sevenperforce
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Posts posted by sevenperforce
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22 hours ago, YNM said:
On Topic : I've only saw the Moon at the day, never planets. Once through a telescope, quite nice actually.
If you have a clear sky tonight and live anywhere near the Eastern Time Zone, try looking a bit west of the zenith for a faint spot of light around 8 pm. Jupiter should be visible.
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On 4/30/2016 at 5:08 PM, Veeltch said:
Don't know about the jovian moons, but the last time is saw the ISS I could swear I saw more than one source of light by it. Thought that it might've been the Dragon capsule on it's approach, but then went back home and learnt that the launch was scrubbed. Was probably the solar arrays on the both sides of the station that gave the impression of two dots flying in a formation.
I forgot to do this for CRS-8, but it would be really cool for someone to figure out when the Dragon and the ISS will both be separately visible to the naked eye. Typically the ISS is only visible once per day at any given point if it is at all, so there would only be a few places where the two dots of light would be clearly visible and separate.
That Dragon is supposed to be departing soon...on the 8th I think?...so maybe we will get lucky.
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On 5/1/2016 at 8:26 AM, lajoswinkler said:
There's absolutely no need for that. In no circumstances will we get solid or liquid hydrogen and oxygen from electrolysis of water. It's a silly pleonasm from US education system.
I only called them GOX and GH2 in contrast to LOX and LH2 because this forum has a LOT of discussion of the latter and not much of the former.
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13 hours ago, K^2 said:
"Stable" is a subjective matter. A black hole that's "only" a few million tons would take decades to evaporate. And if you only need one for a few days, you can get away with almost reasonable mass.
Of course anything that could be made at LHC energies will only just last long enough to detect.
Then again, any black hole made at any energy will only just last long enough to detect...because if it's large enough to last any longer than it takes to detect, it is large enough to destroy the detector. And you.
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2 minutes ago, John JACK said:
Accelerating extra dead weight of tankage, ducts and redundant engines with payload 7000 m/v to orbit needs much more fuel than decelerating near empty first stage (without payload) less than 1000 m/s. SSTOs may be wicked cool, but they are ineffective.
Wouldn't it be Liquid Flyback Booster?
Carrying that additional weight only makes sense if you can maintain a high payload fraction (actual payload / total mass to orbit). Adding an airbreathing engine to an SSTO is useless if you end up with only 10-20% of your total orbital mass as payload...but if you're looking at 40-50% of your orbital mass being payload, then you don't care so much about the extra weight.
And yeah, the biamese approach would be the equivalent of a parallel liquid flyback booster.
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Speculation on first stage uprated Merlin 1D thrust and specific impulse...
Elon stated that the F9 would be launching at 1.71 million pounds thrust with a vacuum thrust on the first stage of 1.9 million pounds. Doing the math, that comes to a whopping 845.6 kN launch thrust and 938.9 kN vacuum thrust on the first-stage Merlin 1D engines.
If you're tracking, that's a vacuum TWR of 204. Absolutely astounding.
Anyway, I think we should see Isp come up a bit due to increased pressure thrust, since the nozzle diameter is unchanged. In other words, the increased flow means the exhaust is now underexpanded at launch. Assuming no increase in actual exhaust velocity, we are looking at a mass flow increase. 846/642 = 132%, so a very broad estimate using the ideal gas law suggests a 32% increase in gas pressure.
We can figure out what Isp increase this represents by using the difference between published vacuum Isp and published SL Isp. Assume for simplicity that earlier SL thrust matches atmospheric pressure (even though it probably was already slightly underexpanded). 311 s from 282 s corresponds to a 10.2% increase in specific impulse due to pressure thrust. This suggests that going up by one atmosphere of pressure is a 10.2% increase in mass-specific impulse.
So, going up by 32/100ths of an atmosphere is probably going to be a 3.26% increase in specific impulse. That means a SL specific impulse of roughly 291.2 seconds and a vacuum specific impulse of roughly 320.3 seconds. Vacuum specific impulse of the Merlin 1D Vacuum will remain the same because it uses a better expansion ratio so its pressure thrust will be largely unaffected...maybe 0.5-1.5% increase at best.
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If they lose a periphery engine shortly after launch, can they gimbal the other engines to maintain COM/COT, or would they have to shut down the opposite engine entirely to balance?
If the latter, then I expect that the writing and plumbing associated with the upper parts of the engine and turbopump is probably done in pairs, so that if they lose two engines it is more likely to be an opposite pair.
And as far as specific impulse is concerned, any improvements would be very minor, I think. I can't remember if it is possible to calculate the backpressure of the exhaust flow based on published ISP differences or not, but if so then I ought to be able to get a general idea of the increased pressure thrust associated with the higher flow.
Also, unless I'm mistaken the uprated thrust should now come with a larger throttling capability as a percentage of total thrust.
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25 minutes ago, Nikolai said:
And then there's stuff that doesn't seem to showcase any talent and doesn't seem to aspire to anything. I don't have a name for it, but to my tastes, it tends to be painfully bad.
Starwarzianholidazic?
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With the newly-released FT numbers for the Merlin 1D, these margins end up looking quite a bit better. I'll run the numbers again shortly.
One advantage of this design would be a very simple biamese setup. Connect two of these belly-to-belly on launch with crossfeed and you can carry something to GTO with ease.
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2 hours ago, MatterBeam said:
How is the air-augmented Merlin engine better than an aerospike?
Every possible way.
An aerospike is an altitude-compensating nozzle. It allows you to use a greater expansion ratio without risking flow separation, but it does nothing else. (Also, a Merlin can't be easily mated to an aerospike.)
An AAR, on the other hand, serves to collect and compress and airflow, then combine it with the exhaust flow from the ordinary engine in the center. The net exhaust velocity drops, but the thrust increases because you are using the same kinetic energy with a greater mass flow. Most importantly, this new mass flow doesn't have to be carried by the rocket, so the thrust-specific fuel consumption drops, causing specific impulse to skyrocket. Even with just a simple tapering cylindrical stainless steel shroud, static thrust goes up by 15% and thrust at speed goes up by 50%, without requiring any additional fuel.
15 hours ago, wumpus said:All it needs are 2km/s (of actual speed provided by the first stage) and vacuum and it handles the rest.
I'd be worried about all the extraneous size. I doubt that this entire structure could be anywhere near F9's second stage dry mass.
I could probably streamline it a little better; integrate the ducting into the body a bit.
Also, I think I would cut landing gear down considerably since there really is no need for horizontal takeoff. This increases payload fraction noticeably.
3 hours ago, Elukka said:While I agree SSTO doesn't generally make a lot of sense it's an interesting design.
I wonder if the ducts could really be as light as you expect though. 25% of two Merlins is only 235 kilograms. Surely they'd be rather heavier than that. I tried a quick google for existing design but the best I got was an uncited claim on Wikipedia that you'd expect a duct-augmented rocket engine to be five to ten times heavier. I can't judge the accuracy of that number but if you made them five times heavier you would still have a payload.if your isp expectations hold.
A stainless steel duct can't be THAT heavy.
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7 minutes ago, PB666 said:
You could have a solar powered recycling system.
Sure, though that adds mass and means pretty substantial modification to the FH upper stage.
I bet that one of the things we see in the next few months is a GTO mission where the upper stage has enough remaining propellant to raise its perigee above 100 km, allowing it to remain in orbit for a few weeks, so that they can test extended-delay inflight restarts.
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Here's that last image, flipped and superimposed:
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6 minutes ago, SuperFastJellyfish said:
Callisto and Ganymede orbit well outside of 3 or 4 radii of Jupiter at their apparent furthest when viewing from Earth. Maybe that's when I've seen it.
Jupiter's radii: ~70,000km
Ganymede's periapse: ~1,070,000km
Callisto's periapse: ~1,850,000km
Many times have I looked at Jupiter over the years, and have been able to see a pronounced 'line' of light through Jupiter with my naked eye. Using Stellarium or my telescope/binoculars, I've confirmed that the pronounced line is in the direction of the plane of the moons. Maybe I'm seeing what I want to see, but there it is.
Wow, I had assumed all this time that the moons were a great deal closer to Jupiter, but apparently I was wrong.
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8 minutes ago, CatastrophicFailure said:
Since you mentioned it, what would happen to kerosene after six months in space? I'd always understood it stores and transports pretty well, hence it's popularity with militaries (also why I drive a diesel
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The kerosene would probably be frozen solid. Unless they painted the kerosene-tank portion of the body black, in which case it may well boil off.
NASA's current GR&As for cislunar mission architecture predict 0.2% propellant boil-off for kerolox each day. Over six months that means you lose about 31% of whatever fuel you started with.
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6 minutes ago, Rakaydos said:
Out of curiosity, has there been any suggestion of putting a satelite-type payload in the trunk, to be abandoned in martian orbit before entry? I mean, it's already got the solar panels and it's already going to be in position in orbit...
A great idea, but it won't be in orbit. Falcon Heavy can deliver a Dragon V2 to a Martian aerobraking trajectory but it can't deliver it to Martian orbit; the dV requirements are too high and I doubt the kerosene would be usable after cruising for that long. The trunk has no RCS of its own.
One possibility would be to put a very RCS-capable SEP module in the trunk and have the Dragon eject the trunk a few thousand km out from re-entry, on a high aerobraking trajectory, while the Dragon itself plunges to a lower aerobraking trajectory and enters. The trunk might conceivably be able to get a very small aerobrake for an eccentric orbital capture and then deliver just enough of an SEP pulse at apoapse to circularize above the Martian atmosphere. Would require some pretty hefty design work, though, and it would be mass-limited because the Falcon Heavy is already straining to put an empty V2 on the aerobraking trajectory.
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1 minute ago, PB666 said:
Also generates chlorine gas instead of oxygen
Chemistry was never my strong suit.
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25 minutes ago, Rakaydos said:
I dont think the 2018 schedual will slip, though, because it's not based on anything spaceX can control, so there's no room for optimistic elon-guesses. If they get Falcon Heavy ready anytime in the next 2 years, and if they get a Dragon 2 ready anytime in the next two years, then at the appropriate time in 2018, SpaceX will tell everyone to hold their calls, reserve a window of launch schedule, and just go.
I'm inclined to agree.
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1 hour ago, Kryten said:
Nope. It's clearly a long exposure/contrast-stretched/both, just look how many stars there are and how bright the background colour is.
But not too long or the stars would be star trails.
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59 minutes ago, livefree75 said:
I've been wanting to build my own electrolyzer to produce hydrogen and oxygen, but I'm having difficulty choosing an electrolyte, and have seen many conflicting sources. What would be the best and most common electrolyte to use for this application?
Epsom salt. Sodium chloride tends to corrode.
Also, be careful. GOX and GH2 are not that energetic compared to most rocket fuel but they are rather horribly energetic compared to ordinary household items.
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2 hours ago, tater said:
As I recall, best seeing is a little under half an arcsecond. Usually it's going to be higher, probably approaching an arcsecond. While you might not resolve it naked eye, the usual impression is that there is much less (or even no) twinkle because they are extended objects.
Yeah, lack of central twinkle is the most immediate indicator that you're looking at a planet rather than a star.
Still get a shimmering diffraction pattern, though, due to the suture lines in your eyes.
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In a restricted three-body problem, there are some pretty tight upper limits on the masses of the Trojan and the secondary in comparison to the primary. For the purposes of this explanation, I'll let m1 represent the mass of the primary, m2 represent the mass of the secondary, and m3 represent the mass of the Trojan.
For cases where m2 >>> m3 (e.g. where the secondary is a planet and the Trojan is a satellite or a tiny asteroid), then m1 must be more than 25x m2 in order for m3 to have a stable Lissajou orbit. For the case where m3 and m2 are closer in mass, or where m2/m3 is on the order of m1/m2, then both of those ratios must be greater than 26 in order for m3 to have a stable Lissajou orbit.
For the extreme case where m2 ≈ m3, a co-trojan (e.g. Earth and Venus in the same orbit), the mass of the primary must be more than 51 times the mass of either secondary.
For the "ultimate solar system" arrangement, Hill spheres need to be taken into account. Jupiter's Hill sphere is massive due to its dramatic distance from the sun, but if it was within the sun's habitable zone, its Hill sphere would be far far smaller. Its moons, in turn, need to have non-overlapping Hill spheres even if they are in resonance. So you can't actually pack nearly as many world-sized moons around a gas giant in the habitable zone as you might think.
The best chance of having a high-world-density system form is to have a system with several gas giants which collides with a very massive protostellar accretion disk, so that you start with numerous accretion points already "primed".
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20 minutes ago, PB666 said:
At a high elevation you can accelerate to max q much more quickly, reducing the hoovering time.
Yeah, one of the benefits of living on a mountain in Ecuador is much less dust, greatly reducing hoovering time.
Sorry, couldn't resist.
43 minutes ago, wumpus said:The reason for my question is that the data I had for MEC (main engine cutoff) for stage 1 of Falcon9 was 2km/s for recovery and ~3.5-4 km/s for non-recovery situations. I plugged in the numbers for Falcon Heavy and became convinced that the booster was going to be going faster than the non-recovery situations (although quite possibly with the needed fuel reserve).
Is the 2 km/s vs 4 km/s break taking into account RTLS vs downrange recovery?
I have a sneaking suspicion that fuel reserve, not re-entry speed, is the break point for the core. SES-9 seemed to survive re-entry just fine; from what I've been able to tell it only failed because it lacked sufficient fuel reserves for a one-engine landing burn and had to go with a three-engine landing burn, which failed either because it was uncontrollable or because it didn't even have enough reserve fuel for that. I cannot think of any reason for a re-entry burn unless they are doing an exhaust shield.
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12 hours ago, SuperFastJellyfish said:
I doubt you'd be able to make out Jupiter as more than a point of light, but you can definitely see a hint of the 4 Galilean moons on clear nights with your naked eye.
Really? That would be difficult, I think. Once the sky fully darkened, Jupiter's brightness against the black sky caused a strong diffraction pattern that washed out everything within 3 or 4 radii; I can't imagine that I would have ever been able to spot its moons.
The only reason I thought I could see a disc was because the sky was still bright blue and so it prevented image washout.
7 hours ago, Shpaget said:What if we include the atmospheric effects?
It's a rare sight but I have personally seen the Moon rising absolutely huge (larger than my fully spread fingers at arms length, and I don't have tiny hands). It was very pale, but huge nevertheless.
It was out of ordinary, the very next evening and many later evenings around the same time, from the same location the effect was not present, so I'm not attributing it to an illusion.
Sorry if the question seems rude, but did you actually extend your hand, or is this just based on your recollection?
The effect of the horizon on perspective causes the image you see to be distorted at your focal point, so your brain sees a much larger image (almost like a fisheye effect) than when the moon is directly overhead, even though the image actually entering your eye is the same size. It is not possible for the moon to occupy that much angular distance at the horizon.
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13 hours ago, fredinno said:
You know, because NASA is paying SpaceX for Dragon V2 to land people on the Moon, and not send people to the ISS...
That was a great laugh. Wait, you were serious?
Dragon V2 was sold as a crew capsule but was designed as an interplanetary lander, and Elon has specifically discussed its use for landing on the moon and elsewhere.
13 hours ago, fredinno said:Also, what you suggest (internal aux. tanks) is more than enough to validate building a new Spacecraft due to the modifications needed.
SpaceX has said they would add additional fuel capacity for selected applications rather than building a new lander, so...
13 hours ago, fredinno said:You need to modify that top docking port to withstand the heat of launch.
Only if you want the top docking port to survive. Which isn't really necessary in this case.
Not that there was any actual intent to do a sample return on this first mission.
How do you know? You mean the dragonv2 suit? or a mars mct suit?
The Dragon V2 suit. I wouldn't typically cite PopSci, but the article claims to be based on actual SpaceX contracts so I think it's a safe source.
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Planets in daylight
in Science & Spaceflight
Posted
Ah well. I did not look at that.