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#940 by GreenShrike on 07 Mar, 2017 18:21
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First, what do you need? Looking at the numbers for Apollo 12, we see 3150 m/s for TLI, 880 m/s to enter lunar orbit, and 923 m/s to return to Earth. So we need 4953 m/s delta-V from LEO orbit.
To get this, we can use the second stage engine (ISP=348) and the super Dracos (vacuum ISP roughly 240). The division of labor helps even with the SuperDraco's worse ISP, since they don't need to push the empty second stage. To make this work, we'll need about 4 tonnes of Dragon fuel. This is more than they normally carry (1.3 tonnes) but they are only carrying two people. This gives a total Dragon mass of 11 tonnes.
Is the SD's high thrust actually required in this instance? From what I've read, Dracos have a somewhat better ISP of about 300s, which would obviously increase available delta-V.Assuming the empty Dragon + occupants = 7 tonnes, the Dragon adds 240*9.8*ln(11/7) = 1063 m/s. That's a total of 4953 m/s, just what is needed. The Dragon now needs to land in the ocean since there is no delta-V left for a propulsive landing, or even an assist.
With the rest of your figures constant, a 300s ISP nets you 300*9.8*ln(11/7) = 1328 m/s, about 265 m/s more, for margin on the flight. Or with about 3 tonnes of fuel, you can keep the delta-V the same (300*9.8*ln(10/7) = 1048 m/s.
Ah, but I'm forgetting the second stage. Hmm.
Okay, so with 3t of fuel the Dragon is 10t, so the S2 gets 348*9.8*ln((33+4.5+10)/(4.5+10)) = 4046 m/s, for a total of 4046 m/s + 1048 m/s = 5094m/s, about 140m/s more than the 4953 m/s needed.
But if you reduced Dragon fuel to 2t, then 348*9.8* ln((33+4.5+9)/(4.5+9)) = 4217 m/s and 300*9.8*ln(9/7) = 738 m/s, for a total of 4955 m/s.
So it looks like using Dracos with 4t of Dragon fuel gets you 265m/s for landing, and 3t gets you 140m/s... which would be with 240s SuperDracos, not 300s Dracos. Ack.
Fine, more math. Uh...
Okay, with 4t of fuel, and 11t total initial mass and S2 delta-V of 3890 m/s, Dragon needs to provide 1063 m/s to hit the required 4953 m/s. 300*9.8*ln(11/7.65) = 1067 m/s, so there's ~650kg left in the tanks. 240*9.8*ln(7.65/7) = 208m/s for landing on SuperDracos. That seems like lots for a propulsively-assisted parachute ground landing, but I don't know if it's enough for a fully propulsive landing, especially when you start adding in mission margins.
With 3t of fuel, S2 DV of 4046m/s, 300*9.8*ln(10/7.345) = 907m/s, leaving 345kg in the tanks, and 240*9.8*ln(7.345/7) = 113m/s for the SDs.
2.6t fuel leaves 200kg of propellant which is 66m/s for the SDs.
So, if you can use Dracos for on-orbit maneuvers, it looks like just doubling the normal 1.3t fuel load would get the job done.
Of course, lunar tourism would need to become a significant market for SpaceX to put any R&D effort into modifying the S2 and Dragon for such a purpose. It's nice to dream, but... -
#941 by manoweb on 07 Mar, 2017 18:45
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So Elon was being disingenuous when he said no one from Hollywood?
Mr. J. Cameron is from Canada
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#942 by MP99 on 07 Mar, 2017 18:56
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It's likely easier to add tanks mounted on hard points in the trunk, and run plumbing to Dragon's service section. Using Dracos instead of SuperDracos would help as well, they appear to have a specific impulse of 300 seconds. Dragon 2 looks to have 8 forward-pointing Dracos, which could fire simultaneously to get 3.2 kN of thrust.
Note that there is a constraint on the engines on CPS for the SLS (IIRC) that the thrust could not be too high for certain manoeuvres, as the short burn time, and uncertainty in exact residual thrust after shutdown cause too large large uncertainties in the resulting orbit.
Dracos are a better choice for this application.
Cheers, Martin -
#943 by Space Ghost 1962 on 07 Mar, 2017 19:06
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It's likely easier to add tanks mounted on hard points in the trunk, and run plumbing to Dragon's service section. Using Dracos instead of SuperDracos would help as well, they appear to have a specific impulse of 300 seconds. Dragon 2 looks to have 8 forward-pointing Dracos, which could fire simultaneously to get 3.2 kN of thrust.
Note that there is a constraint on the engines on CPS for the SLS (IIRC) that the thrust could not be too high for certain manoeuvres, as the short burn time, and uncertainty in exact residual thrust after shutdown cause too large large uncertainties in the resulting orbit.
Also, you can do a capture orbit and repeat brief perilune burns for optimal efficiency if you are indifferent to time and simply want best props usage. You can even time/place burns to use mascons to improve still further props usage.QuoteDracos are a better choice for this application.
Yes. And, you could overexpand a few of them for even more. -
#944 by envy887 on 07 Mar, 2017 20:03
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I wonder how difficult it would be to duplicate the Dragon service section tanks and plumbing used to feed the SuperDracos, and put them in a package that mounts on the trunk internal hard points. With a single highly-expanded (and highly-speculative) SuperDraco playing the role of the Apollo SPS, that could easily put LLO within the reach of a single partially reusable FH launch.
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#945 by Kaputnik on 07 Mar, 2017 21:16
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The problem with a trunk mounted propulsion system is what happens in the event of launch abort.
Can a trunk payload be reliably jettisoned or left behind?
Has a figure been published for maximum trunk payload on a crewed launch? -
#946 by DAZ on 07 Mar, 2017 22:27
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The problem with a trunk mounted propulsion system is what happens in the event of launch abort.
Can a trunk payload be reliably jettisoned or left behind?
Has a figure been published for maximum trunk payload on a crewed launch?
This kind of stuff has been discussed before in other threads. The general consensus would be that SpaceX is not going to do this as it doesn't lend directly to going to Mars but if they were to do this it probably could be made to work relatively straightforwardly. It would essentially just be a breakaway system that when the trunk was pulled away by the Dragon capsule it would leave behind the very heavy fuel and rocket engine kit. In fact, it would take considerable work to make these connections strong enough to stay with the trunk during the abort situation. So as long as this kit was supported from beneath adequately during a normal launch the fact that these connections are relatively weak for the rest of the trip should not be much of an issue. -
#947 by IainMcClatchie on 08 Mar, 2017 00:09
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The RED is smaller, but is prone to artifacts caused by cosmic radiation. A recent IMAX documentary was shot on Station with the RED, but needed a ton of post production cleanup to deal with the cosmic radiation noise that was present in the footage. On a trip to the Moon the problem will be worse.
Where can I read about that directly?
The image sensor itself should be less prone to radiation that the electronics behind it, just because the sensor is physically smaller. Once through the A/D converters on the sensor, the data can be protected with ECC (error correction coding). When it gets written to flash memory, for instance, this is standard. If I'm building a camera for space work, I could keep the entire datapath protected by ECC if that's really necessary.
I have not yet built a camera that has flown in space, but my received understanding is that with a few mm of aluminum shielding, the remaining radiation will cause latch-up events that require an immediate power cycle of the system. You'd definitely notice this, but if necessary it should be possible to make a camera do a complete power cycle back to filming with just a couple of dropped frames.
The thing is, I've been told these latch up events are a once-a-year thing. No point in worrying about it, aside from making the power supply able to detect and shut down very fast, and storing all data under ECC. So I'd like to read about what the IMAX/Red team experienced.
I'm struggling to imagine how these radiation-induced defects can mess up an image sensor but not the digital electronics, in particular the flash and DRAM, behind it. Flash memories and DRAM also store bits in small number of electrons and have analog electronics which greatly amplify small errors.
From a Wired article, I got this:Quote“You can see missing pixels in the footage,” Ivins says. “They’re like stars. Particularly in a low-light image, you can look at a frame and I can tell you pretty much to the month how old the camera is.”
This suggests the radiation is causing charges to get stuck somewhere in the pixel, perhaps in the gate oxide of the pass transistor between the capacitor and the bitline. Stuck pixels only visible in low-light images suggest very small amounts of leakage that can be overcome with either active logic or more rapid refresh in the case of flash or DRAM.
And frankly, exchanging cameras for a bunch of stuck pixels is kind of whiny. People have been interpolating over stuck pixels for so long that it's a standard part of an imaging pipeline. If you get big globs of stuck pixels, that'll be challenging, but we're already missing two out of the three color channels at each pixel anyway.
Bottom line is that for a one-week mission, I strongly doubt they'll have much problem with stuck pixels in the camera. -
#948 by MATTBLAK on 08 Mar, 2017 01:01
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Just reiterating an earlier question, part of which we'll find out at some point during a briefing: Is the Dragon going to have a high-gain antenna to assist TV transmissions - what kind of bandwidth and resolution could we expect; 480 or 720p semi-HD?
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#949 by dcporter on 08 Mar, 2017 03:26
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The RED is smaller, but is prone to artifacts caused by cosmic radiation. A recent IMAX documentary was shot on Station with the RED, but needed a ton of post production cleanup to deal with the cosmic radiation noise that was present in the footage. On a trip to the Moon the problem will be worse.
I think watching the film with cosmic radiation artifacts would be pretty cool.
I bet it would get old fast. -
#950 by biosehnsucht on 08 Mar, 2017 08:30
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My understanding is that electronics actually become more sensitive the smaller the features are.
So while a smaller camera sensor vs larger sensor at the same process node might lend to fewer issues with the smaller one (less area to hit with cosmic rays), if it is smaller because it was built on a smaller process (i.e. 28nm vs 130nm) any rays that hit it are going to do much more disruption and more likely to cause actual damage.
This is true of CPUs, DRAM, etc - DRAM error rates are much higher on modern, small processes compared to when they were studied by IBM in the 90s. Really, ECC should be standard along all datapaths and storage mediums by now, but they aren't because it's something to upsell you on (thanks, Intel).
See the many responses to http://stackoverflow.com/questions/2580933/cosmic-rays-what-is-the-probability-they-will-affect-a-program -
#951 by FutureSpaceTourist on 08 Mar, 2017 14:07
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Jim Lovell comments on the SpaceX mission and comparisons with Apollo 8:
http://www.collectspace.com/news/news-030817a-spacex-moon-lovell-apollo8.html -
#952 by WmThomas on 10 Mar, 2017 03:24
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Why does everyone think the travelers on the SpaceX circumlunar mission will be useless baggage?
If, as hints indicate, they are successful Silicon Valley super-nerds, here's what we can expect:
Once timing of the mission is confirmed, they will have 6-12 months to prepare. And can they prepare? Yes, they are super-nerds.
They probably are already thinking of what they will do. I'll bet it will be interesting.
So let's save the snark until there's a reason to be snarky. Be snarky instead about the less-smart people who want to go to space but can't afford to pay for it. Like most NASA Astronauts. -
#953 by LouScheffer on 10 Mar, 2017 11:03
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So let's save the snark until there's a reason to be snarky. Be snarky instead about the less-smart people who want to go to space but can't afford to pay for it. Like most NASA Astronauts.
Foe equal snark on all sides, let's agree that SpaceX is sending tourists, NASA is sending bureaucrats, Russia is sending apparatchiks, and China civil servants. This has the additional advantage that we don't need to invent a new word every time a new country sends someone into space. -
#954 by tater on 10 Mar, 2017 13:34
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The discussion of semantics regarding what to call them is entirely reasonable, although it perhaps deserves a thread to itself.
Going forward, spacecraft are only going to become more, not less automated. In addition, any definition of "astronaut" (or the Russian and Chinese equivalents) must work for anyone at all, and going forward into the future, reliably. If someone is a mission or payload specialist and only flies to space once, but they are not capable of "flying" the spacecraft alone, do they get to be called astronauts? Do they become one merely because they are onboard with at least one person who can (for reasons)? Does "astronaut" require XXX hours of training from NASA, else "tourist" or "passenger?"
That would make no sense, it would be like defining "pilot" to only be people trained to fly by the USAF.
In the future we are also likely to see commercial astronauts. If flying the craft (or that ability) is the definition, then astronaut becomes conflated with pilot, and people who are highly trained, but not in the mechanics of flying the craft should not be astronauts.
I'd think a broad definition makes the most sense. Part of me would want to not include suborbital flights at all, but then Alan Shepard ceases to be an astronaut <edit> at the time of his flight---this becomes a Pluto-like discussion at some level, someone might have to have a status change to secure a more useful definition. -
#955 by Comga on 10 Mar, 2017 13:46
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(Snip)
I'd think a broad definition makes the most sense. Part of me would want to not include suborbital flights at all, but then Alan Shepard ceases to be an astronaut---this becomes a Pluto-like discussion at some level, someone might have to have a status change to secure a more useful definition.
And down the rabbit hole we go.....
Alan Shepard went to the moon.😜
But in general I agree. Let's call them all astronauts and get back to discussing the mission. -
#956 by Jim on 10 Mar, 2017 13:56
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I'd think a broad definition makes the most sense. Part of me would want to not include suborbital flights at all, but then Alan Shepard ceases to be an astronaut--
No, he still flew on Apollo 14 -
#957 by Rocket Science on 10 Mar, 2017 14:14
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Anyone in a U.S. government program that flew over 50 miles in altitude earned their astronaut wings including X-15 pilots...
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#958 by tater on 10 Mar, 2017 14:23
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(Snip)
I'd think a broad definition makes the most sense. Part of me would want to not include suborbital flights at all, but then Alan Shepard ceases to be an astronaut---this becomes a Pluto-like discussion at some level, someone might have to have a status change to secure a more useful definition.
And down the rabbit hole we go.....
Alan Shepard went to the moon.😜
But in general I agree. Let's call them all astronauts and get back to discussing the mission.
I meant at the time of his flight (ditto Grissom), my bad, I should have been more clear.
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#959 by Coastal Ron on 10 Mar, 2017 14:25
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Jim Lovell comments on the SpaceX mission and comparisons with Apollo 8:
http://www.collectspace.com/news/news-030817a-spacex-moon-lovell-apollo8.html
The description Lovell uses for the people going on this trip is "passengers", since the vehicle will be automated and if nothing goes wrong they don't have any mission responsibilities. But even as passengers, he said:
"You have to remember, it's not just the view. It is also the experience. It's the fact that they will come back and at the next cocktail party, they will be the center of attention," he said. "It is the fact that they will have done something that only a few other people have ever done."
I like the term "passenger", and it avoids the sub-categorization challenge we've had between "tourist", "adventurer" and so on.
