SpaceX Falcon 9/Dragon COTS Demo (C2+) GENERAL DISCUSSION

[Jim]

They may also be prepared to spruke a bit but so what, IMO they're entitled. 

That is the point, they are not entitled.  They have yet to back it up with actions.

[oiorionsbelt]

Maybe ULA's comment should read, who's going to the ISS in the next few weeks...not us.

The knives are out for SpaceX...Cesar had it better.

The ULA comment are justified. 

Are you saying the ULA "Who's ready for a launch" comment was deffinitly aimed at SpaceX?


definitely

[Jim]

Maybe ULA's comment should read, who's going to the ISS in the next few weeks...not us.

The knives are out for SpaceX...Cesar had it better.

The ULA comment are justified. 

Are you saying the ULA "Who's ready for a launch" comment was deffinitly aimed at SpaceX?

No, but if he thinks it is, then my statement is applicable.

[Chris Bergin]

Everyone should remember the tweets don't come from Mr Gass, they come from a PAO, and ULA's PAO is a nice lady who really wouldn't use twitter to have a sly dig at another company.

[oiorionsbelt]

Thanks Chris. I guess I was curious because of the Pratt and Whitney Rocketdyne adds which seem to be aimed at SpaceX, rightly or wrongly.

[Chris Bergin]

Thanks Chris. I guess I was curious because of the Pratt and Whitney Rocketdyne adds which seem to be aimed at SpaceX, rightly or wrongly.

Those Futurespace fun people. Yeah, they always retweet my articles, but they advertise on all the other sites. Imagine how well that went down at Chris HQ!

[Space Pete]

Continuing the discussion on the original C2+ updates thread about a potential Dragon internal camera (post linked below), I had another thought that I was hoping to get some feedback on - how about some kind of IR imager to gather data on heat loads on the hatch area during re-entry?

Updated image attached.

http://forum.nasaspaceflight.com/index.php?topic=28486.msg879027#msg879027

[MP99]

Thanks Chris. I guess I was curious because of the Pratt and Whitney Rocketdyne adds which seem to be aimed at SpaceX, rightly or wrongly.

Those Futurespace fun people. Yeah, they always retweet my articles, but they advertise on all the other sites. Imagine how well that went down at Chris HQ!

Yeah, Chris, but those adverts will attract people from those sites to sign up for FS's tweets.

Then when FS re-tweet you, they're advertising your site to those people from the other sites.

cheers, Martin

[Jorge]

OK. So if I get this right, when Dragon is momentarily motionless in this ISS-centered frame, and is in a co-planar orbit directly below ISS, z is positive and thus z-double-dot is positive and thus if the Dragon were in free drift it would accelerate down towards the Earth? That makes sense since its angular velocity is matching that of ISS in its higher (slower) orbit. So Dragon at that moment is at the apogee of an elliptical orbit.

That's right. Such an apogee is sometimes referred to as a "cusp".

To hold in the ISS frame Dragon will thus thrust directly upwards towards ISS. And if Dragon thrusters could provide exactly the right amount of thrust the vehicle could hold its position without thrusting at all along the x (Vbar) axis? So that's the ideal (minimum) amount of thrust required. And it grows only linearly with z?

Correct in theory. The C-W equations are a linear approximation to the true motion.

But looking at the CW equation for x (which I think is x-double-dot = -2*n*z-dot), if any net acceleration along the Rbar is experienced, then z-dot becomes non-zero and there is acceleration along the Vbar. And things get really whacky, and Dragon better have a darn good control system deciding which way to thrust to get back to the hold point, or it could consume plenty of propellant in the process!

Right. So the control law for forced-motion approach along the Rbar still has to account for all three axes.

[sdsds]

z-double-dot = 3*n^2*z

where n is the mean motion (orbital rate), about 0.0011 rad/sec for ISS orbit.

So Dragon holding at 250m seems to require a minimum of 0.0009 m/s/s, or 3.3 m/s of delta-v per hour of hold time. Out at the 1.4km point, remaining stationary requires 18 m/s per hour; in at the 30m hold point, only 0.4 m/s per hour.

It seems heading directly up the Rbar at a constant rate requires a constant thrust along the Vbar to stay on track, because without thrust CW says:
x-double-dot = -2*n*z-dot

Countering that doesn't require much delta-v; if Dragon is closing at a z-dot of 1 m/s the Vbar acceleration needed is only 0.0022 m/s/s or 0.55 m/s total for the entire 250 second trip.

Has SpaceX or NASA said whether Dragon will maintain a fixed attitude during the approach? Conceivably they could get the right amount of thrusting along each axis using only one set of thrusters, rotating the spacecraft as it approaches....

[Jorge]

z-double-dot = 3*n^2*z

where n is the mean motion (orbital rate), about 0.0011 rad/sec for ISS orbit.

So Dragon holding at 250m seems to require a minimum of 0.0009 m/s/s, or 3.3 m/s of delta-v per hour of hold time. Out at the 1.4km point, remaining stationary requires 18 m/s per hour; in at the 30m hold point, only 0.4 m/s per hour.

It seems heading directly up the Rbar at a constant rate requires a constant thrust along the Vbar to stay on track, because without thrust CW says:
x-double-dot = -2*n*z-dot

Close. I think the sign on the z-dot term should be positive if you're working in the NASA LVLH frame. Positive z-dot (radial in) should result in positive x-double-dot (acceleration toward the +Vbar).

That's a substantial issue with the literature on the C-W equations; all the papers seem to use different coordinate frames. In particular, one must attend carefully to the signs.

Countering that doesn't require much delta-v; if Dragon is closing at a z-dot of 1 m/s the Vbar acceleration needed is only 0.0022 m/s/s or 0.55 m/s total for the entire 250 second trip.

Has SpaceX or NASA said whether Dragon will maintain a fixed attitude during the approach? Conceivably they could get the right amount of thrusting along each axis using only one set of thrusters, rotating the spacecraft as it approaches....

Fixed attitude. Outside 250 m, capsule toward +Vbar, trunk toward -Vbar. 180 degree yaw maneuver at 250 m to place capsule toward -Vbar while maintaining LIDAR lock on ISS (the LIDARs are behind the door that holds the Dragon FRGF, so this axis must remain pointed at ISS). All visiting vehicles (VVs) to ISS have limited sensor fields-of-view and sensor lock is required prior to GO inside 250 m; therefore, approach attitude is limited.

[Jorge]

What's the reason that the station rotates anyway?  Why not stay fixed to the sun so the arrays don't have to track.  Docking is harder.

Docking is easier, not harder, because visiting vehicles can perform an approach along a fixed LVLH axis. For unmanned berthing vehicles, the +Rbar approach maximizes safety because the trajectory is "free-drift safe". That is, the trajectory requires continuous acceleration toward ISS to continue approach, so a VV failure that leaves the VV inert will result in the VV drifting safely away from the station.

And doesn't the acceleration mess with experiments? 
What's the benefit?

Accelerations relative to the station center of mass are a function of LVLH attitude, not inertial attitude. The station attitude is optimized for microgravity science. Keeping the main line of modules aligned with the LVLH X axis minimizes the tidal (Rbar) acceleration (the z-term in the C-W equations) within the station and therefore minimizes perturbations on the microgravity environment.

Jim's statement about attitude control is also true. A flight attitude with the line of modules along the +Vbar is close to a torque equilibrium attitude, which allows efficient non-propulsive (CMG) attitude control.

[Jorge]

What's the best approach to calculating the delta-v per second consumed by Dragon as it holds position a fixed distance below the station on the r-bar?

Clohessy-Wiltshire equations. If using the NASA LVLH frame (Rbar = z axis), take the z equation, set x-dot to zero, and solve for z-double-dot as a function of z:

z-double-dot = 3*n^2*z

where n is the mean motion (orbital rate), about 0.0011 rad/sec for ISS orbit.

Note that for most spacecraft, including Dragon, the same delta-v in different axes may result in different propellant consumption due to the different canting of RCS thrusters.

I was thinking of the same question. anda FABULOUS answer... A quick question, I'm assuming x the distance to the station?
jb

Not in the frame I'm using (NASA LVLH frame). This frame is described as follows:

Origin - center of mass of target (ISS)
+z - points toward center of Earth (+Rbar)
+y - points out-of-plane "starboard", opposite the angular momentum vector (-Hbar)
+x - completes a right-handed system, points in the direction of the velocity vector (+Vbar)

And of course, dot means velocity and double-dot means acceleration.

OK, time for a fun application of these principles to a literary SF example. Moving OT, obviously. I'm flagging this post so the mods can move this subthread appropriately. Not sure where it will end up.

Reading assignment: "The Integral Trees" by Larry Niven.

Homework questions:

1a) Relate the cardinal directions of the Treedwellers to the axes of the NASA LVLH frame described above. Use the diagrams at the front of the book for reference. b) What is the origin of the frame, from the perspective of the Treedwellers?

2a) Relate the lines of the Treedwellers' saying:

East takes you out.
Out takes you west.
West takes you in.
In takes you east.
Port and starboard bring you back.

to the terms of the C-W equations (attached). 2b) Which term in the C-W equations is not accounted for in the saying? 2c) What force in the Treedwellers' daily experience is represented by the "missing" term? 2d) Why do you think the missing term is not included in the saying?

3) (extra credit) Compare and contrast the dynamics of the integral trees with the severed tether from TSS-1R on STS-75. In particular, what force accounts for the different behavior of the "inward" tuft of the Trees compared to the inward end of the severed tether?

Homework answers due Friday 1700 CDT (I'm feeling generous).

[MP99]

OK, time for a fun application of these principles to a literary SF example. Moving OT, obviously. I'm flagging this post so the mods can move this subthread appropriately. Not sure where it will end up.

In the same vein (mods - perhaps you could move with the above post?) this reminds me of another orbital mechanics example from SF, which probably couldn't be tested on ISS due to air currents, but wouldn't it be nice...

What's the reason that the station rotates anyway?  Why not stay fixed to the sun so the arrays don't have to track.  Docking is harder.  And doesn't the acceleration mess with experiments? 
What's the benefit?

...neglecting coriolis forces, there still ought to be a zero-gravity point somewhere on the station (where the "artificial gravity" of spinning slowly cancels out the fact that not all points on the station are at the center-of-gravity (or equivalent).

Our heros are trapped in orbit without any external sensors, but how to determine the direction to the body they're orbiting?

They suspend objects across the length of their craft, which each settle into independent orbits. Once air resistance damps movement relative to the craft/air, they form a (very) subtle arc, the centre of which is the barycentre of the craft's orbit.

Would seem to rely on the craft being settled into a parent-facing attitude (like ISS) in order for this to work.

So ... if the air was still enough, could it work?

cheers, Martin

[Herb Schaltegger]

OK, time for a fun application of these principles to a literary SF example. Moving OT, obviously. I'm flagging this post so the mods can move this subthread appropriately. Not sure where it will end up.

Reading assignment: "The Integral Trees" by Larry Niven.

Homework questions:

1a) Relate the cardinal directions of the Treedwellers to the axes of the NASA LVLH frame described above. Use the diagrams at the front of the book for reference. b) What is the origin of the frame, from the perspective of the Treedwellers?

2a) Relate the lines of the Treedwellers' saying:

East takes you out.
Out takes you west.
West takes you in.
In takes you east.
Port and starboard bring you back.

to the terms of the C-W equations (attached). 2b) Which term in the C-W equations is not accounted for in the saying? 2c) What force in the Treedwellers' daily experience is represented by the "missing" term? 2d) Why do you think the missing term is not included in the saying?

3) (extra credit) Compare and contrast the dynamics of the integral trees with the severed tether from TSS-1R on STS-75. In particular, what force accounts for the different behavior of the "inward" tuft of the Trees compared to the inward end of the severed tether?

Homework answers due Friday 1700 CDT (I'm feeling generous).

Crap. I read that book sometime fairly early in high school, way before I'd had enough math to appreciate it, and I didn't like it nearly as much as the RINGWORLD books.  And since then, I've had the math and then had 20+ years to forget it. I'm gonna get some popcorn and wait for the answers.

[jabe]

OK, time for a fun application of these principles to a literary SF example. Moving OT, obviously. I'm flagging this post so the mods can move this subthread appropriately. Not sure where it will end up.

Reading assignment: "The Integral Trees" by Larry Niven.

Homework questions:

1a) Relate the cardinal directions of the Treedwellers to the axes of the NASA LVLH frame described above. Use the diagrams at the front of the book for reference. b) What is the origin of the frame, from the perspective of the Treedwellers?

2a) Relate the lines of the Treedwellers' saying:

East takes you out.
Out takes you west.
West takes you in.
In takes you east.
Port and starboard bring you back.

to the terms of the C-W equations (attached). 2b) Which term in the C-W equations is not accounted for in the saying? 2c) What force in the Treedwellers' daily experience is represented by the "missing" term? 2d) Why do you think the missing term is not included in the saying?

3) (extra credit) Compare and contrast the dynamics of the integral trees with the severed tether from TSS-1R on STS-75. In particular, what force accounts for the different behavior of the "inward" tuft of the Trees compared to the inward end of the severed tether?

Homework answers due Friday 1700 CDT (I'm feeling generous).

ok..I'm up for challenge.. now to buy book and read it and post answer by friday..dam..not possible.. dam conference...
I have a hunch of answer..bu need to rethink
thanks for the challenge..
jb

[Jorge]

Crap. I read that book sometime fairly early in high school, way before I'd had enough math to appreciate it, and I didn't like it nearly as much as the RINGWORLD books.  And since then, I've had the math and then had 20+ years to forget it. I'm gonna get some popcorn and wait for the answers.

Niven was never one to allow trifles like plot or characters to get in the way of a good world-building concept. It's a good thing he included a "dramatis personae" at the end of his books; otherwise, I would have no clue about the characters I'd just read about.

[Lars_J]

Yep,
Cool concept + bland characters + sprinkling of sex = any Niven novel.

I have yet to find a novel of his that does not match that formula. Not that there is anything wrong with that, I suppose.

[TOG]

OK, time for a fun application of these principles to a literary SF example. Moving OT, obviously. I'm flagging this post so the mods can move this subthread appropriately. Not sure where it will end up.

Reading assignment: "The Integral Trees" by Larry Niven.

Homework questions:

1a) Relate the cardinal directions of the Treedwellers to the axes of the NASA LVLH frame described above. Use the diagrams at the front of the book for reference. b) What is the origin of the frame, from the perspective of the Treedwellers?

2a) Relate the lines of the Treedwellers' saying:

East takes you out.
Out takes you west.
West takes you in.
In takes you east.
Port and starboard bring you back.

to the terms of the C-W equations (attached). 2b) Which term in the C-W equations is not accounted for in the saying? 2c) What force in the Treedwellers' daily experience is represented by the "missing" term? 2d) Why do you think the missing term is not included in the saying?

3) (extra credit) Compare and contrast the dynamics of the integral trees with the severed tether from TSS-1R on STS-75. In particular, what force accounts for the different behavior of the "inward" tuft of the Trees compared to the inward end of the severed tether?

Homework answers due Friday 1700 CDT (I'm feeling generous).

Crap. I read that book sometime fairly early in high school, way before I'd had enough math to appreciate it, and I didn't like it nearly as much as the RINGWORLD books.  And since then, I've had the math and then had 20+ years to forget it. I'm gonna get some popcorn and wait for the answers.

I've got it!   42!     

[sdsds]

From the updates thread:

Some good views of the Dragon CCP (Crew Command Panel) near the Lab RWS.

http://www.spaceflight.nasa.gov/gallery/images/station/crew-30/html/iss030e250643.html

http://www.spaceflight.nasa.gov/gallery/images/station/crew-30/html/iss030e250651.html

FRGF Sep? Like the crew can command Dragon to ... ? ... do what exactly with its grapple fixture? Is the FRGF held to Dragon with frangible nuts?