Countdown event timing

[ka9q]

The STS SRB's when firing seem to me to be just barely short of an explosion. Many observers have been critical of their use on safety grounds (for the reasons stated by ka9q and AllanF) yet ones of a similar design look set to be used in the SLS that will put the Orion spacecraft in orbit with a crew in the next decade   

Yes, and this also bothers me. To see why, watch this video of a failed Delta-II/GPS launch (you may have already seen it):

This is a much more typical SRB failure than what happened on STS-51L (Challenger). I believe one of the things that doomed the Aries 1 was an Air Force study that showed this kind of SRB failure would probably be unsurvivable even with a working LES because the parachutes would likely be destroyed by burning chunks of solid propellant. I don't know if there's been a corresponding analysis of the Orion atop SLS.

[ka9q]

Of course, not every failure of a rocket with SRBs has been their fault. This 1986 Delta failure was caused by premature shutdown (at 1:55) of the liquid fuel engine in the core stage. Its gimbal provided the only roll and pitch control. This is an excellent demonstration of what happens when the angle of attack gets too high around the time of max-Q.

[smartcooky]

The STS SRB's when firing seem to me to be just barely short of an explosion. Many observers have been critical of their use on safety grounds (for the reasons stated by ka9q and AllanF) yet ones of a similar design look set to be used in the SLS that will put the Orion spacecraft in orbit with a crew in the next decade   

Yes, and this also bothers me. To see why, watch this video of a failed Delta-II/GPS launch (you may have already seen it):

This is a much more typical SRB failure than what happened on STS-51L (Challenger). I believe one of the things that doomed the Aries 1 was an Air Force study that showed this kind of SRB failure would probably be unsurvivable even with a working LES because the parachutes would likely be destroyed by burning chunks of solid propellant. I don't know if there's been a corresponding analysis of the Orion atop SLS.

Holy Moly!!

I suppose the "fireworks" display is the aforementioned chunks of burning solid propellant falling back to earth.

[Echnaton]

IMHO the main reason for using large solids is to form a subsidy to Thiokol, keeping them in the large solid rocket business so they can manufacture storable SLBMs. Many would disagree with that opinion though.

Not sure it is the main reason, but probably a consideration.  There is a need in government contracting to spread the money around among various states and to appeal to various congress critters with broader agendas than the single project.  And in this case, the work can fund research and other needs that seem sure to be applicable to the Defense department through the NASA budget. In the same view as your comment, the entire space program can be viewed as a corporate welfare/jobs program for aerospace engineers to help maintain the defense program.

[Bryanpoprobson]

Wasn't the option of SRB's the only economically viable option to get around the thrust required for the shuttle? I expect it wasn't a decision that NASA necessarily liked. 

Apology's for taking this thread a little bit away from the OP.

[Sus_pilot]

Of course, not every failure of a rocket with SRBs has been their fault. This 1986 Delta failure was caused by premature shutdown (at 1:55) of the liquid fuel engine in the core stage. Its gimbal provided the only roll and pitch control. This is an excellent demonstration of what happens when the angle of attack gets too high around the time of max-Q.

I'm going to use this one to explain why they should pay attention to the NOTAM's around KSC and Vandenberg,  among other places. It'd suck to have all that fall out the sky on to your aircraft.

[ka9q]

Here are two more launch failures involving launchers with SRBs:

18 April 1986, Titan 34D, Vandenberg AFB, KH-9 payload:

This was another SRB failure, caused by separation of insulation within one of the SRBs near a field joint and burn-through during flight, somewhat similar to the Challenger STS-51L failure.


12 Aug 1998, Titan IVA, Cape Canaveral AFS, NRO payload:

This one was more like the Delta/GOES-G failure in 1986 in that the cause was an electrical short, not an SRB failure. Power was momentarily lost to the guidance computer, which in turn provided control signals to the guidance platform. The platform tumbled. When power came back, the guidance computer commanded a hardover of the engines to compensate for a large, illusory attitude error and aerodynamic forces ripped the vehicle apart starting with one of the SRBs. This triggered the destruct system.

Destruct ordnance can be fired not only by a command from the ground, but automatically by the vehicle itself when breakwires detect that it's coming apart anyway.

Since these were both Titans that use hypergolic propellants in their core stages, the explosions produced huge clouds of toxic brown N₂O₄ gas (actually a mixture of NO₂ and N₂O₄). This is the same stuff that gives smog its yellow-brown color. I don't think any western launch vehicle lower stages use it anymore, though it's still common in upper stages and in spacecraft.

[Echnaton]

Here are two more launch failures involving launchers with SRBs:

Ya gotta luv rocket porn.

[Glom]

I seem to remember a shuttle launch being scrubbed, after the shuttle engines were lit.. Or am I imagining that? Must have been pretty late on the countdown clock.

Mike Mullane described such an event in his memoirs. In particular, he described the effect on his wife who was watching several miles away.

You hear the ignition then all goes quiet, so it sounds a bit like an explosion. Not pleasant for the wife.

[Luke Pemberton]

Ya gotta luv rocket porn.

I've typed my witty reply once, but in good taste I'm not sure I can post it, so I won't risk a reprimand from LO.

[cjameshuff]

The STS SRB's when firing seem to me to be just barely short of an explosion. Many observers have been critical of their use on safety grounds (for the reasons stated by ka9q and AllanF) yet ones of a similar design look set to be used in the SLS that will put the Orion spacecraft in orbit with a crew in the next decade   

The initial versions are to use Shuttle-derived solids. The later versions are to have higher performance boosters, two of the proposed options being liquids, one using F1B engines derived from the F1 used on the Saturn, one using a domestic, uprated version of a NK-33. The third option is another ATK solid.

[ka9q]

Whatever happened to earth orbit rendezvous? In the 1960s, you could make a reasonable argument that rendezvous is a tricky, difficult procedure you do only when you have to, and the one in lunar orbit was enough since it greatly reduced the total mass you had to lift into space.

But rendezvous is now utterly routine, unlike launching Saturn-V-class (and larger) rockets, and you could assemble a spacecraft of any desired size from enough launches on smaller (and already available and proven) launch vehicles. Case in point: the ISS.

[Glom]

I thought it was a cost thing. Having a large LV capable of launching your entire payload in one go means cheaper operating costs that having to manage too missions.

[Luke Pemberton]

I thought it was a cost thing. Having a large LV capable of launching your entire payload in one go means cheaper operating costs that having to manage too missions.

I thought it was both factors to be quite honest, cost and lift. Without LOR it would have required two or more lifts to get the lunar package into space (mass factor), which would have cost more.

[JayUtah]

That has always seemed somewhat dangerous to me, given the consequences of either an umbilical disconnect failure or failure of an arm to retract. These risks would be avoided by retracting the arms just before liftoff so you could still abort the launch if one fails to do so.

The swingarms are an important part of stability under unexpected wind loads right up to the instant of flight and engine thrust disagreement during ignition and runup.  Umbilical disconnection is an art.  Which is to say, it's an engineering discipline within rocket science in and of itself.  Understandably it's biased toward fail-safety.  Detanking is a big issue, as you guessed.  But also with cryogenic propellants you typically want the option of restarting a topoff procedure if you need to cycle the countdown.  At a certain point you close off the venting and allow the tanks to naturally pressurize, but for a recycle you may want to reopen the vents.  That means you need the propellant lines connected right up to the scrub point.

Consider also deadfacing, which is often best accomplished with one-time actuators.  This leads to the larger issue in that umbilical mating is a laborious process.  It's not just a matter of swinging the arm back into place.

But why was the CM access arm retracted so soon? Was this to get out of the way of an LES firing in the event of a sudden emergency in which they wouldn't have time to climb out of the cabin anyway?

My impression is that the service swingarms (including the CM's) were very much more massive and couldn't benefit from the retraction methods used for the umbilical swingarms.

I can see why you wouldn't want to do this too early, as it would allow the platform to drift, but why T-17 sec, specifically?

You need several seconds of good data to know that the platform is released and responding.  And beginning a few seconds prior to ignition, the telemetry bus starts getting saturated with propulsion system events.  Even today we stagger individual engine ignition commands.  And at T-7 seconds or so the vehicle is in full ignition rattle -- not a time when you want to be trying to calibrate the guidance platform.  If you aren't sure by then whether you've got a good guidance system, then you don't want to light off the engines.