Scale model. Skylon is a British design for an unmanned Single Stage to Orbit (SSTO) spaceplane designed to reach Earth orbit where it would deploy and retrieve satellites. It was intended to be completely reusable, with the objective of cutting the cost of launches dramatically. It was envisaged that some 30 Skylon vehicles would be built, and operated by commercial companies, and it was planned that Skylon could be adapted for carrying passengers, or ‘space tourists’. At the heart of the Skylon concept were its two innovatory Synergestic Air Breathing and Rocket Engines (SABREs) which were designed to draw oxygen directly from the atmosphere. Skylon is one example of several SSTO launcher concepts considered worldwide in the 1990s.
Rarely does something get developed which is a real game changer in space exploration. One example is the Skylon reusable single-stage-to-orbit spaceplane. Powered by the hypersonic SABRE engine it operates like a jet engine at low altitude and more like a conventional rocket at high altitude. Sadly, ‘Reaction Engines’ the company that designs the engines has filed for bankruptcy.
An artist's conception of Reaction Engines' Skylon spacecraft. Credit: Reaction Engines
When it comes to the future of space exploration, the name of the game is “save money”. To do this, space agencies and aerospace companies around the world are investing in things like reusable rockets, single-stage-to-orbit (SSTO) rockets, and reusable space planes. This last concept builds on the tradition established by the Space Shuttle and Buran spacecraft, two reusable vehicles designed to make space launches more affordable.
The one drawback of these spacecraft was the fact that it still took two rocket boosters and a huge external fuel tank to put them into orbit. This is where the Synergetic Air Breathing Rocket Engine (SABRE) comes into play. With the help of the European Space Agency (ESA) and the UK Space Agency (UKSA), this revolutionary hypersonic engine recently took a big step towards fruition.
Now, don’t get me wrong, Science Fiction is awesome. Like almost everyone working in the field of space and astronomy, I was deeply influenced by science fiction. For me, it was Star Trek and Star Wars. I had a toy phaser that made this awesome really loud phaser sound, and I played with it non-stop until it disappeared one day. And I was sure I’d left it in the middle of my floor, like I did with all my toys, but I found it a few years later, hidden up in a closet that I couldn’t reach. And I always wondered how it got there.
Anyway, back to science fiction. For all of its inspiration, science fiction has put a few ideas into our brains which aren’t entirely helpful. You know, warp drives, artificial gravity, teleportation, and rockets that take off, fly to space, visit other planets orbiting stars, land again.
The Millennium Falcon, Firefly, and Enterprise Shuttles are all examples of single stage to orbit to orbit spacecraft, or SSTOs.
Consider the rockets that exist in reality, you know, the Atlases, Falcons and Deltas. They take off from a launch pad, fly for a bit until the fuel is used up in a stage of the rocket, then they jettison that stage and thrust with the next stage. The mighty Saturn V was so powerful that it had three stages, as it made it’s way to orbit.
Diagram of Saturn V Launch Vehicle. Credit: NASA/MSFC
As we discussed in a previous article, SpaceX is working to make the first stage, and maybe even the second stage reusable, which is a vast improvement over just letting everything burn up, but there are no rockets that actually fly to orbit and back in a single stage. In fact, using the technology we have today, it’s probably not a good idea.
Has anyone ever worked on a single stage to orbit? What technological advances will need to happen to make this work?
As I said earlier, a single stage to orbit rocket would be something like the Millennium Falcon. It carries fuel, and then uses that fuel to fly into orbit, and from world to world. Once it runs out of fuel, it gets filled up again, and then it’s off again, making the Kessel Run and avoiding Imperial Blockades.
This concept of a rocket matches our personal experience with every other vehicle we’ve ever been in. You drive your car around and refuel it, same with boats, airplanes and every other form of Earth-based transportation.
But flying into space requires the expenditure of energy that defies comprehension. Let me give you an example. A Falcon 9 rocket can lift about 22,800 kilograms into low-Earth orbit. That’s about the same as a fully loaded cement truck – which is a lot.
SpaceX Falcon 9 poised for Jan. 14, 2017, Return to Flight launch from Vandenberg Air Force Base in California carrying ten Iridium NEXT comsats to orbit. Credit: SpaceX
The entire fueled Falcon 9 weighs just over 540,000 kg, of which more than 510,000 kgs of it are fuel, with a little extra mass for the engines, fuel tanks, etc. Imagine if you drove a car that was essentially 95% fuel.
The problem is specific impulse; the maximum amount of thrust that a specific kind of engine and fuel type can achieve. I’m not going to go into all the details, but the most efficient chemical rockets we have, fueled by liquid hydrogen and oxygen, can just barely deliver enough thrust to get you to orbit. They have a maximum specific impulse of about 450 seconds.
Because the amount of fuel it takes to launch a rocket is so high, modern rockets use a staging system. Once a stage has emptied out all its fuel, it detaches and returns to Earth so that the second stage can keep going without having to drag along the extra weight of the empty fuel tanks.
After stage separation of the Falcon 9 rocket, flames are barely visible around nozzle as the second stage engine ignites and the first stage falls back to the Earth below. Credit: SpaceX
You might be surprised to know that many modern rockets are actually capable of reaching orbit with a single stage. The problem is that they wouldn’t be able to carry any significant payload.
At the end of the day, considering the chemical rockets we have today, the multi-staged profile is the most efficient and cost-effective strategy for carrying the most payload to space for the lowest cost possible.
Has anyone tried developing SSTOs in the past? Definitely. Probably the most widely publicized was NASA’s X-33/VentureStar program, developed by Lockheed Martin in the 1990s.
The proposed X-33 spacecraft. Credit: NASA
The purpose of the X-33 was to test out a range of new technologies for NASA, including composite fuel tanks, autonomous flight, and a new lifting body design.
In order to make this work, they developed a new kind of rocket engine called the “aerospike”. Unlike a regular rocket engine which provide a fixed amount of thrust, an aerospike could be throttled back like a jet engine, using less fuel at lower altitudes, where the atmosphere is thickest.
The test of twin Linear Aerospike XRS-2200 engines, originally built for the X-33 program, was performed on August 6, 2001 at NASA’s Sternis Space Center, Mississippi. The engines were fired for the planned 90 seconds and reached a planned maximum power of 85 percent. Credit: NASA’s Marshall Space Flight Center
Lockheed Martin was working on a 1/3rd scale prototype, but they struggled with many of the new technologies. In the end, their failure to be able to build a composite fuel tank that could contain the liquid oxygen and hydrogen forced them to abandon the project.
Even if they could get the technology working, so the X-33 was fully reusable, its ability to carry a payload would have been dramatically lower than a traditional multi-staged rocket.
In order to really achieve the dream of single stage to orbit, we need to step away from chemical rockets and move to a type of engine that can deliver thrust more efficiently.
We know that jets work more efficiently than rockets, because they only need to carry fuel. They pull oxygen in from the atmosphere, to burn the fuel. So one intriguing idea is to make a rocket that acts like a jet engine while in the atmosphere, and then acts like a rocket once it’s out in space.
And that’s the plan with the British Skylon rocket. It would take off from a regular runway, accelerate to about 6,600 km/h reaching an altitude of 26 kilometers. All this time, its SABRE engine would be pulling in oxygen from the atmosphere, combining it with hydrogen fuel.
An artist’s conception of Reaction Engines’ Skylon spacecraft. Credit: Reaction Engines
From this point, it would switch over to an internal liquid oxygen tank to provide oxidizer, and complete the flight to orbit. All the while using the same flexible SABRE engine. Once in orbit, it would release its 15-tonne payload and then return to Earth, landing on a runway like the space shuttle orbiter did. It’s a really creative idea.
Unfortunately, the development of the Skylon has taken a long time, with shrinking budgets limiting the amount of tests they’ve been able to do. If everything goes well, the first prototype might fly within a few years, so stay tuned to this story.
Another idea which has had some testing is the idea of a nuclear rocket. Unlike a chemical rocket, which burns fuel, and blasts it out the back for thrust, a nuclear rocket would carry a reactor on board. It would heat up some kind of working fuel, like liquid hydrogen, and then blast it out the back for propulsion.
The key elements of a NERVA solid-core nuclear-thermal engine. Credit: NASA
NASA did some tests a few decades ago with a nuclear thermal rocket called NERVA, and found that they could sustain high levels of thrust for very long periods of time. Their final prototype, provided continuous thrust for over 2 hours, including 28 minutes at full power.
NASA calculated that a nuclear-powered rocket would be roughly twice as efficient as a traditional chemical rocket. It would have a specific impulse of more than 950 seconds. But flying a nuclear rocket into space comes with a significant downside. Rockets explode. It’s bad when a chemical rocket explodes, but if a nuclear reactor detonated while making its way up through the atmosphere, it would rain down radioactive debris. For now, that’s considered too much of a risk; however, future interplanetary missions may very well use nuclear rockets.
There’s one more exotic fuel system that’s really exciting – metallic hydrogen. This solid form appears naturally at the heart of Jupiter, under the incredible pressure of the planet’s gravity. But earlier this year, researchers at Harvard finally created some in the lab. They used a tiny vice to squeeze hydrogen atoms with more force than the pressures at the center of the Earth.
Microscopic images of the stages in the creation of atomic molecular hydrogen: Transparent molecular hydrogen (left) at about 200 GPa, which is converted into black molecular hydrogen, and finally reflective atomic metallic hydrogen at 495 GPa. Credit: Isaac Silvera
It took an enormous amount of energy to squeeze hydrogen together that tightly, but in theory, once crafted, it should be relatively stable. And here’s the best part. When you ignite it, you get that energy back.
If used as a rocket fuel, it would provide a specific impulse of 1700 seconds. Compare that to the mere 450 from chemical rockets. A rocket powered by metallic hydrogen would easily get to orbit with a single stage, and travel efficiently to other planets.
Single Stage to Orbit rockets would be awesome. Science fiction has foretold it. That said, at the end of the day, whatever gets the most amount of payload into orbit for the lowest price is the most interesting rocket system. And right now, that’s staged rockets.
However, a bigger issue might be reliability and reusability. If you can get a single vehicle that takes off, travels to orbit and then returns to its launch pad, you can’t get anything simpler than that. No rockets to restack, no barges to navigate. You just use and reuse the same system again and again, and that’s a really exciting idea.
Right this moment, reusable staged rockets like SpaceX has the edge, but if and when the Skylon gets flying, I think we’ll have some serious competition.
Once we master metallic hydrogen, spaceflight will look very very different. Science reality will nearly match science fiction, and I’ll finally be able to fly my own personal Millennium Falcon.
Reaction Engine's Skylon spaceplane explained in the '10 hundred words' that people use the most. Credit: celerycoloured on reddit.
Just about everyone can claim to being inspired by the xkcd webcomic — or at least enjoying a shared nerd moment. A favorite comic of many space fans is the Up Goer Five poster, which explains the Saturn V rocket in the “10 hundred” (i.e. 1,000) English words that are used most frequently.
Now, one fan of xkcd, celerycoloured on reddit has created a poster of the single-stage-to-orbit Skylon spaceplane being built by the British company Reaction Engines Limited (REL). With some great hand-drawn graphics, this homage poster uses — again — the most commonly used words in the English language to explain Skylon:
“The first space car that may go to space without parts falling off” describes the single stage to orbit idea, and “this space car can burn the air outside so it doesn’t have to take so much breathing type air or drop parts” explains the air-breathing SABRE engines.
“I’d just like people to know more about REL,” celerycoloured told Universe Today. “Their efforts towards reusable SSTO spacecraft is one of the coolest things I’ve seen!”
In other space plane news today, ESA’s Intermediate eXperimental Vehicle IXV successfully launched on a Vega rocket from the Kourou space center for a one hour forty minute test flight. This is a lifting body vehicle similar to the X37B, however the IXV splashed down in the ocean for this first flight.
Artist impression of ESA’s Intermediate eXperimental Vehicle (IXV). Credit: ESA – J. Huart.
An artist's conception of Reaction Engines' Skylon spacecraft. Credit: Reaction Engines
Air-breathing rocket engine. Need we say more?
The technology, which sounds straight out of a science-fiction movie, has enough reality to it for the United Kingdom government to offer $90.62 million (£60 million), in stages, to a company looking to develop the engine.
The money will go to Oxfordshire-based Reaction Engines, which we’ve seen on Universe Today before. They’re also developing an unpiloted and reusable spacecraft called Skylon, which is intended for low Earth orbit after leaving the planet from a conventional runway.
Skylon isn’t flight-ready yet, but so far the project did pass a United Kingdom Space Agency technical assessment. If completed, the UK Space Agency says Skylon is just one of many vehicles that could use this engine, which is called Sabre.
“The unique engine is designed to extract the oxygen it needs for low atmosphere flight from the air itself, paving the way for a new generation of spaceplanes which would be lighter, reusable and able to take off and launch from conventional airport runways,” the agency stated.
The money, stated Reaction Engines founder Alan Bond, will fund “the next phase in the development of its engine and heat management technology.” More specifically, this is what the company plans to use the funds for:
– Engine technical design work;
– Improving lightweight heat exchanger technology and manufacturing;
– Performing wind tunnel and flight testing of engine components;
– Doing a “ground demonstration” of the engine.
If all stays to schedule, Reaction Engines expects a Sabre prototype will be ready in 2017, with flight tests commencing in 2020.
A cutaway view of the proposed Sabre engine, which is being developed by Oxfordshire-based Reaction Engines. Credit: Reaction Engines
The major goal of Sabre is to use hot air entering the engine to obtain the required oxygen for operations, rather than carrying the gas separately on board. The engine is supposed to switch to a “rocket mode” at 26,000 feet in altitude.
“This advantage enables a spaceplane to fly lighter from the outset and to make a single leap to orbit, rather than using and dumping propellant stages on the ascent – as is the case with current expendable rockets,” the UK Space Agency stated.
Reaction Engines promises Skylon would give “reliable access to space” through carrying payloads of up to 15 tonnes, but at only 2% of the cost of more conventional launch vehicles — namely, rockets. It remains to be seen if they will achieve that cost goal, but the funding is welcome news nonetheless for the company.
An artist's conception of Reaction Engines' Skylon spacecraft. Credit: Reaction Engines
After 30 years of development, the UK and European space agencies have given a go for the Skylon Spaceplane.
The Skylon, which is being developed at the Oxfordshire-based Reaction Engines in the UK, is an unpiloted and reusable spacecraft that can launch into Low Earth Orbit after taking off from a conventional runway.
Looking like something out of Star Wars, Skylon is a self contained, single stage, all in one reusable space vehicle. There are no expensive booster rockets, external fuel tanks or huge launch facilities needed.
The vehicle’s hybrid SABRE engines use liquid hydrogen combined with oxygen from the atmosphere at altitudes up to 26km and speeds of up to Mach 5, before switching over to on-board fuel for the final rocket powered stage of ascent into low Earth orbit.
The Skylon is intended to cut the costs involved with commercial activity in space, delivering payloads of up to 15 tons including satellites, equipment and even people into orbit at costs much lower than those that use expensive conventional rockets.
Once the spacecraft has completed its mission, it will re-enter Earth’s atmosphere and return to base, landing like an airplane on the same runway, making it a totally re-usable spaceplane, with a fast mission turn around.
Skylon has received approval from a European Space Authority panel tasked with evaluating the design. “No impediments or critical items have been identified for either the Skylon vehicle or the SABRE engine that are a block to further development,” the panel’s report concludes.
“The consensus for the way forward is to proceed with the innovative development of the engine which in turn will enable the overall vehicle development.”
The UK Space Agency says that Reaction Engines will carry out an important demonstration of the SABRE engine’s key pre-cooler technology later this summer.
