Aero wrote:From the spread sheet it is hard to see how that could be right but the documents were both from late 2012 so that's not it.
Actually, based on the metadata the spreadsheet looks like it was created in 2004. It describes the Skylon C1 configuration, meaning the engine is probably the SABRE 2. The SABRE 3 incorporates numerous technological advances, and the SABRE 4 is a cycle rearrangement that uses a lot less excess hydrogen. So the engine isn't the same now as it was then.
It is odd, though, that the website says 300 tonnes per engine in rocket mode, when the spreadsheet indicates pretty much exactly half that...
calculating the thrust from mass times acceleration given
What I meant by "net thrust" is just the gross thrust minus the inlet drag. In this case, since the inlet is shock-on-lip, there is no spillage drag, and the inlet drag is therefore simply Mcap*U, the captured mass flow rate times the vehicle forward speed.
It still doesn't get up to 200 tonnes per engine, but it does get past 140. Note that Skylon D1 is close to 20% heavier than C1; maybe the engine is bigger...
...but no, it turns out that in the spreadsheet, the time-averaged net thrust during the airbreathing phase is almost exactly 200 tonnes. So we have the same story as with rocket mode...
I think the website is wrong.
And I did find another source for the mass, that being just over seven tonnes per engine.
That sounds like the engine mass plus nacelle mass from my figures.