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On Jan/5/2020 at 09:19, Jeff Findley wrote :
In article , says... On 2020-01-04 21:40, Greg (Strider) Moore wrote: No, you're missing the point. The propellant was for one OR the other. I understand that. Rephrasing my question: Propellant tanks would be sized for the biggest needs of abort OR landing. If landing requires more fuel than an abort, then the tanks would be sized to support landing. In such a scenario, removing propulsive landing would allow SpaceX to shrink the propellant tanks for super dracos to only be able to do aborts. If both functions need the same amount of fuel, then removing landing doesn't change tank size. (same if Abort uses more fuel than landing at which point those tanks are sized to meet the needs of the abort function). In either case having a propellant reserve is a "good thing". You might rephrase the question as which would have had a bigger reserve. But you won't get an answer. This is because SpaceX will have tested both a pad abort and an in flight abort, so they'll have solid numbers for that. But, for propulsive landing, they'll only have simulations and some ground testing to determine how much propellant is required. Without actually testing propulsive landing (repeatedly in different situations like varying wind speed), they can't be 100% confident in exactly how much reserves they'd have in a landing scenario. The big picture question: Say Dragon2 has proven itself. How difficult would it be for SpaceX to build a prototype that can propulsively land? If tanks already big enough, then it is a matter of adding the landing gear and changing software. Yes, but they *won't* do this because the customer, NASA, simply doesn't want this and won't pay for this. NASA is one customer. Bigelow or mister so and so might become other customers. I think Mr Mezei's question here is pertinent. Other customers might very well think that landing (on land) is preferable to splashdown. Initially SpaceX was talking about having their Dragon capsules do landings. I think they switched to splashdowns at the request of NASA and not because there was some difficulty preventing landings. I, like Mr Mesei, would like to know if going back to landing is a possibility. Even if there were no difficulty preventing landings early on in the design, it doesn't mean that going back to that scenario now wouldn't basically implicate designing a new capsule. I don't know, but I suspect it would mean a big design change. But possibly SpaceX kept in mind all along that other customers might later ask for landing. That said, I think that if SpaceX had kept its design compatible with going back to landings, we would know about it. You don't design your spacecraft with some interesting capabilities and don't tell anyone. If you want to have customers, you shouldn't keep secret the capabilities of your product. You keep secret how you managed to have those capabilities but not the capabilities themselves. Alain Fournier |
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In article , says...
On Jan/5/2020 at 09:19, Jeff Findley wrote : In article , says... The big picture question: Say Dragon2 has proven itself. How difficult would it be for SpaceX to build a prototype that can propulsively land? If tanks already big enough, then it is a matter of adding the landing gear and changing software. Yes, but they *won't* do this because the customer, NASA, simply doesn't want this and won't pay for this. NASA is one customer. Yes, but more importantly, NASA is the most important customer for Dragon 2. In fact, it's the only customer who's paying for actual Dragon 2 missions. All other customers are currently *potential* customers. Bigelow or mister so and so might become other customers. Agree, *might*. I think Mr Mezei's question here is pertinent. Other customers might very well think that landing (on land) is preferable to splashdown. Yes, but only if they're going to actually *pay* to develop propulsive landing. SpaceX proposed testing this using Dragon 2 cargo missions. So to SpaceX, the cost of testing propulsive landings would have been very small since they were going to fly those cargo missions *anyway*. But because NASA is paying for those Dragon 2 cargo missions, and they want to make sure they get valuable hardware back like EMU suits, NASA balked at that proposal. The problem is you can't really test propulsive landings with a crew on board. So customers who want Dragon 2 for crewed flight would have to pay for entire test flights of Dragon 2 propulsive landings. Maybe if a customer like Bigelow agreed to do this with returning cargo missions to their space stations it would work. And in the meantime, any crewed missions paid for by Bigelow would still be splashing down int the ocean. Initially SpaceX was talking about having their Dragon capsules do landings. I think they switched to splashdowns at the request of NASA and not because there was some difficulty preventing landings. I, like Mr Mesei, would like to know if going back to landing See above. They switched because the customer, NASA, didn't want SpaceX testing propulsive landings using returning cargo missions. If you can find a source for other returning cargo missions where a different customer will allow this sort of testing, it *may* be possible to develop without spending a fortune on dedicated test flights. That said, I think that if SpaceX had kept its design compatible with going back to landings, we would know about it. You don't design your spacecraft with some interesting capabilities and don't tell anyone. If you want to have customers, you shouldn't keep secret the capabilities of your product. You keep secret how you managed to have those capabilities but not the capabilities themselves. The biggest change was when SpaceX announced that cargo Dragon 2 would *not* even have Super Dracos installed. This allows for more cargo mass to be delivered to ISS. Any potential customer for Dragon 2 cargo flights would have to sacrifice some cargo delivery capability on every flight in order to test propulsive landings. At that point, it becomes a cost trade. In other words, is the extra cost of testing propulsive landings (in terms of less cargo delivered on each flight) worth it in the long run to this (currently hypothetical) new customer? On top of that, since this would be a development program, there isn't certainty over how many test flights would be needed to make the system safe enough to return people. So there is risk that money would be spent to develop a capability that might not ever mature. And finally, you're talking about developing a capability NASA has already made a hard pass on. That's an uphill battle to fight in and of itself since they're the "experts" in crewed spaceflight. So, you've got to find investors who will bet *against* NASA's "experts". This has been a problem in the industry for decades. It has taken billionaires with deep pockets to even start to break this cycle. Jeff -- All opinions posted by me on Usenet News are mine, and mine alone. These posts do not reflect the opinions of my family, friends, employer, or any organization that I am a member of. |
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On 2020-01-06 14:19, Jeff Findley wrote:
SpaceX clearly sees Dragon 2 as a stepping stone to Starliner, ... Ahem. I assume you mean "Starship", don't you? (Damn this hyperbolic fashion of using "star" in naming spacecraft not designed for interstellar travel.) -- Niklas Holsti Tidorum Ltd niklas holsti tidorum fi . @ . |
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Niklas Holsti wrote:
On 2020-01-06 14:19, Jeff Findley wrote: SpaceX clearly sees Dragon 2 as a stepping stone to Starliner, ... Ahem. I assume you mean "Starship", don't you? (Damn this hyperbolic fashion of using "star" in naming spacecraft not designed for interstellar travel.) Heh. The Starship is more of a Starliner than the Starliner. The Starliner would more properly be named Starrowboat. -- Mvh./Regards, Niels Jørgen Kruse, Vanløse, Denmark |
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