One comment I heard from XCOR was that VG developed the vehicle in parallel with engine while industry norm is to build vehicle after engine is developed. VG is discovering the hard way why this is industry norm.
Quote from: meadows.st on 05/26/2014 02:28 amWow! Serious instability at 20 seconds then again between 26-27 seconds and all shock diamonds are gone by 40 seconds.For those of us that may not be up on flow dynamics of rocket exhaust, are shock diamonds a good thing? At least for this fuel combination? What sorts of engines are they a good (or at least acceptable) sign?https://en.wikipedia.org/wiki/Shock_diamond says they are unburnt fuel and a sign of either over or under expansion. No way a non variable nozzle can be correctly expanded at all altitudes so? normal?? Is it worth running non fuel-rich to avoid them?
Wow! Serious instability at 20 seconds then again between 26-27 seconds and all shock diamonds are gone by 40 seconds.
http://paleofuture.gizmodo.com/over-a-decade-of-virgin-galactics-failed-space-age-prom-1575643484
For those of us that may not be up on flow dynamics of rocket exhaust, are shock diamonds a good thing? At least for this fuel combination? What sorts of engines are they a good (or at least acceptable) sign?https://en.wikipedia.org/wiki/Shock_diamond says they are unburnt fuel and a sign of either over or under expansion. No way a non variable nozzle can be correctly expanded at all altitudes so? normal?? Is it worth running non fuel-rich to avoid them?
I would place bets on how long it will take for them to admit they have to go all liquid propulsion, something like LOX/IPA.Why have they not sent out an RFP with desired total system mass and volume constraints is beyond me.I hear SS67B-3 kits are still available to purchase ..
Quote from: TrevorMonty on 05/27/2014 04:13 amOne comment I heard from XCOR was that VG developed the vehicle in parallel with engine while industry norm is to build vehicle after engine is developed. VG is discovering the hard way why this is industry norm.Well the X-15 program did the same back in the days. And the vehicle also had to begin its flight tests with a substitute engine (xlr-11) while waiting for the definite one (xlr-99).Also for solids it is the norm to develop the motor in parallel with the vehicle. STS and Ariane 5 were developed in parallel with their boosters, and that is also true for ballistic missiles.Hydrid are in this aspect akin to solids: you first need a vehicle design to tailor the rubber/nylon block to your needs.
Is it possible that the thrust drop could be an intentional throttle-down built into the design of the fuel grain? Perhaps there is some structural limit (dictated by SS2's design) that they need to keep within and running the engine at full throttle exceeds that limit later in the burn?It also looks to me like the shock diamonds do disappear, but then the exhaust looks like it's getting "dirtier" (ie. more opaque) and so they might still be there, but obscured by the glow of the exhaust products. To my amateur mind that suggests that the engine is throttled down. Maybe they were just testing the throttle limits of the engine?
“We’ve been planning for this,” Whitesides said, adding that Scaled’s permit application to the Federal Aviation Administration includes use of both types of fuel grains. “We’ll be able to go able to go straight up to our PF (Powered Flight) 4 goals … and we’ll just keep expanding the envelope out to about 60 seconds” of engine burn time.
And from a followup article , repeats what was reported before, but this is new i thinkQuote“We’ve been planning for this,” Whitesides said, adding that Scaled’s permit application to the Federal Aviation Administration includes use of both types of fuel grains. “We’ll be able to go able to go straight up to our PF (Powered Flight) 4 goals … and we’ll just keep expanding the envelope out to about 60 seconds” of engine burn time.www.spacenews.com/article/launch-report/40719virgin-galactic-hoping-for-spaceshiptwo-altitude-boost-with-new-fuelThey will skip re-testing the new engine in flight incrementally. So the next flight, whenever that happens, would burn longer than 20 seconds.
Just to confirm, do you read Whitesides' statement as "for the first PF using the new polyamide-based grain, we will perform a burn of XX seconds which is what we have always planned for PF4"? If so, do you think that PF 3 was always planned to be the same burn duration as PF 2?
Quote from: meadows.st on 05/28/2014 09:20 pmJust to confirm, do you read Whitesides' statement as "for the first PF using the new polyamide-based grain, we will perform a burn of XX seconds which is what we have always planned for PF4"? If so, do you think that PF 3 was always planned to be the same burn duration as PF 2?The way i read it was that they'll continue powered flight test program where they left off - and this will mean either the same or longer duration burn in flight. I do not think that PF 3 was planned to be the same length as PF 2, i think it was planned to be longer but they had to reset that based on some PF 2 results, but that is just guessing.However, the spin master that Whitesides is, you can construct his statement to mean whatever later, such as that PF 4 was always planned to make a small burp only and they always planned a series of drop test for some random reason between PF 3 and 4. The man is good at his job.