Besides what Doug wrote, if I gather correctly from you, Rand; the reason for feathering the wing was due to concerns about the engine. That sounds like a germane aspect as to why the pilot committed the “error”.
You mean early feathering? Not really. It’s more concerns about accelerating all the way to suborbital velocity if they couldn’t feather. They wanted to be sure it would work at the earliest safe moment. But Alsbury depolyed earlier than that.
Yes, early feathering. Thanks for the clarification.
Could someone help me understand their engine philosophy?
Here’s what’s perplexing me; hybrid engines (liquid oxidizer, solid fuel) were chosen for simplicity and reliability, right? They made a trade off; simplicity and reliability for a performance hit, so obviously, they valued the hybrid concept.
However… now they are using liquid fuel too? Doesn’t this give you all the drawbacks of a true liquid engine (something pressure fed and simple, like Kestrel) without any of the benifits? And doesn’t doing this take away ever advantage a hybrid has?
So, my question is, why do this (as seen on this flight) bastardized approach that can’t even really be called a hybrid? Wouldn’t it be easier and cheaper (and safer) to go with a solid, or simple pressure fed liquid, design? Even a solid would have higher ISP, and they do seem to be having a shortfall in that area.
It might also be worthwhile to go for some engine commonality with their planned smallsat launcher. I’d be willing to bet they won’t be going with hybrid for that.
So, my question is, why do this (as seen on this flight) bastardized approach that can’t even really be called a hybrid?
Colloquially, such abominations are sometimes called tribrids.
Wouldn’t it be easier and cheaper (and safer) to go with a solid, or simple pressure fed liquid, design?
If you’re starting from clean sheet, I say yes. But VG decided to build a spaceship and then design the engine around it. SS2 form factor is heavily married to the hybrid. I suspect a switch to liquid biprop would require radical, radical redesign of SS2.
So, the nylon engine used methane and helium. That engine was built by Scaled Composites, who are supposed to be out the project since the crash last year. You might recall that they were really talking up how well the engine performed for the 12 seconds or so before the ship broke up.
Anyway, they now seem to be going back to the rubber engine that Virgin Galactic is working on internally. You will recall that the previous rubber engine had been developed by Sierra Nevada before Virgin blindsided the company by switching to a nylon engine due to supposed better performance. That announcement came on a Friday in May at the beginning of Memorial Day weekend in the U.S. and a bank holiday over in the UK.
The rubber engine that SNC worked on was only fired for 16 and 20 seconds in the first three powered flights. That’s because it caused oscillation and vibration problems. SNC had solved that problem by injecting helium. They conducted a burn in December 2013 that Virgin Galactic released a video of on YouTube. However, it had weight, performance and passenger capacity limitations (tanks in the wings, 50 miles, 4 passengers in the back). And VG was paying SNC a lot of money. The switch to nylon saved them money and brought more money into Scaled Composites.
My guess is despite the horse race claim, Scaled is actually out of the picture and Virgin has resuscitated the rubber engine along the lines of what SNC was working on when they got dumped. (The part about Scaled is a guess; I’m not sure.) Virgin may have been able to coax some enhancements out of the rubber hybrid. However, I think the ship is getting heavier due to modifications required on the feather mechanism. So, I don’t know what sort of performance they will get out of the vehicle.
Doug, first off, thank you for the superb article, plus many others you’ve done.
I’m intrigued that they’re going back to rubber (if they do), given that they had performance issues with it.
I’d sure love to see performance specs for these engines, though I suspect we won’t.
The solid fuel with separate oxidizer prevents large amounts of mixed and unburnt fuel + oxidizer, thus preventing an explosive failure mode you might get from a “hard” start with liquids or certain failures with premixed solids and cracks in the fuel grain.
… unless the oxidizer is temperamental and prone to explosion without mixing with the fuel, in which case this advantage is largely mute.
Besides what Doug wrote, if I gather correctly from you, Rand; the reason for feathering the wing was due to concerns about the engine. That sounds like a germane aspect as to why the pilot committed the “error”.
You mean early feathering? Not really. It’s more concerns about accelerating all the way to suborbital velocity if they couldn’t feather. They wanted to be sure it would work at the earliest safe moment. But Alsbury depolyed earlier than that.
Yes, early feathering. Thanks for the clarification.
Could someone help me understand their engine philosophy?
Here’s what’s perplexing me; hybrid engines (liquid oxidizer, solid fuel) were chosen for simplicity and reliability, right? They made a trade off; simplicity and reliability for a performance hit, so obviously, they valued the hybrid concept.
However… now they are using liquid fuel too? Doesn’t this give you all the drawbacks of a true liquid engine (something pressure fed and simple, like Kestrel) without any of the benifits? And doesn’t doing this take away ever advantage a hybrid has?
So, my question is, why do this (as seen on this flight) bastardized approach that can’t even really be called a hybrid? Wouldn’t it be easier and cheaper (and safer) to go with a solid, or simple pressure fed liquid, design? Even a solid would have higher ISP, and they do seem to be having a shortfall in that area.
It might also be worthwhile to go for some engine commonality with their planned smallsat launcher. I’d be willing to bet they won’t be going with hybrid for that.
So, my question is, why do this (as seen on this flight) bastardized approach that can’t even really be called a hybrid?
Colloquially, such abominations are sometimes called tribrids.
Wouldn’t it be easier and cheaper (and safer) to go with a solid, or simple pressure fed liquid, design?
If you’re starting from clean sheet, I say yes. But VG decided to build a spaceship and then design the engine around it. SS2 form factor is heavily married to the hybrid. I suspect a switch to liquid biprop would require radical, radical redesign of SS2.
So, the nylon engine used methane and helium. That engine was built by Scaled Composites, who are supposed to be out the project since the crash last year. You might recall that they were really talking up how well the engine performed for the 12 seconds or so before the ship broke up.
Anyway, they now seem to be going back to the rubber engine that Virgin Galactic is working on internally. You will recall that the previous rubber engine had been developed by Sierra Nevada before Virgin blindsided the company by switching to a nylon engine due to supposed better performance. That announcement came on a Friday in May at the beginning of Memorial Day weekend in the U.S. and a bank holiday over in the UK.
The rubber engine that SNC worked on was only fired for 16 and 20 seconds in the first three powered flights. That’s because it caused oscillation and vibration problems. SNC had solved that problem by injecting helium. They conducted a burn in December 2013 that Virgin Galactic released a video of on YouTube. However, it had weight, performance and passenger capacity limitations (tanks in the wings, 50 miles, 4 passengers in the back). And VG was paying SNC a lot of money. The switch to nylon saved them money and brought more money into Scaled Composites.
My guess is despite the horse race claim, Scaled is actually out of the picture and Virgin has resuscitated the rubber engine along the lines of what SNC was working on when they got dumped. (The part about Scaled is a guess; I’m not sure.) Virgin may have been able to coax some enhancements out of the rubber hybrid. However, I think the ship is getting heavier due to modifications required on the feather mechanism. So, I don’t know what sort of performance they will get out of the vehicle.
Doug, first off, thank you for the superb article, plus many others you’ve done.
I’m intrigued that they’re going back to rubber (if they do), given that they had performance issues with it.
I’d sure love to see performance specs for these engines, though I suspect we won’t.
The solid fuel with separate oxidizer prevents large amounts of mixed and unburnt fuel + oxidizer, thus preventing an explosive failure mode you might get from a “hard” start with liquids or certain failures with premixed solids and cracks in the fuel grain.
… unless the oxidizer is temperamental and prone to explosion without mixing with the fuel, in which case this advantage is largely mute.
I wrote about this from another angle back in August: Virgin Galactic Misled Ticket Holders, Public on Complexity of Engine Change