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I have read a lot about Skylon which is a single stage to orbit concept space plane with a SABRE engine designed by the UK think tank Reaction Engines limited. However, in recent months it seems like the project has stalled. Last year, the key engineer behind the project, Alan Bond, retired which seems to have led to a reduction in activity and media attention for it.

I think the SABRE engine and Skylon project is also interesting because it could potentially provide a concept for a supersonic high altitude airliner with zero emissions if powered by hydrogen. Reaction engines developed the A2 as a result of the research work done on the Skylon project.

Also, Elon Musk has talked recently about an electric supersonic VTOL aircraft which could fly at very high altitude where there is little air resistance. However, it still seems battery energy density is not sufficient to allow this. Such an aircraft would have to be powered by liquid hydrogen or methane in its launch stage like the Skylon and then land. If it were VTOL it would not need the 6km runway which is required by the Skylon.

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    $\begingroup$ "with zero emissions if powered by hydrogen" -- H2O is a greenhouse gas; SABRE may be zero-carbon, but won't be zero-emission. $\endgroup$
    – Russell Borogove
    Jan 30, 2021 at 23:54
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    $\begingroup$ Yet another reason to fear H2O - the silent killer! $\endgroup$
    – Organic Marble
    Jan 31, 2021 at 1:25
  • $\begingroup$ @RussellBorogove H20 is water. What do you mean H20 is a greenhouse gas? From what I have read hydrogen cars are considered zero emissions because water comes out. I am not sure how it would be with Skylon, but I would be very surprised if it is producing emissions. $\endgroup$
    – The Rocket fan
    Mar 15 at 12:46
  • $\begingroup$ Yes, water vapor is a greenhouse gas. For legal purposes hydrogen powered vehicles are considered “zero emission”, but it’s not technically true. $\endgroup$
    – Russell Borogove
    Mar 15 at 15:04
  • $\begingroup$ @RussellBorogove from what I saw, hydrogen cars release water when used. I do not under why H20 would be considered a green house gas $\endgroup$
    – The Rocket fan
    Mar 16 at 8:31

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The new management at Reaction Engines are distancing themselves from the founders insistence on Skylon as the only way forward.

One concept, the Skylon spaceplane, has been “a little bit of a distraction” in the public eye from the company’s engine development
[according to a spokesperson]
The firm is considering “a number” of concepts, and will speak to vehicle developers after testing of Sabre.

Skylon remains Reaction Engines' reference design, and their software model of it was used, with modifications, in mid 2020 joint papers with CNES investigating SABRE in a TSTO HTOL configuration as art of ESA's future launch program.

Unfortunately there's only so far one can go with an aircraft design before the characteristics of its propulsion system are finalised.

The company is very actively developing the SABRE engine, they completed the high temperature flight scale heat exchanger tests in autumn 2019, simulating the conditions experienced at Mach 5 at their TFT test facilities in Colorado (funded by the USAF) north view

The company is having test facilities built at Westcott (TF1) to test the air-breathing engine, which are expected to be started in 2021 - leading up to this they've had Airborne Engineering test fuel injectors for the preburners (they're somewhat novel, in that they have to work across a large range of hydrogen/air ratios) and they've (separately) tested the preburner heat exchangers.

Work is underway at Cranfield on specifying an engine test vehicle, and Beyern-Chemie are continuing to work on the nacelle equipment (intake, sealing of intake, ducting, ramjets)

While Musk may be working on an improbable electric airplane, Alan Bond, at his new company Mirror Quark, is working on an electric spaceship, pinning his hopes on someone making lightweight, high-power inertial electrostatic confinement fusion a thing in order to produce an electric launch vehicle.

Alan Bond's Discus

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    $\begingroup$ "Electric plane" doesn't really describe it. He apparently wants to build a 15 t flying saucer propelled by plasma thrusters powered by a 1 gigawatt inertially confined fusion reactor that produces 155 MW of x-rays and 7 MW of gamma radiation. epic-src.eu/wp-content/uploads/1.-Keynote-Speech-Alan-Bond.pdf $\endgroup$
    – Christopher James Huff
    Dec 13, 2018 at 1:09
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    $\begingroup$ he's likened it to the Millenium Falcon @ChristopherJamesHuff, I was perhaps mischievous in calling it an electric plane - in response to Musk's electric plane in the OP - it is, instead, a single-state to anywhere... $\endgroup$
    – user20636
    Dec 13, 2018 at 9:26
  • $\begingroup$ Is that thing just a torchship? Seems somewhat absurd. $\endgroup$
    – ikrase
    Apr 13, 2020 at 1:31
  • $\begingroup$ not really @ikrase, in space it has to use internal propellant stores. $\endgroup$
    – user20636
    Apr 13, 2020 at 1:48
  • $\begingroup$ @JCRM so do most sci-fi torchships. $\endgroup$
    – ikrase
    Apr 13, 2020 at 2:10

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