the volume-scaling interpretation), fits onto the specified boosters.
Super Heavy Booster
The Super Heavy booster, designed to loft the current 9-meter-diameter Starship, also has a 9-meter diameter. Mounting a 5-meter-diameter mini Starship atop it requires an adapter or interstage to bridge the diameter difference. This is a common practice in rocketry—boosters often use tapered interfaces to accommodate upper stages or payloads of different sizes. The Super Heavy’s immense lift capacity (designed for over 1400 tons, including the full Starship) far exceeds the needs of a 264-ton mini Starship, making it technically feasible, though potentially inefficient due to overcapacity. With an appropriate adapter, the mini Starship fits onto the Super Heavy booster.
Falcon 9 Booster
The Falcon 9 has a uniform diameter of 3.7 meters across its first and second stages. Its second stage, also 3.7 meters, uses one Merlin engine and has a mass of about 100 tons when fueled. The mini Starship, at 5 meters wide and 264 tons, is both wider and heavier. Directly replacing the Falcon 9’s second stage with a 5-meter-diameter upper stage is problematic, as the booster’s interstage and structural design are tailored to 3.7 meters. Modifying the Falcon 9 to accommodate a wider upper stage would require significant redesign—altering the interstage, aerodynamics, and load paths—which isn’t practical for standard configurations. The mini Starship does not easily fit onto the Falcon 9 booster as an upper stage without extensive modifications.
New Glenn Booster
The New Glenn booster, developed by Blue Origin, has a 7-meter diameter and is designed with a reusable first stage and a detachable second stage. Its payload fairing is also 7 meters, but as an upper stage, the mini Starship would connect via an interstage. A 5-meter-diameter mini Starship is smaller than New Glenn’s 7-meter base, allowing it to fit with a conical adapter tapering from 7 meters to 5 meters, similar to how some rockets (e.g., Atlas V’s Centaur stage) use narrower upper stages. New Glenn’s lift capacity to low Earth orbit is around 45 tons in reusable mode, but as a booster paired with a 264-ton upper stage (including propellant), its performance depends on the mission profile. Assuming sufficient thrust, the mini Starship could be adapted to fit onto New Glenn with an appropriate interstage, making it a feasible pairing with custom engineering.
Conclusion
The reusable mini Starship with five times less mass, would use two Raptor engines to ensure sufficient thrust and landing redundancy. It can fit onto the Super Heavy booster with an adapter to reconcile the 9-meter to 5-meter diameter difference. It can potentially fit onto the New Glenn booster with a 7-meter-to-5-meter adapter, leveraging New Glenn’s wider base, though integration would need tailored engineering.
Brian Wang is a Futurist Thought Leader and a popular Science blogger with 1 million readers per month. His blog Nextbigfuture.com is ranked #1 Science News Blog. It covers many disruptive technology and trends including Space, Robotics, Artificial Intelligence, Medicine, Anti-aging Biotechnology, and Nanotechnology.
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Why not build a spaceship in space. The stresses the vessel has to overcome and the energy required launching from earth limit its ability to travel in space.
I had a little dialog with GROK 3- on the “concrete tornado” and high speed debris problem for both moon and Mars. There is a big hole or gap in the plans for precursor missions to scout and prepare a safe landing site. This is hard to get around for Mars with the limited launch windows. SpaceX has a fast iteration approach but it will only work for unmanned missions.
Realistically, there will be no manned missions until there are safe landing/takeoff sites. This will take scouting as a minimum and paving equipment in the longer run. All this will need precursor missions.
Is someone working on probes or scout vehicles for Mars ?
GROK 3 says – “Not really.”
Take a look at the SpaceX Moon lander. They placed smaller engines in a ring near the top, so that by the time the exhaust plume reaches the surface it won’t be concentrated enough to dig holes. These engines are used for only a short while during the final landing phase.
There’s no reason the same approach can’t work on Mars.
Fascinating stuff, but hopefully whatever ship is used is adaptable to NTP or NEP propulsion; it may be 5 or more years before the current development reaches fruition, but it is coming. It is coming.
Is the Blue Origin’s Project Jarvis actually a ministarship?
Where are vacuum raptors?
Raptor engines that are optimized to work in the vacuum of space
But, what’s the use case, if they actually get Starship working?
Use it as a lander for Mars
I think it’s just Zubrin adapting Starship to his scale vision for Mars. The only reason he really offers is it’s easier to fill with ISRU propellant on Mars, but SpaceX doesn’t care. They’re sending a lot more mass than his plans and with Starships being cheap, leaving a lot of them on Mars. It’s mostly what I think Musk would consider a lot of distractions and unnecessary engineering. I think Musk would build bigger Starships to be more efficient tankers for orbital refilling and send more cargo to Mars for ISRU propellant if needed.
Pretty much what I think he’s likely to do: Once the current generation of Starship is flying, he’ll built an even bigger one to function as a fuel tender. There’s room to make it at least twice the diameter, 4 times the capacity.
Though I think they’d really need to build those off shore launch towers if they go down that route; A 4x Starship stack exploding at the launch tower would be about as big a bang as a small hydrogen bomb, you really want something like that to be launching in a very isolated location.
Superheavy booster is already significantly suboptimal in being as tall and skinny as it is – the Raptor engines have had to be made smaller than optimal diameter for efficiency.
They should go to an equatorial launch site with drone boat booster landings for fuel tanker flights – to eke out every scrap of efficiency and avoid having to build for hurricane survivability. Perhaps Ariane’s site in Guiana, could also consider doing it at 4-5km elevation in Ecuador for higher engine efficiency.