2. Elimination of solar arrays by replacement with methane-LOX fuel cells.
Don't get sarcastic about this thread.
Question for the aerospace-knowledgeable types: what are the problems in scaling up a shape like that of Dragon (or Soyuz or Apollo, for that matter)? Do you gain anything in structural weight versus volume, or do other considerations such as heatshield mass and design make scaling a dead-end? Is there any rule-of-thumb for when you are better off abandoning a shape and trying something completely different?
Quote from: darkenfast on 01/28/2014 04:14 pmQuestion for the aerospace-knowledgeable types: what are the problems in scaling up a shape like that of Dragon (or Soyuz or Apollo, for that matter)? Do you gain anything in structural weight versus volume, or do other considerations such as heatshield mass and design make scaling a dead-end? Is there any rule-of-thumb for when you are better off abandoning a shape and trying something completely different?Certainly not aerospace-knowledgeable, but common sense says that as the mass increases, the square-cube law dictates that the capsule's diameter must increase more quickly than the height, if you want the terminal velocity to remain the same. So a significantly larger Dragon might look more like the MSL entry shape:
A Dragon-? My call is that the diameter would be the same 5.2m as the Falcon 9 fairing and would be designed for full re-usability. However, I would also expect Dragon-3 to be only an Earth-to-orbit vehicle. The Mars orbiter and lander would be totally different machines, although the lander might have some software commonality and I'd expect them all to use Dracos for RCS purposes.Dragon 3 might not be a ballistic capsule.
Quote from: darkenfast on 01/28/2014 04:14 pmQuestion for the aerospace-knowledgeable types: what are the problems in scaling up a shape like that of Dragon (or Soyuz or Apollo, for that matter)? Do you gain anything in structural weight versus volume, or do other considerations such as heatshield mass and design make scaling a dead-end? Is there any rule-of-thumb for when you are better off abandoning a shape and trying something completely different?According to some, this is part of the problem with Orion's mass budget. They thought that scaling up the Apollo shape would be easy and simple, but it is proving more difficult than expected. It is much heavier than originally planned, and will have to go on a diet to avoid overloading the parachutes.
Why would it be 5.2m? If we're talking about a larger, more advanced crew module, we're probably talking about something beyond the FH generation, which Dragon 2 will cover just fine. Might as well go for the 7-9 meter MCT version and carry a crowd of passengers.
is there a reason you couldn't launch a 7m capsule on FH?
Why would it be 5.2m? ... Might as well go for the 7-9 meter MCT version and carry a crowd of passengers.
Also, I remember someone (not me) fretting pretty hard about egress issues if too many people are in a capsule.
I can't help thinking about all this "escaping from a Dragon" to where? You won't get me to open the hatch in space unless it is properly attached to a habitat of some sort. On the way up? Nope. After an abort and you are in the water, yes, but not in a panic then, just waiting for rescue.
I think the capsule form is nearing its practical end. 3.6m, 5.2m, or 7-8m still have limitations that are sub-optimal. I would hope that a longer, narrower shape would be used with the heat shield on long axis -- maybe an ellipsoidal or modified, flattened cylinder reentering like Shuttle or Dream Chaser, but without wings. Control surfaces of some sort would be probably needed, maybe deployable. The long aspect ratio would seem to be much more scalable to large volumes than the capsule form. High speed electronic stability controls (of thrusters, engines, control surfaces, or some combination) can make a rock flyable as evidenced by the F117 Nighthawk.
Somewhere along the line it will be time for a 2001 style space plane. With ~50 tonne LV payload we may be able to get ~25 people to orbit.
If Orion is already exceeding it's parachute weight limits, is that a hard limitation for capsule weight? Above a certain weight do you need to use a winged vehicle or lifting body instead of parachutes?Or are their other approaches to extend the weight limits with parachutes?
Quote from: randomly on 01/30/2014 01:49 amIf Orion is already exceeding it's parachute weight limits, is that a hard limitation for capsule weight? Above a certain weight do you need to use a winged vehicle or lifting body instead of parachutes?Or are their other approaches to extend the weight limits with parachutes?I'm not sure but I think Orion has run into a combination of factors that's limiting its parachute weight. If I recall correctly, there isn't enough room in the nose bay for a fourth 'chute because of the steep sides of the vehicle imposed by using a scaled-up Apollo geometry. The Orion is almost too heavy to land safely in the event of the failure of one of the three 'chutes (which happened to Apollo 12, IIRC). So, the Orion design has come up to a hard limit of mass. However, alternate capsule geometries wouldn't have this problem. A capsule with bigger 'chute bays should be able to get to higher weights.
Quote from: AncientU on 01/30/2014 12:55 amI think the capsule form is nearing its practical end. 3.6m, 5.2m, or 7-8m still have limitations that are sub-optimal. I would hope that a longer, narrower shape would be used with the heat shield on long axis -- maybe an ellipsoidal or modified, flattened cylinder reentering like Shuttle or Dream Chaser, but without wings. Control surfaces of some sort would be probably needed, maybe deployable. The long aspect ratio would seem to be much more scalable to large volumes than the capsule form. High speed electronic stability controls (of thrusters, engines, control surfaces, or some combination) can make a rock flyable as evidenced by the F117 Nighthawk.Perhaps something like the IXV.
Quote from: Ben the Space Brit on 01/30/2014 11:38 amQuote from: randomly on 01/30/2014 01:49 amIf Orion is already exceeding it's parachute weight limits, is that a hard limitation for capsule weight? Above a certain weight do you need to use a winged vehicle or lifting body instead of parachutes?Or are their other approaches to extend the weight limits with parachutes?I'm not sure but I think Orion has run into a combination of factors that's limiting its parachute weight. If I recall correctly, there isn't enough room in the nose bay for a fourth 'chute because of the steep sides of the vehicle imposed by using a scaled-up Apollo geometry. The Orion is almost too heavy to land safely in the event of the failure of one of the three 'chutes (which happened to Apollo 12, IIRC). So, the Orion design has come up to a hard limit of mass. However, alternate capsule geometries wouldn't have this problem. A capsule with bigger 'chute bays should be able to get to higher weights.Chutes are an anachronism... as are 60s era LASs that are jettisoned (after wasting lots of fuel).
I'm not sure but I think Orion has run into a combination of factors that's limiting its parachute weight. If I recall correctly, there isn't enough room in the nose bay for a fourth 'chute because of the steep sides of the vehicle imposed by using a scaled-up Apollo geometry. The Orion is almost too heavy to land safely in the event of the failure of one of the three 'chutes (which happened to Apollo 12, IIRC). So, the Orion design has come up to a hard limit of mass. However, alternate capsule geometries wouldn't have this problem. A capsule with bigger 'chute bays should be able to get to higher weights.
The F9R upper stage reentry concept implies that SpaceX has a solution for reentering and propulsively landing cylindrical vehicles from earth orbit. If they can do that, then why not make "Dragon 3" a cylindrical pressure vessel with aft-mounted propulsion? Simple to manufacture, efficient lightweight structure, and relatively low ballistic coefficients at high angles of attack. It would be characteristic of SpaceX to integrate the upper stage and design its propellant system to also drive RCS thrusters.Remember, Elon said that the exciting thing about Raptor isn't the engine, it's the spaceship it's attached to...
Ok, so can you describe it a little better? Would it fit a vacuum-exposed payload bay? Do you think cryo (methalox) oms/rcs is a necessity?Would it functionally be a third stage that does mundane tasks like releasing satellites at GTO and then returning to earth or is it for human transport only?
Quote from: butters on 01/31/2014 01:52 amThe F9R upper stage reentry concept implies that SpaceX has a solution for reentering and propulsively landing cylindrical vehicles from earth orbit. If they can do that, then why not make "Dragon 3" a cylindrical pressure vessel with aft-mounted propulsion? Simple to manufacture, efficient lightweight structure, and relatively low ballistic coefficients at high angles of attack. It would be characteristic of SpaceX to integrate the upper stage and design its propellant system to also drive RCS thrusters.Remember, Elon said that the exciting thing about Raptor isn't the engine, it's the spaceship it's attached to...So you're thinking of something with a layout similar to the old t/Space CEV concept, but on a much larger scale?
