[eksith]

This is a dubious investment IMO.

Rockets that have shrouds have been thrown about for a long time, but they have one fundamental weakness. The shroud only works while in the atmosphere. After that, it's dead weight (since no significant aerodynamic forces come into play further up).

It's not quite the same as the SR-71 Blackbird engine which is a turbojet engine that morphs mid-flight into a ramjet. That's quite a spectacular piece of engineering especially since this was before the advent of computer-aided modeling. But the SR-71 was still very much an air breathing engine. The outer engine casing was still an essential part of its function.

Don't get me wrong; I wish these guys success and I hope my assessment is incorrect, but I just don't see how added weight to win over gravity lower in the atmosphere would help in the upper atmosphere and in space.

[NickPollard]

I don't know the maths involved, but the idea presumably is that they save more weight not carrying (as much) oxygen than they lose by adding the rest of the engine. By being able to draw oxygen from the air whilst in the atmosphere, they only need to store a smaller amount of oxygen for when they hit space. This is the main saving I believe.

[pja]

Indeed. O2 is much heavier than H2 so if you can scavenge the O2 out of the atmosphere on the way up then the amount you need to carry for the final burn in space is much smaller & the weight savings are huge.

Consider that the Space Shuttle external fuel tank contained 650,000kg of O2 and only 100,000 kg of H2. Save 10% of your O2 needs and suddenly you've got 60,000 kg to play with. The Shuttle payload was only about 25,000 kg.

[cpleppert]

There are a lot of weaknesses in the design that haven't been worked out regardless of the technical merit of the engine. Reusable vehicles are tremendously expensive and Sabre has to peform perfectly not only on the engine but the vehicle as well to get any return on its investment.

Although the sabre is more efficient operating in the atmosphere than a conventional rocket engine the vehicle itself will have to spend the cost of having wings, strengthening the vehicle to operate in several orientations on ascent, operating in the atmosphere for longer than a conventional rocket(which costs fuel and requires the vehicle to be shaped in a way that just isn't as efficient as a conventional rocket), and being able to withstand reentry temperatures(more weight) not to mention not being able to get rid of dead weight on ascent. The reusable components of the engine also weigh a lot more as well. If you go through everything it doesn't look like that great a win; and that is if everything works perfectly.

The space shuttle never came close to its projections and this vehicle is far more technically ambitious in every possible way. I just don't think that their projections of even $10-15 billion in development costs are even remotely realistic. Last time I checked their business plan suggested they could break even if satellite launch costs didn't come down and the volume of launches of large satellites increase.

[chromaton]

There's also the drag effects to worry about which pure rockets don't have to deal with.

This engine could end up being useful in the long run, but the clear path to cheap access to space involves reusability, and a more complex motor works against that.

[stcredzero]

However, the ability to avoid staging is also a huge win.

[disputin]

It'll be fine - "Mr Osborne himself has inspected the test rig"!? The UK gov will jump at any potential economy booster ('scuse) right now.