In September, Ariane Group announced the SUSIE spacecraft at the International Astronautical Conference in Paris. You can watch the whole presentation below.
Cue the usual incoherently negative hot-takes - if you believed everything you read on social media then the concept was dead a thousand different ways before the presenters had even left the stage.
I'm going to try and give a more considered view on SUSIE here.
What is SUSIE?
SUSIE stands for "Smart Upper Stage for Innovative Exploration" and is a fully reusable spacecraft. The intention is to create an all-purpose "space truck". Initially SUSIE would carry cargo, but later variants would carry crew. Eventually Ariane wants it to be able to travel across the solar system to land on other planets and moons. Ariane indicated a first launch in 2031.
SUSIE is sitting on top of the rocket in this photo of a scale model.
SUSIE gets into space on top of a rocket. It has been designed to be compatible with Ariane 6 and future Ariane rockets. Once in orbit, SUSIE has its own engines and thrusters to travel and manoeuvre in space. To return to Earth, it points the black heat shield on one side of the nose into the atmosphere to protect itself from heating, and then uses its engines and thrusters to navigate toward its target landing pad and complete a powered landing on its tail.
It's not clear how much refurbishment SUSIE would need before flying again.
To travel further in space, SUSIE is attached to the front of another rocket which provides thrust, power and the other essentials needed for longer trips. Ariane calls this configuration a "space train".
Specifications provided so far
Length: 12m
Diameter at base: 5m
Payload capacity: 7 tonnes and 40 cu m
Future crew capacity: 5
Vehicle weight: 25 tonnes
The initial cargo version has two large doors to open up the entire payload bay and enable space release/capture.
What do I think of SUSIE?
Design
In some ways SUSIE reminds me of the Space Shuttle. The two-door bay. Gliding home on a belly of thermal protection tiles.
Another similarity is its size. SUSIE is only 20% the mass of the Shuttle, but is still much bigger than any other capsules and spaceplanes currently active or under development. However these vehicles are big for very different reasons. Shuttle was designed to deliver and retrieve large, sensitive military payloads in space with minimum scrutiny from rivals, and needed a huge payload bay to achieve this. I think SUSIE is large to make it more viable for longer duration interplanetary travel - crew will need more room to move, and will need to take more supplies and equipment with them.
Then there are the differences with Shuttle.
Firstly, the shape.
SUSIE is a cone that's been pinched into the shape of a four sided pyramid, with the flattened sides providing some lifting body characteristics for easier atmospheric re-entry. I expect the overall symmetry of the vehicle means that aerodynamically it behaves a lot like a pair of payload fairings during launch and atmospheric flight, with little need for special flight programming. This would mean SUSIE could launch on many heavy lift rockets. By contrast, the size and shape of the Shuttle meant there was only one specially designed launch system that could get it to space.
The upcoming Dream Chaser is asymmetric and highly optimised as a lifting body for Earth re-entry. A future crewed variant could not be enclosed in fairings at launch because this would prevent operation of a launch escape system, and therefore the rocket will experience aerodynamic challenges at launch due to the shape of the Dream Chaser. I'm not suggesting these challenges would be insurmountable since Shuttle and Buran have already demonstrated the viability of asymmetric launch systems, but this could increase costs and reduce the versatility of Dream Chaser.
Secondly, propulsive landing.
Instead of being optimised solely for Earth re-entry, SUSIE is also able to land on planets or moons with little to no atmosphere. It may even be able to launch by itself from bodies with sufficiently low gravity.
Summary and comparison to other reusable spacecraft
Overall I think the design is well considered given the stated objectives for the vehicle. The goal is not just to fly a single mission type or win a specific contract, but to create a versatile reusable spacecraft that can fulfil a wide range of purposes for a long time into the future. Rocket Lab claims their upcoming Neutron rocket is designed for 2050. SUSIE also appears to be designed for a similar timeframe.
SUSIE is quite different to other capsules and spaceplanes. Crew Dragon, Starliner, Dream Chaser and Space Rider are not designed to go beyond Earth orbit. Orion is designed to leave Earth orbit, but it won't be capable of landing on other planetary bodies.
To me Starship is the closest in concept, being capable of interplanetary travel and landing.
If successful, Starship is very likely going to occupy a dominant role of "space semi-trailer" or "space widebody aircraft". It's likely to be our apex vehicle until spacecraft start being manufactured in space, at which point we will start seeing "space container ships" and "space cruise liners".
However from our experience on Earth we know that such a large vehicle is not always the most appropriate for a particular trip or load. I can see SUSIE being successful as a "space van" for the many situations where Starship would be overkill.
Both Starship and SUSIE are all-terrain vehicles and will be effective in a solar system with little or no space infrastructure. Eventually we can expect space infrastructure such as space ports and fuel depots to be built out over time which will gradually shrink the role of ATVs and shift the advantage to vehicles that are optimised for (and only operate within) their specific regimes. But this may take a very long time - there's a lot of solar system out there!
Feasibility
A lot of the online critiques went straight to things like the weight of the vehicle being greater than what Ariane can currently lift. I'm not going to get too worked up about that. I expect some of these numbers are indicative and subject to change.
I'm more concerned about the following.
Funding. This is going to be expensive to develop. European space agency budgets are dwarfed by NASA and unlike their American counterparts, European companies and billionaires seem happy to sit on their hands instead of splurging cash on megaprojects like this.
Flight heritage. SUSIE would be ambitious for even the most adept and experienced in the field. For a space agency that has only reached the experimental stage for reusable spacecraft and has never performed a crewed launch, this is the stretch goal of a lifetime.
Track record of product development. ESA previously tried to develop their own version of the Dream Chaser called Hermes, and that project was ultimately cancelled after significant expenditure. Will SUSIE follow the same path of failure, or will it be successful like the Ariane rocket series? One potentially promising sign is Space Rider which is similar to SUSIE but much smaller. It is currently under development with a planned launch date of 2023, and would validate a lot of the theory applied in designing SUSIE.
