Author Topic: SpaceX Falcon 9 CRS/SpX-10 Dragon - Feb. 19, 2017 - Discussion  (Read 266678 times)

Online macpacheco

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In a worst case scenario, do we know how many berthing attempts the Dragon's fuel capacity can provide for, before having insufficient propellant left for a successful return to Earth?
Consider Dragon arrives at the 500 meters distance to ISS. If it then moved at 1m/s directly it would take just 5 minutes to arrive.
Approach speeds are likely in the 0.2m/s range (just an educated guess). That means nearly inconsequential fuel requirements even considering all the intermediary start/stops along the way.
I recall from memory the Dragon de orbit burn is 100m/s.
I would be really surprised if Dragon doesn't have enough hydrazine for ten attempts at normal speeds.
Additional consideration, if fuel were short, Dragon could use even slower speeds to save fuel. Let the approach take 2 hours.
Of course that's just a poorly educated guess.

Any attempt to do linear calculations on orbital maneuvers is futile.  NOTHING up there works like that.
Plus we can safely assume that the biggest limitation is not fuel.  The abort started a procedure for another attempt the next day, 24 hours out.  They will run out of time and patience long before they run out of fuel.
Are my assumptions about fuel consumption actually wrong or you're just pointing out that other resources / limits would run out before they make that many approach attempts (which I agree with) ? I would appreciate a more educational answer rather than to censure the non rocket engineer trying to thing. After all I did say it was a poorly educated guess.
Yes, the bottom line is there's an implicit assumption that such problems won't repeat themselves consecutive times. They should be really rare such that even single approach problems would happen less than 10% of the time. But that wasn't the question.
« Last Edit: 03/04/2017 08:48 AM by macpacheco »
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Offline Comga

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The answer to all of your questions is yes
The math is not extremely high level, but it's not linear. You can't divide distance by time to get velocity. All velocities are perturbations to the ~7 km/sec orbital velocity and everything rotates every orbit.
Yes there are other considerations
I don't know the fuel capacity and total delta-V capability. While some here do, that's not the limit.
But like the wise owl in the 1960's Tootsie Pop ad, our answer to your initial question is "two" 😉
What kind of wastrels would dump a perfectly good booster in the ocean after just one use?

Offline Greg Hullender

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What might help your understanding is to remember that all these objects are in elliptical orbits around the Earth (counting a circle as a special ellipse). At perigee (closest to Earth), an object in an elliptical orbit moves faster than an object in a circular orbit at the same height. At apogee (furthest from Earth) an object moves slower than an object in a circular orbit.

If you change velocity at perigee that changes the altitude of apogee (highest point), which is clear on the other side of the planet. And vice versa. If you were in a circular orbit and you sped up, you'd now be in an elliptical orbit, at the perigee.

What all this means is that when you accelerate toward the space station, you are now in a different orbit. If you miss it, you'll gradually move higher above the Earth, moving slower, and it'll pass under you and leave you behind. By the time of your next perigee, it'll be way ahead of you.

Since the orbit time of the space station is 90 minutes, anything that happens in a small fraction of that time can generally ignore the orbital effects. Five minutes is probably fairly safe, assuming you do some course correcting when you get close. But 25 minutes is probably not safe. (It could be a fun problem to work out the exact amounts of error involved.)
« Last Edit: 03/05/2017 06:03 PM by Greg Hullender »

Offline yokem55

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What might help your understanding is to remember that all these objects are in elliptical orbits around the Earth (counting a circle as a special ellipse). At perigee (closest to Earth), an object in an elliptical orbit moves faster than an object in a circular orbit at the same height. At apogee (furthest from Earth) an object moves slower than an object in a circular orbit.

If you change velocity at perigee that changes the altitude of apogee (highest point), which is clear on the other side of the planet. And vice versa. If you were in a circular orbit and you sped up, you'd now be in an elliptical orbit, at the perigee.

What all this means is that when you accelerate toward the space station, you are now in a different orbit. If you miss it, you'll gradually move higher above the Earth, moving slower, and it'll pass under you and leave you behind. By the time of your next perigee, it'll be way ahead of you.

Since the orbit time of the space station is 90 minutes, anything that happens in a small fraction of that time can generally ignore the orbital effects. Five minutes is probably fairly safe, assuming you do some course correcting when you get close. But 25 minutes is probably not safe. (It could be a fun problem to work out the exact amounts of error involved.)
One of the most counter intuitive things about orbital maneuvers is that if you do a prograde burn forward, you actually will be moving more slowly relative to an object in the same orbit that didn't do that burn. So to catch up to an object ahead of you, you born retrograde, to slow down to allow an object behind you to catch up, you burn prograde.  Then once you've rendezvoused to within a few hundred meters, you null out your relative velocities, and then more or less you can translate to docking in a more intuitive manner. 

Offline Greg Hullender

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One of the most counter intuitive things about orbital maneuvers is that if you do a prograde burn forward, you actually will be moving more slowly relative to an object in the same orbit that didn't do that burn. So to catch up to an object ahead of you, you born retrograde, to slow down to allow an object behind you to catch up, you burn prograde.  Then once you've rendezvoused to within a few hundred meters, you null out your relative velocities, and then more or less you can translate to docking in a more intuitive manner. 
On average, yes, you'll be moving slower, but immediately after your burn, you'll be moving faster because you are at the perigee of your new orbit (assuming the original orbit was circular). This is why you can ignore the orbital effects on a time scale that's small compared to your orbital period.

Offline sewebster

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We're we able to see if they kept the rocket horizontal as it climbed the launch mount hill or did they let it go slightly inverted?

If it was kept horizontal they must have modified the late load vehicle because it would be much higher above the tracks.

I don't know if we got an answer to the above question in this thread, but I found the following discussion from a couple years ago that says the vehicle is kept roughly level.

http://forum.nasaspaceflight.com/index.php?topic=36100.msg1390980#msg1390980

Offline Comga

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We're we able to see if they kept the rocket horizontal as it climbed the launch mount hill or did they let it go slightly inverted?

If it was kept horizontal they must have modified the late load vehicle because it would be much higher above the tracks.

I don't know if we got an answer to the above question in this thread, but I found the following discussion from a couple years ago that says the vehicle is kept roughly level.

http://forum.nasaspaceflight.com/index.php?topic=36100.msg1390980#msg1390980

The text on that link includes
Quote
There is a hydraulic system to keep the vehicle "roughly level" as it goes up the ramp to the pad. This is not intrinsically necessary, but they raise the payload end to keep it from hitting the ground as the rocket transitions from level to the ramp.

The 39A TE will retract all the way back to horizontal prior to launch. This is "a big benefit" with the enhanced Merlins running at 100%. This will be for both the Heavies and the F9 "single stick".

We know that the second statement is not what happened with the TEL at LC-39A for CRS-10 and won't be true for future launches.  Both details, the timing and angle, are at best modified since that June 2015 post.
That says we cannot rely on the first statement.
What kind of wastrels would dump a perfectly good booster in the ocean after just one use?

Online cscott

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Well, if we do the geometry we can figure out what angle is necessary to keep the fairing from hitting the ground.  It's probably a safe bet the TEL hydraulic system allows at least that amount of levelling.

Of course, if the math shows that 0 degrees is sufficient to keep the fairing from scraping the ground, then perhaps the whole levelling system was omitted.  But if not, then there definitely has to be *some* truth to the first statement.

Offline Jim

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Lights at LC-39A are on. Did SpaceX keep the xenon lights that lit up the Shuttle?

the range owned the lights

Offline manoweb

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Lights at LC-39A are on. Did SpaceX keep the xenon lights that lit up the Shuttle?

xenon lights? Interesting. Xenon lights are typically not used as floodlights but in applications that require a high CRI... I did some research but I cannot find more details why xenon lights were (are?) used

Offline CameronD

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Lights at LC-39A are on. Did SpaceX keep the xenon lights that lit up the Shuttle?

xenon lights? Interesting. Xenon lights are typically not used as floodlights but in applications that require a high CRI... I did some research but I cannot find more details why xenon lights were (are?) used

Given the age and application, I would expect they'd be fairly standard industrial Mercury Vapour lamps - not Xenon..  Just FWIW.
With sufficient thrust, pigs fly just fine - however, this is not necessarily a good idea. It is hard to be sure where they are
going to land, and it could be dangerous sitting under them as they fly overhead.

Offline RonM

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Lights at LC-39A are on. Did SpaceX keep the xenon lights that lit up the Shuttle?

xenon lights? Interesting. Xenon lights are typically not used as floodlights but in applications that require a high CRI... I did some research but I cannot find more details why xenon lights were (are?) used

Given the age and application, I would expect they'd be fairly standard industrial Mercury Vapour lamps - not Xenon..  Just FWIW.

C'mon, guys. A quick Google search will confirm NASA used xenon lights.

Now back to our SpX-10 discussion.

Offline launchwatcher

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Quote
A birthday surprise arrived with @SpaceX Dragon, hidden by my teammates until 27 Feb in collusion with Houston! You can not trust anyone ;)
https://twitter.com/thom_astro/status/841042883721408513]https://twitter.com/thom_astro/status/841042883721408513[/url]
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Offline Brian45

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A quick return cargo question -- I read that when Dragon leaves the ISS there will be some material placed in the un-pressurized trunk. How does that work? Isn't the trunk jettisoned before entry exposing the heat shield? Is that just a way of disposing junk?

Offline NASAGeek

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A quick return cargo question -- I read that when Dragon leaves the ISS there will be some material placed in the un-pressurized trunk. How does that work? Isn't the trunk jettisoned before entry exposing the heat shield? Is that just a way of disposing junk?

Yeah the trunk is jettisoned  before re entry. The items placed in the trunk are meant to be disposed by burning up in the atmosphere.

Offline mvpel

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Per a spaceflightnow story, 104 kg of EVA equipment was brought home on Dragon. Does anyone (hi, Pete?) know what that was? Substantial - sounds like an EMU or at least a PLSS or HUT or something

Would it have been the leaky Parmitano / Kopra suit equipment, maybe? Or would that have come down on CRS-9?
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Offline Norm38

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I hadn't seen any of these "Flight Club" videos before, don't recall them being mentioned here, though there are a lot of them.  The animations are fun to watch, especially for RTLS missions like this one.
Being able to watch the various burns occur along the trajectory, their timing with respect to velocity, the RTLS maneuver makes so much more sense now, seeing it like this.  It really is quite elegant.

« Last Edit: 06/18/2017 11:13 PM by Norm38 »

Offline deruch

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I hadn't seen any of these "Flight Club" videos before, don't recall them being mentioned here, though there are a lot of them.  The animations are fun to watch, especially for RTLS missions like this one.
Being able to watch the various burns occur along the trajectory, their timing with respect to velocity, the RTLS maneuver makes so much more sense now, seeing it like this.  It really is quite elegant.

For those interested, go to the website:  https://flightclub.io/

http://www.popularmechanics.com/space/rockets/a18289/choose-your-own-spacex-adventure-with-this-website/
« Last Edit: 06/19/2017 12:03 AM by deruch »
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