As a space nerd, I love learning about the craziest rocket concepts that have come up over the years (and there certainly have been quite a lot). One category of concept intrigues me in particular: single-stage-to-orbit, or SSTO. This type of spacecraft can reach orbit without the need for detachable boosters or other staged hardware. This is quite easy to achieve on a body like the Moon, given its low gravity and lack of atmosphere. SSTOs are also feasible on Mars, as Mars has lower gravity and a very thin atmosphere. However, Earth’s higher gravity and thicker atmosphere make SSTOs fairly tricky to pull off; nevertheless, it’s doable, and so there have been numerous designs proposed.

Many SSTO designs have a traditional rocket form factor, with the exception that the fuel tank is conical rather than cylindrical (the DC-X is a good example of this). However, the spaceplane form factor is also common and arguably sports some benefits: at takeoff, you can use the atmosphere to your advantage as it generates lift, and you can just land on a runway instead of having to fool around with parachutes or retropropulsive landings (a la Falcon 9). In fact, as far as I can tell, the spaceplane-style SSTO is the more popular SSTO style among Kerbal Space Program players; browsing the KSP subreddit shows both spaceplanes and rockets, but spaceplanes are far more common.

Of course, what goes in KSP doesn’t necessarily go in real life. While Kerbin’s gravity and atmospheric pressure are similar to Earth’s, it is only about a tenth of the size of Earth, making it far easier to get to orbit. This makes it very easy to take a large fuel tank, slap a Vector on the bottom, and get 4 km/s of delta-V, which is enough to get to orbit and back with a single stage.

Skylon

Skylon stands out among spaceplane SSTO designs for various reasons. For starters, its SABRE engines can run in an airbreathing mode, which reduces the amount of liquid oxygen that Skylon has to carry (which in turn allows it to carry more hydrogen fuel), but also function just fine when burning hydrolox. Furthermore, it’s the only SSTO spaceplane that is in active development; while not much news is forthcoming from its developer Reaction Engines, it appears that they are still working on SABRE and Skylon.

However, Skylon has been in development since the 1980s. It has its roots in the HOTOL spaceplane; while the HOTOL project was ultimately cancelled by the British government, the people behind it didn’t give up the idea of a spaceplane with a dual-mode engine, eventually starting Reaction Engines to develop Skylon as a sort of second iteration on the concept. Since then, we’ve seen the end of the space shuttle and the rise of Falcon 9, with Starship looming on the horizon. We’re no longer in an era where rocket launches are prohibitively expensive; in fact, if you just want to send a CubeSat into orbit, you can do so quite cheaply. SpaceX currently estimates a launch cost of $300,000 for a 1U CubeSat weighing up to fifty kilograms on one of their rideshare missons. In other words, if you have three hundred grand burning a hole in your pocket, you can get sweet bragging rights by launching your own satellite! And it will only get cheaper: when SpaceX starts offering rideshare launches on Starship, costs will likely drop significantly due to Starship’s improved efficiency (not to mention its reusable second stage, which represents an enormous savings on its own).

At this point, then, should we be throwing in the towel on Skylon? Does it even have a place in today’s landscape, or does Starship completely obsolete Skylon?

By the numbers

I’m not going to use mass-to-orbit as a deciding factor, because Starship would beat every other rocket out there, and besides, just because we need super-heavy-lift rockets for some payloads doesn’t mean we should use them for every single satellite that is ever launched. Instead, I’ll try to make a more fair comparison based on the payload fraction and launch costs of each vehicle. Please keep in mind that while I’ve tried to get accurate numbers here, you should treat everything said here as a rough calculation, as Skylon is still operation in the theoretical realm and the numbers given for Starship are likely not extremely precise. Additionally, SpaceX is continually iterating Starship for more performance, so we could easily see the numbers change in another year or so. I’ve also converted every number to freedom units, so there could be slight precision losses from that as well.

According to its manual, Skylon has a payload capacity of 16.5 tons, while SpaceX says that Starship can launch up to 150 tons. Given Skylon’s 345 ton mass and Starship’s 5000 ton mass (Wikipedia), we can calculate a payload fraction of 4.8% for Skylon and 3% for Starship. While this seems to point to Skylon, the cost per launch tells a different story: Skylon’s estimated cost per launch is $10 millon. While we don’t know the exact numbers for Starship, Elon Musk has claimed that costs could fall as low as a few million dollars per launch. Even if Starship cost the same to launch as Skylon, it still has a clear win in cost per pound to orbit: in this scenario, Skylon would cost $302.39 per pound to orbit, while Starship would cost just $33.33 per pound to orbit.

Since numbers don’t lie, you’d be forgiven if you immediately wrote off Skylon at this point. However, I think there’s one more thing we need to consider.

Apples to oranges

Skylon and Starship are both orbital launch vehicles that aim to decrease the cost to launch while promoting reusability, but that’s about where the similarities end, as Skylon leans into the SSTO spaceplane design while Starship embraces traditional rocket designs alongside retropropulsive landing. Clearly, we can’t properly compare these vehicles without weighing these differences, and I think this is where Skylon gets an opportunity to shine.

Traditional reentry capsules (think Apollo) tend to pull many Gs on reentry, which causes a certain level of discomfort to the crew. I couldn’t find numbers on the projected G-forces on either Starship or Skylon, but Skylon has a big advantage in this department: its wings. As a craft that is capable of gliding, Skylon is capable of taking a much shallower reentry path that reduces the overall stress on the vehicle. In an age where spacecraft are being proposed for use as long-range passenger transport vehicles and various companies are developing space stations for commercial or tourist use, Skylon could become the perfect crew transport vehicle to reduce stress on passengers, both at launch and reentry. Given that Reaction Engines claimed an intended crew capacity of 30 or 40, Skylon could become the luxury space travel option, while a trip on Starship would be a bit more like flying economy: less expensive and less comfortable. And it’s not necessarily just people; Skylon could potientially enable us to launch equipment that is much more delicate to further scientific research.

It’s up to Reaction Engines now

With all this in mind, I’m optimistic that there is still a place for Skylon in Starship’s world. At this point, we just need Reaction Engines to pull through and actually build the thing. Until then, we’ll just have to keep flying the traditional rockets.

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