Delta IV launch question

[BazBear]

Indeed, I was having an entire day where I was constantly thinking one thing and saying/writing another.  I finally just went home to pet the cat.  I mean the dog.

It's almost a shame those t-shirts are mythical.

With their rarity, a mercenary or desperate winner could probably sell their authentic one to the Blunder or Adrian etc. , for a nice premium price.

[Sus_pilot]

Indeed, I was having an entire day where I was constantly thinking one thing and saying/writing another.  I finally just went home to pet the cat.  I mean the dog.

or the macaw...

[ka9q]

This is  considered a component-level design factor, not a system-level design factor.

In the sense that this was dependent on electrical properties of the stage design and staging sequence design, this was a system-level design failure.

I guess this is a matter of semantics. If a given pyro fails to fire despite being installed correctly and undamaged at the moment of firing, that is clearly a component failure. This seems extremely rare in modern spacecraft and launch vehicle pyros.

But if the pyro fails to fire because it's damaged by something else, it's kind of hard to ascribe that to a pyro component failure. Sure, a separation system might have been designed to withstand a specific kind of damage, e.g., by initiating a linear shaped charge at both ends simultaneously, but it wouldn't have failed without that external damage. Any component can fail if sufficiently damaged, and it's hard to blame that strictly on the component. So I'd classify this as a system problem.

Regarding the effect of the second-plane separation failure on Skylab, I had understood it did not trigger an abort because there was so much mass margin that the vehicle had no trouble making it to orbit. My understanding is that there was so much less mass margin on a lunar mission that just the mass of the interstage would have prevented a successful TLI. You seem to be saying this is not true, is that right?

[JayUtah]

I guess this is a matter of semantics.

Yes, or more specifically a matter of scope.  That's why I wrote two paragraphs with a different perspective and a different answer.  You can consider each pyro initiator a component along with the ordnance load, or the whole installed pyro assembly together a component.  It really matters only when you want to have a specific kind of engineering discussion to exclude the other paradigm.

As I said, I believe there's a system-level argument to be had based on how the separation sequence was designed to proceed.  There are holes in that design (no pun intended) that would properly be system-level design questions.  And as you note, there's a system-level design argument to be had at a broader scope that incorporates not only general hazards from the environment and the rest of the vehicle but also the specific hazards from the payload.  If you want to launch on a Delta, you spend a lot of time in the payload integration and integration testing phase.

You seem to be saying this is not true, is that right?

No, I'm not making the mass-budget argument, although I know that the Skylab launch did indeed have a pretty huge margin.  The mission report says there were no guidance anomalies aside from a slightly longer S-II burn.  But I'm not sure about the margin for the typical Apollo stack.  My impression, talking to the ordnance engineers from Boeing, was that it was strictly for safety -- the abort for a manned mission.  An immediate abort wouldn't be a mission rule if it were just a performance issue.

The concerns I've always heard from Boeing are (1) the aft skirt heating -- which happened on Skylab 1 -- and (2) the various aerodynamic, structural dynamic, and J-2 interference issues from a partially-separated interstage.  Ostensibly you'd have a few minutes of flight following an indication of S-II interstage separation failure to decide on the abort, but the gist of what I heard was that unless it separated cleanly, things could conceivably go very bad very quickly.  Better to abort when you can command a clean S-II shutdown and not give the LES too much to do, instead of trying to separate from a possibly tumbling, possibly damaged and burning vehicle.

[JayUtah]

Here's the joint in question.  Or rather, on a test fixture.  The charge is the white band being fitted to the joints.  The tension straps are the short pieces of metal that connect the stringers (the ridges that stick out).  For flight, the charge was covered with an aerodynamic fairing that can be seen as two narrow white bands near the top of the S-1C, but that's essentially it -- no significant protection from debris.



And here you can see the tension straps severed after the ordnance has fired.

[Allan F]

Is that a shaped charge? Looks more like a piece of quite heavy det-cord.

[JayUtah]

I agree it looks like det-cord, but I'm told it's the linear shaped charge by the person who supplied the photos, who was on the Boeing ordnance team that designed and installed it.  As I'm sure you're aware, ordinarily the shaped charge for this application would have a V-shaped cross section with the concave side facing the straps.  However, the encapsulation of the charge need not have the same cross section.  There are rectangular and beveled encapsulations -- plastic, epoxy, etc.

You're used to the thin copper casings used in building demolition.  Keep in mind the encapsulation doesn't have to contain pressure or contribute to the brisance of the ordnance.  It just keeps the plastic explosives in the proper shape and protects it from light damage and contamination until it's needed.  If you build the linear shaped charge (LSC) as a copper-clad, form-fitting component, it would have to be a circle in the same circumference as the S-1C and the S-II interstage.  If you install it as an epoxy- or elastomer-clad unit, it has the same effect but it's easier to handle and install.  In the top photo you can see the spool from which the ordinance package is being unwound.

[Allan F]

Is there a marking on it which shows the cutting direction?

[JayUtah]

For rectangular cross sections, yes.  For beveled cross section, the side away from the cutting direction bears the bevels.  The cutting side remains flat.

[ka9q]

I know this picture is from way back in 1963, but doesn't it look just a little strange to see a stereotypical engineer in white shirt, black tie and pocket protector handling explosives?

Well yeah, the guy to his left is actually holding the spool, and we can't really see what he's wearing...

Oh, and they're still working while the rest of the country was watching JFK's funeral...

[JayUtah]

Oh, and they're still working while the rest of the country was watching JFK's funeral...

I hadn't noticed, but you're right.

[smartcooky]

Oh, and they're still working while the rest of the country was watching JFK's funeral...

That has a nice sort of symmetry to it; engineers pressing ahead on the project that JFK started even on the day of his funeral.

[cjameshuff]

They seemed as concerned about engine gimbal hardovers as premature shutdown, and some of the failure scenarios looked pretty bad no matter what you did. Still, they took some precautions such as canting the four outboard engines slightly outward. This was said to make it easier in case of an engine hardover, but I'm not sure how.

Are you sure that was for the case of a hardover and not an engine shutdown? Pointing the direction of thrust of each engine closer to the center of mass rather than parallel reduces torques if one underperforms or unexpectedly shuts down.

[ka9q]

I'd have to go back and find the actual discussion, but I know they were concerned about both hardovers and shutdowns.  The IU would quickly try to compensate, but the gimbals on the remaining engines probably couldn't move fast enough to avoid some momentary but catastrophic bending moments on the vehicle structure.

Not being a mechanical or aerospace engineer, it wasn't until I read some of the Saturn V flight reports that I realized just how critical those bending stresses in the vehicle structure really were. Imagine a car with an engine so powerful that merely losing traction on a tire or turning the steering wheel just a little too quickly or too far would rip your car apart even before it had a chance to roll over and crash.

Then again, judging from their subjective reports of what it felt like on top of a Saturn V as it lifted off the pad, the astronauts probably had a very keen awareness of these possibilities...

[JayUtah]

Not being a mechanical or aerospace engineer, it wasn't until I read some of the Saturn V flight reports that I realized just how critical those bending stresses in the vehicle structure really were.

A bending moment of 80,000 lb-ft was typical in a Saturn V flight.  When you look at the construction, the miracle is how something that looks so flimsy can be so frakking strong.