John,
On the 24" engine costs, is this cost mostly in the catalysts?
About half of it is catalyst.
Is the cost of a nozzle machined out of solid bar stock becoming a major cost? At this size does the nozzle cost make other fabrication methods more attractive, such as spin forming or forging?
Our current fabricator (Entek) will max out at about 17" diameter billets. We can still make the entrance profile, throat, and expansion cone for an 8" throat nozzle inside that, but we will have to weld it to an intermediate section of dished tank end to connect to a 24" diameter catalyst pack.
Spin forming doesn't seem like it is going to have a winning point over machining today. The tooling cost is more expensive than machining several engines, and it still has to be made in two sections, so even mass produced it may be more expensive than billet machining. None of the companies I spoke to would be able to spin form half inch thick 316 stainless into a 24" engine, either.
For larger nozzles, it is going to have to be rolled and welded plate, either with a separately machined nozzle throat section, or possibly machining the rolled and welded joint.
We are considering an alternate nozzle strategy: lots of little 1" throat nozzles welded between two water jet cut plates. This has cost and fabrication benefits, but it also has another really significant benefit: any flow separation from low chamber pressures would tend to be randomly distributed, rather than the entire engine thrust heading off at a 15 degree vector, and we can position nozzles so the core jet is right over the jet vanes, even at very low chamber pressures. Moving the jet vanes out to the diameter of the catalyst pack instead of the diameter of the nozzle exit would also give us 50% more roll authority.
The engine temperatures look low enough that perhaps the last section of nozzle could be made with composite materials, perhaps saving some cost and weight.
Because of the relatively low chamber pressures, we don't have much expansion ratio, so the exhaust hasn't cooled off all that much by the end of the nozzle. It is still around 1000 F. Sometimes the bottoms of our nozzle look cool because of flow separation inside the nozzle.
Now that you may not be so tight on time for development, have you considered doing more ground bench tests for the engines, to work out engine characteristics more?
Yes, but flight testing is more important. If the engines are working "good enough", we want to concentrate on vehicles. We could spend an arbitrary amount of time improving and testing the engines.
What is your prospects for the flight waver? How difficult is it to get and how much might it allow you to do?
It looks like we should have it fairly soon, but only for flights at the southwest regional spaceport in New Mexico, which is a long trip. We are probably going to get a 120 second burn time waiver, which will cover us for everything we can do under the 200,000 lb-sec impulse limit. AST isn't requiring a full environmental assesment for the waiver, but they are requiring some level of official environmental work, which is why we can't get one for our property in Texas.
With out the waver would you be allowed longer burn times as long as the vehicle is tethered? Have you looked at using some of the new really strong but light and felxable cabling that could be spooled out to allow a long relatively high flight with minimal interference?
I don't think that would be looked on with favor, and we don't want to irritate AST.
In many ways Bert beating everyone may be a good thing as I've been a bit concerned that the X-Prize cutoff date may have been pushing people a bit too hard and forcing people to take too much risk and jeopardize a future industry.
Really, I don't think there are any other teams in a position to fly. I doubt any other vehicles will even be capable of flight this year, and even with statements to the contrary, I don't think anyone is dumb enough to actually get in a vehicle on its first flight.
John Carmack
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