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#2780 by wolfpack on 10 Jan, 2013 16:55
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How did the TPS cover the forward attach point after ET sep? In the photos I have seen there isn't an apparent door.
There is an "arrowhead" shaped piece of TPS that has a hole in it for the bolt that separates the forward ET bipod. The whole bipod remains attached to the ET, and the bolt is captured in the Orbiter. The hole is a bit below the mold line of the TPS so thermal issues are mitigated on re-entry. -
#2781 by Fequalsma on 10 Jan, 2013 20:54
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Arrowhead is RCC, per this:
http://www.columbiassacrifice.com/techdocs/RCC_Design.pdfHow did the TPS cover the forward attach point after ET sep? In the photos I have seen there isn't an apparent door.
There is an "arrowhead" shaped piece of TPS that has a hole in it for the bolt that separates the forward ET bipod. The whole bipod remains attached to the ET, and the bolt is captured in the Orbiter. The hole is a bit below the mold line of the TPS so thermal issues are mitigated on re-entry. -
#2782 by JayP on 11 Jan, 2013 01:24
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How did the TPS cover the forward attach point after ET sep? In the photos I have seen there isn't an apparent door.
There is an "arrowhead" shaped piece of TPS that has a hole in it for the bolt that separates the forward ET bipod. The whole bipod remains attached to the ET, and the bolt is captured in the Orbiter. The hole is a bit below the mold line of the TPS so thermal issues are mitigated on re-entry.
The "Arrow Head" simply surrounds the fitting itself. The fitting is a metallic spherical bearing with a hole thru it for the connection bolt. The bolt has pressure charges at each end and is specifically designed to break exactly flush with the outer face of the spherical bearing. (Personally, I think this is one of the most impressive engineering feats on the whole shuttle) the bearing is sprung loaded to rotate flush with the outer surface after separation. Both the faces of the outer race and spherical ball as well as the broken face of the bolt remain exposed to the slipstream and heating effects of re-entry, but the mounting of the entire system is a very heavy metal structure that acts as a heat sink so there is no thermal damage to the parts. -
#2783 by spacecane on 11 Jan, 2013 12:25
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Does anybody have a link to a good closeup picture of the "arrowhead" either with ET attached or without?
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#2784 by wolfpack on 11 Jan, 2013 14:02
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Does anybody have a link to a good closeup picture of the "arrowhead" either with ET attached or without?
I have one of Discovery with the bipod mount still attached after the ferry flight to DC. You can see it well because it is painted red. I'll post when I get home tonight. -
#2785 by wolfpack on 12 Jan, 2013 12:14
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As promised.
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#2786 by JayP on 13 Jan, 2013 00:09
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Does anybody have a link to a good closeup picture of the "arrowhead" either with ET attached or without?
Here's one from STS-133
http://www.nasa.gov/images/content/521274main_iss026e029923_hires.jpg -
#2787 by spacecane on 13 Jan, 2013 01:39
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That STS-133 picture is great but now I'm confused. I pictured the ET bipod having a ball at the end and this ball going into the hole in the arrowhead and then an explosive bolt holding the ball in. I figured upon ET separation the bolt would blow and the ball would come out of the hole leaving a hole in the orbiter belly.
In that picture, it appears as if the silver colored metal is almost flush with the bronze/gold colored ring.
So what exactly is (was) the attachment/separation method of the forward ET addach point. -
#2788 by JayP on 13 Jan, 2013 03:53
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That STS-133 picture is great but now I'm confused. I pictured the ET bipod having a ball at the end and this ball going into the hole in the arrowhead and then an explosive bolt holding the ball in. I figured upon ET separation the bolt would blow and the ball would come out of the hole leaving a hole in the orbiter belly.
In that picture, it appears as if the silver colored metal is almost flush with the bronze/gold colored ring.
So what exactly is (was) the attachment/separation method of the forward ET addach point.
Here is a drawing of the attachment point including a section thru the middle of it. The ball is shown rotated to the maximum extent away from its nuetral position and the bolt is not shown.
The "bolt" is a specially manufactured component of course. What would normaly be the "head" of the bolt is actually a wider cylinderical section with threads on the out side that screws into the upper part of the ball. You can see the threads in the drawing below. The other end of the bolt sticks thru the ET bi-pod head and is secured with a nut from the bottom.
As it was explained to me, The bolt has a pressure charge intalled at each end. When the charges detonate, they create two pressure waves that travel towards each other down the length of the bolt. When the waves meet, they reinforce each other and the tension forces peak causing the bolt to snap flush with the bottom face of the ball. The upper half stays in the ball in the orbiter and the bottom half is retained in the ET bipod fitting.
The clever thing about this system is that it doesn't produce any debris that need to be contained (unlike the sytem used for the aft fittings and the SRB hold downs) and that it leaves a flush metal surface with no voids or exposed edges that could dissrupt the plasma flow on reentry and create a localized hot spot.
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#2789 by AnalogMan on 13 Jan, 2013 19:54
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JayP, the explanation you were given on the how the frangible bolt works appears to be from another application rather than the one used on the orbiter forward attachment.
I've attached a cross-sectional drawing of the head-end of the bolt showing the pyrotechnic mechanism that shears the bolt and results in the clean flush outer surface shown in the STS-133 RPM photograph.
The left-hand side of the drawing shows the mechanism before separation, and the right-hand side after separation.
The head of the bolt (colored yellow) is a hollow cylinder whose internal diameter very similar to the outer diameter of bolt's shank where it passes through the spherical bearing (green) to the outside world. Within the bolt is a free piston (light blue) made of Inconel 718, as are the bolt and bearing parts.
Two pyrotechic pressure cartridges (for redunancy) rapidly force the piston downwards causing the bolt to shear along a cylindrical inner surface. The piston and spherical bearing act as a punch and die set shearing the bolt, rather like a paper punch does acting on a stack of paper. The difference is that the end of the piston is dead flat rather than having an acute shaped cutting edge like the paper punch has.
After the piston shears the bolt, an attenuator skirt on the upper part is slammed and flanges outwards into a 45 degree circular groove - this absorbs excess energy from the piston in a controlled fashion ensuring the end of the piston ends up flush with the flat outer faces of the spherical bearing (ball and mount).
The aerothermal requirement was that there be no more than a ±0.017 inch (±0.43mm) step between the piston and the bearing surfaces, and a gap no greater than 0.035 inch (0.89mm) around the piston.
The pyrotechnic bolt used on the Approach and Landing Test (ALT) flights also used a piston arrangement, but the bolt was fractured in tension around a hollow part of the head. It was found that the exposed fracture surface of the bolt (which surrounded the piston) created a 0.1 inch wide x 0.2 inch deep groove (2.5 x 5.1mm) failing to meet the requirements of re-entry from orbit. This led to the redesign for orbital flights.
Robert Pearlman took a photo of the metallic parts of Enterprise's arrowhead assembly, complete with bolt - its about half-way down this web page:
http://www.collectspace.com/news/news-031510b.html
(click images to enlarge)
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#2790 by JayP on 13 Jan, 2013 22:09
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Thank you for the clarification. I thought it was a pretty impressive piece of engineering, getting the pressure waves to coincide at exactly the right point. Oh well, this does seem a lot more reliable.
I noticed that the flange around the head of the piston, in addition to attenuating the motion, also serves to lock the piston in the extended position and keep it from sliding back up inside the body durring the flight. -
#2791 by Fequalsma on 13 Jan, 2013 22:37
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Note also that the "fwd" orientation in the drawing JayP posted from:
http://www.columbiassacrifice.com/techdocs/RCC_Design.pdf
is opposite from the RPM photo at:
http://www.nasa.gov/images/content/521274main_iss026e029923_hires.jpg
AnalogMan - what is the source for your excellent drawing?
F=ma -
#2792 by AnalogMan on 14 Jan, 2013 14:08
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AnalogMan - what is the source for your excellent drawing?
I found the original monochrome drawing in:
http://hdl.handle.net/2060/19780024234 -
#2793 by spacecane on 14 Jan, 2013 14:41
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AnalogMan,
Thanks for posting that. It makes sense now. I have 3 other questions related to this:
1) What locked the piston in the down position so that it stayed flush with the ball?
2) Related to #1, what locked the ball in the "flat" position?
3) What was the process of mating this attach point? Was it something like half of the bearing plate and the piston assembly in attached to the orbiter while the other half of the bearing plate, the bolt and the ball attached to the ET and then the two halves joined with fasteners?
This was an amazing piece of engineering, I guess that's why I'm so interested into such a small part fo the system. -
#2794 by Jim on 14 Jan, 2013 15:24
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3) What was the process of mating this attach point? Was it something like half of the bearing plate and the piston assembly in attached to the orbiter while the other half of the bearing plate, the bolt and the ball attached to the ET and then the two halves joined with fasteners?
The yoke is attached to the orbiter before it rolls over to the VAB. Each leg of the bipod is then attached to the yoke during Orbiter ET mate. -
#2795 by sdsds on 14 Jan, 2013 20:19
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I've attached a cross-sectional drawing of the head-end of the bolt
Thanks for the great diagram! Are the "dual pressure cartridges" to make the mechanism single fault tolerant? Also, it's clever how the shear section is only 0.79 cm yet the loads nonetheless get carried around it. -
#2796 by wolfpack on 14 Jan, 2013 20:25
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1) What locked the piston in the down position so that it stayed flush with the ball?
2) Related to #1, what locked the ball in the "flat" position?
I'd be willing to bet it's a pretty decent interference fit once that piston travels downward.
Edit: Analog Man already had the answer.
"After the piston shears the bolt, an attenuator skirt on the upper part is slammed and flanges outwards into a 45 degree circular groove - this absorbs excess energy from the piston in a controlled fashion ensuring the end of the piston ends up flush with the flat outer faces of the spherical bearing (ball and mount)."
The deformed metal into that groove will prevent upward travel of the piston. -
#2797 by AnalogMan on 14 Jan, 2013 23:59
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AnalogMan,
Thanks for posting that. It makes sense now. I have 3 other questions related to this:
1) What locked the piston in the down position so that it stayed flush with the ball?
2) Related to #1, what locked the ball in the "flat" position?
3) What was the process of mating this attach point? Was it something like half of the bearing plate and the piston assembly in attached to the orbiter while the other half of the bearing plate, the bolt and the ball attached to the ET and then the two halves joined with fasteners?
This was an amazing piece of engineering, I guess that's why I'm so interested into such a small part fo the system.
1) If you look at the earlier diagram I posted you will see something labelled "piston attenuator skirt" on the underside of the upper part of the piston. This is a thin cylindrical shell that runs all the way around the piston. When the piston slams down shearing the bolt, this skirt is deformed and forced into a channel between the 45 degree chamfer on the outer top edge of the bolt (yellow) and a corresponding chamfer on the housing (pink). This helps slow down the piston and also, as JayP pointed out, locks it into its final position so it can't move.
2) There are two powerful spring loaded plungers on the forward side of the body that act against the upper surface of the bearing plate and which pivot the whole assembly from the tilted orientation to one where the flat on the ball/piston is aligned with the bearing plate surface. A post on the opposite side acts as a locating stop for the assembly. See the attached drawing.
3) The bolt mechanism (bolt shank/piston/body/cartridges) is fitted into the spherical bearing from behind, and the the whole arrowhead assembly is bolted back into the orbiter.
Then, as Jim has mentioned, the yoke (top part of the ET support structure, and actual flight hardware) is clamped to the orbiter using the bolt with a nut on the end. This all happens in the Orbiter Processing Facility (OPF). The yoke is used to support the orbiter on the transporter during rollover to the VAB.
Once in the VAB the yoke is unbolted from the transporter support structure ready for the vertical lift and bolting to the ET struts.
I've attached part of a photo taken by Larry Sullivan (NSF photographer) that shows the flight yoke in use during the rollover (orange structure is part of transporter), and a drawing of the ET forward struts/yoke for comparison (courtesy of fequalsma via L2).
(click images to enlarge)
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#2798 by Fequalsma on 18 Jan, 2013 02:11
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AnalogMan, I think that you omitted a really interesting part of the figure (III-21) from the SLWT SDH. The enclosed portion of that figure shows the side view of the Forward Bipod. It shows that the center of the forward orbiter fitting is 5.5 inches forward of vertical for the loaded ET. I estimate that the unloaded ET's orbiter fitting is 1.0 inches aft of vertical, for a total nominal range of motion of 6.5 inches. So why does this motion occur?
The answer is thermal contraction of the loaded ET. The unloaded ET is nominally at room temperature (70F), and the loaded ET LH2 tank is at -423F, for a total temperature difference of -493F. The length of the LH2 tank barrel is (2058 - 1130), or 928 inches, and the coefficient of thermal expansion (CTE) for aluminum is 13.1e-6 in/inF. The equation for thermal contraction is CTE x length x temp. diff., or 6.0 inches. So the LH2 tank shrinks ~6 inches after it's loaded - wow.
Another interesting tidbit is how the Orbiter is aligned relative to the ET. The center of the forward orbiter fitting is 220.34 inches above the ET center line, and the centers of the aft orbiter fittings are 204.06 inches above the ET center line. So the front is 16.28 inches above the back and, when divided by the 928 inch horizontal separation, trigonometry shows that the Orbiter attachment plane sits 1.0 degree nose-high w.r.t. the ET center line.
F=ma
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#2799 by JayP on 18 Jan, 2013 13:32
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Another interesting tidbit is how the Orbiter is aligned relative to the ET. The center of the forward orbiter fitting is 220.34 inches above the ET center line, and the centers of the aft orbiter fittings are 204.06 inches above the ET center line. So the front is 16.28 inches above the back and, when divided by the 928 inch horizontal separation, trigonometry shows that the Orbiter attachment plane sits 1.0 degree nose-high w.r.t. the ET center line.
True, but the forward attachment is at the outer mold line while the aft attachments are recessed in the umbilical wells so the angle in regards to the belly of the orbiter (which is slightly convex anyways) would be greater than 1 deg. Does anyone know the actual angle of attack of the wings in relation to the ET CL?