A waiver has been requested by Space Shuttle engineers, ahead of Atlantis’ launch on STS-115 this weekend, to mitigate risk associated with Space Shuttle Main Engine (SSME) exhaust in the event of the orbiter’s computers issuing a shutdown after engine ignition on the pad.
The rationale is based on the risk of Hydrogen in the air – which could cause an explosion in certain conditions.
**Information taken from acquired documentation and four associated videos – available on L2**
At T-6.6 seconds, the three SSMEs ignite in sequence. In the event of – for example one SSME failing to reach full power – the orbiter’s computers signal for shutdown – that can leave Hydrogen in the air around the aft of the vehicle. If the presence of Hydrogen is of a volume of four percent or higher, combining with Oxygen, it can be a combustion source.
Recently, NASA Marshall Space Flight Center (MSFC) issued a waiver for the system that deals with Unburned Hydrogen. This issue of backflow of Hydrogen appears to remain a problem based on this waiver, only issued for STS-115.
‘With urgent engine shutdown, the hydrogen in the air must be dispersed or burned to prevent a fire or explosion on the pad,’ noted the document. ‘This led to a Hydrogen ‘burn off ‘ system for the following situations.’
1. On pad SSME start sequence. 2. Normal On pad SSME firing. 3. FRF Shutdown. 4. On pad abort shutdown.
However, a Hydrogen Burn Off Igniter (HBOI) system is not available for shutdowns late in the firing sequence, hence the need for a waiver. Numbers three and four are not covered by the HBOI.
HBOIs are activated at T-10 seconds and burn for 8 +5/-0 seconds. Based on that burn time, the current Hydrogen Burn Off System is not be active during a late SSME abort situation.
During this later phase, the Hydrogen that is not burned in the combustion chamber of the SSMEs will mix with atmospheric Oxygen to be consumed in the plume flame. This burning of Hydrogen in the exhaust plume flame continues until the Hydrogen concentration is below levels that will cause them to burn, thus the SSMEs themselves act as the Hydrogen burn off igniters.
Again, if the shutdown occurred earlier in the sequence, the HBOIs would have burned the excess hydrogen, keeping it from exploding. However, the HBOI does not work this late in the sequence as noted in the concerns.
Speculation has considered that a computer simulation still recommends that the HBOI will work during late shutdowns. There has been no report of damage, to the contrary, the report notes more than 1250 late shutdowns on the test stand without detonation.
The waiver request itself only notes that since the HBOI does not work late in the sequence, a waiver is needed. It does not go into why the waiver has become an issue now.
The intent of the HBOI requirement is being fulfilled by the SSMEs and other methods, such as Helium being pumped in to decrease the temperature and purge the Hydrogen. If Hydrogen concentrations are high the Firex sensors alarm – and pump in more water. This eliminates ignition sources by cooling surfaces below Hydrogen auto ignition temperatures. Hydrogen is quickly dispersed and concentrations fall below flammable levels (less than four percent).
The waiver request goes on to say that the permanent fix is to ‘fulfil or rewrite the requirement.’ Fulfilling the requirement would enable the HBOI system to continue late in the sequence. Rewriting the requirement would essentially make this waiver permanent. The issue of flammable Hydrogen concentrations and detonations late in the sequence would be handled by the SSMEs, Firex, Helium and dispersion into the air until Hydrogen concentrations are less than four percent.
Looking at the diagram to the left (click to enlarge), before the SSMEs ignite, hundreds of thousands of gallons of water (flame hole water), and the HBOIs are working to deal with the impending build up of Hydrogen.
To the left of the diagram, the ROFI Fire Command refers to the start of the HBOI system. Looking at the center of the diagram, typical burn and maximum burn are referring to the HBOI system. That ends at T+3 at best.
Looking to the right of the diagram, the Helium purges, Firex sensor alarm with subsequent additional water deluge (separate from the flame hole water at T-10 seconds) and the burning of the Hydrogen in the SSME exhaust takes care of the issue of elevated levels of hydrogen and possible explosion. The waiver seeks approval for this system.
Currently, flight rules require the continued burning of the HBOI system to function throughout this critical time.
Proper disposition of unburned Hydrogen in the SSME exhaust is a critical event. It carries the possibility of fire and explosion on the pad in the face of a late shutdown of the SSMEs.
However, as already noted, with thee 1250-plus test stand events – and no detonations occurring – it remains unclear and unsupported for this waiver to become permanent.