Guest Post: Rockets or Spaceplanes?

Hello! I’ve been absent for a bit—with midterms bearing down on me, I needed a two-week break—but I’m back with a guest post from my good friend Eamon Minges, who wrote about orbital skyhooks last year. He will be making a case for horizontally launched spaceplanes, as opposed to SpaceX’s vertically launched Starship model. Enjoy!

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After the landing of Starship SN-15, we were all engrossed in the splendor of seeing SpaceX’s massive starship prototype soar ten kilometers into the sky, belly flop downward like a skydiver, and then flip to perform a beautiful twin engine landing. Starship might right now be the absolute cutting edge of space transportation. NASA has even selected it for the HLS, contracted to land astronauts on the Moon within five years. With all this hype around Starship, I wanted to turn attention to another question. Yes, Starship is absolutely amazing, but will it indeed be the best way of getting humans frequently, routinely, and safely to Low Earth Orbit? That, in my opinion, is still an open question, given how new and unprecedented Starship is. So what I want to do is a focused analysis of potential benefits and competitive advantages spaceplanes still might have over Starship. Let’s get into it!

Firstly the most critical advantage which space planes—such as BAE Systems Skylon or HOTOL—have over a vehicle like Starship is cross-range capability. Since Starship relies on re-entry and then a controlled belly flop maneuver, there isn’t much margin for error in the landing procedures. The Starship has to slow to the point of terminal velocity almost directly over the landing pad. In the case of bad weather, it wouldn’t have the same capability as Skylon or HOTOL to fly thousands of miles cross range from re-entry for diversions. This tight restriction on landing over the entry zone means a more risky landing profile because Starships can’t fly but merely fall, not having any controlled gliding or powered flight ability.

Even more risky than the precision needed for the landing procedure is the re-firing of the engines during the flip and burn maneuver. Any rocket engine relight during landing will unfortunately add both extra points of failure and potential risk. While it is true that SpaceX has demonstrated engine re-use capability with their Falcon 9 rocket, the acceptable margin of safety for engine relights has a long way to go if it is to approach that of conventional turbojet airline engines—to use a reference Elon is fond of. Since a spaceplane like Skylon or HOTOL wouldn’t need the same engine relight procedure after reentry and can simply fly either powered or by unassisted gliding back to the runway it launched from or somewhere within its overall cross range margin.

Aside from a more liberal landing envelope, the technology necessary for spaceplanes to travel to and from LEO within a single stage may soon be available. British Aerospace or BAE systems has built an air-breathing hypersonic engine capable of reaching Mach 5, at altitudes of over 100,000 feet. Since the engine is multimodal, it then has the ability to switch from using atmospheric oxygen during high-altitude cruise into a more traditional hydrolox rocket engine, with specific impulse in the range of 450 seconds in the near vacuum. While Skylon- or HOTOL-type spaceplanes can only deliver some twenty tonnes of payload into a four hundred kilometer low Earth orbit, that is enough to carry about fifty passengers with carry-on items, so in taking account of how this launch capacity relates to the goal of shuttling humans into space—particularly regarding a reduced-stress launch profile and an increased margin of safety upon landing—it may be time to give these spaceplanes a closer look.

But of course, the advantages don’t end there. Because they launch horizontally and use lift instead of the sheer brute force of a booster stage like Starship, the ascent profile of a spaceplane like Skylon is much gentler than what would be the case on a typical vertical launch rocket system like Starship. Skylon or HOTOL would not require multiple G’s of acceleration since, with atmospheric lift, the engine thrust to weight ratio doesn’t need to exceed 1. This relationship between the lift generated by the spaceplane and the horizontal engine thrust means the trip to orbit is a bit less quick, twenty-two minutes in contrast to the eight minutes a normal rocket system would require, but it is the case that a change in velocity stretched over a greater amount of time will result in less acceleration for the crew and passengers on ascent. For the Skylon system it is something in the range of one g,  instead of the almost three and a half that Starships would likely experience on the way to orbit. While younger, healthier people shouldn’t have too much of an issue with three g’s it could be both unsafe and or extremely uncomfortable for older space tourists, which would just so happen to be the ones wealthy enough to go on the trip in the first place.

While the air-breathing Sabre engine has been in research and development for some time, a recent breakthrough in cooling systems for the engine might mean that Skylon or some derivative of it utilizing the Sabre engine could be making routine flights to orbit within a decade. To me it seems likely that once there are more commercial space stations opening in low Earth orbit, it may be easier for a spaceplane to compete with SpaceX’s Starship in both the markets of orbital transport and tourism.

While we are currently dazzled by the exploits of the Starship testing regime, it is important to note that the more developed the launch industry gets, the more likely it is that competitors arise—and for the myriad reasons stated above, I would not be too surprised if one of those competitors ends up being similar to Skylon. 

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Thank you for reading! I’ll see if I can get Eamon to write some more articles, further down the line. I should be back with another post by next Sunday. Take care, everyone, and I’ll see you then!