Reaction Engines

Point out news stories, on the net or in mainstream media, related to polywell fusion.

Moderators: tonybarry, MSimon

93143
Posts: 1131
Joined: Fri Oct 19, 2007 7:51 pm

Re: Reaction Engines

Postby 93143 » Tue Feb 04, 2014 7:26 am

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.

Aero
Posts: 1200
Joined: Mon Jun 30, 2008 4:36 am
Location: 92111

Re: Reaction Engines

Postby Aero » Tue Feb 04, 2014 5:17 pm

That sounds like the engine mass plus nacelle mass from my figures.


Yes - engine plus nacelle mass. But then the engine won't perform very well without the nacelle. I just lump the masses together. Of course it is true that for mass optimization efforts, etc., they are separate entities.


I think the website is wrong.


Something is wrong, and since the spread sheet shows data allowing us to approach the thrust number from multiple directions while the web page simply lists one cold number in isolation, I too suspect the web page number. But this is not the kind of discrepancy I, or REL for that matter, want to discover in the available data.
Aero

93143
Posts: 1131
Joined: Fri Oct 19, 2007 7:51 pm

Re: Reaction Engines

Postby 93143 » Tue Feb 04, 2014 9:45 pm

No one wants to discover a discrepancy in the available data for a promising project. But this is about as innocuous as they come, isn't it? It appears someone writing publicity copy typed "per engine" without really thinking, or perhaps misunderstood a nebulous statement from someone better informed. It's not like discovering someone fudged a number to make the design close.

Aero
Posts: 1200
Joined: Mon Jun 30, 2008 4:36 am
Location: 92111

Re: Reaction Engines

Postby Aero » Wed Feb 05, 2014 2:09 am

this is about as innocuous as they come, isn't it?


True enough, and probably enough said on that subject. Where were we before we side tracked?
Aero

93143
Posts: 1131
Joined: Fri Oct 19, 2007 7:51 pm

Re: Reaction Engines

Postby 93143 » Wed Feb 05, 2014 4:05 am

People were trying to come up with ways to flight-test (or simulate) SABRE. I was wondering what the problem with ground testing is, considering that it's supposed to be followed by a Y-plane or two even before any production prototypes fly.

GIThruster
Posts: 4686
Joined: Tue May 25, 2010 8:17 pm

Re: Reaction Engines

Postby GIThruster » Wed Feb 05, 2014 4:19 am

93143 wrote:I was wondering what the problem with ground-testing is.


TRL5 requires it be tested in a relevant environment, whether one thinks this is necessary or not. Putting an engine that big in a vacuum chamber could cost more than putting it on a ship, so it looks like jumping from TRL4 to TRL 7 may happen.

If memory serves, a few years ago MSFC finished putting the touches on a new thrust stand, eventually intended for testing fission rockets. That might be big enough, though I still doubt it has vacuum. Pretty darn hard to provide vacuum with something spitting out so much. Now if one wanted to fudge, one could provide the vacuum only to the intake. Not sure that's playing fair though and probably thus still qualifies as TRL4.
"Courage is not just a virtue, but the form of every virtue at the testing point." C. S. Lewis

93143
Posts: 1131
Joined: Fri Oct 19, 2007 7:51 pm

Re: Reaction Engines

Postby 93143 » Wed Feb 05, 2014 6:31 am

You don't actually have to fire a rocket engine in vacuum to test it. Everything downstream of the throat is supersonic (except for the boundary layer, which is pretty thin and dominated by the bulk flow), so pressure waves can't travel upstream in the bell, and the main combustion chamber has no way of knowing whether it's firing into a full-length nozzle in vacuum or a truncated nozzle at sea level. And it's not hard to estimate the vacuum performance pretty accurately given the results of such a test.

The inlet is supposed to keep the compressor face at about 1 atmosphere over the whole trajectory (though it doesn't quite do that in the C1 data). It seems to me that a simple air heater would provide the "relevant environment" (as opposed to "operational environment") for the engine core, which gives you TRL 6.

The Valkyrie sounding rocket is going to test an E/D nozzle with correct exhaust composition over the whole Skylon airbreathing trajectory. But they don't actually need the E/D nozzle for Skylon to work, and a bell nozzle isn't really something you need to raise the TRL of at this point in history...

IIRC, REL only use TRL numbers because they're expected to. They don't like them much; they think they can be misleading.

...

The only part I can think of that might require extreme measures to ground-test correctly is the bypass burner ring. (Maybe I'm just uncreative...) But ramjets in general are TRL 10 and not especially complicated, and in any case they've modelled this one pretty conservatively; they've claimed they're keeping it as a sort of "hidden margin" that they can tweak later to get extra performance... They have ramjet experience from of old on the team.

And if they decide flight-testing the nacelle is not needed before the Y-plane flies (which looks like the way they're going), it probably means they've decided to ground-test it. In light of the recent agreement with the Air Force, this might just involve testing the whole engine at speed and altitude on the ground. Reportedly they don't currently have the capability, but I suppose they could build it if they were sufficiently interested...

Or not. It's a pretty major undertaking. But then, so is designing and building an airplane that big...

Remember, according to the current plan, this engine will fly before a Skylon prototype does. Just probably not before a Skylon pre-prototype does... and there's your answer, IMO, regarding an appropriate test airframe.

93143
Posts: 1131
Joined: Fri Oct 19, 2007 7:51 pm

Re: Reaction Engines

Postby 93143 » Sun Feb 09, 2014 11:58 pm

I messed up. The airbreathing net thrust is not Fgross - Mcap*U. It's Fgross - 2*Mcap*U, because Mcap is per nacelle.

Peak airbreathing net thrust is about 200 tonnes for the whole vehicle.

(Weird how the average was 200 tonnes before, but oh well...)


Return to “News”

Who is online

Users browsing this forum: No registered users and 7 guests