**Lesson Info**

-   **Lesson Number:** FPPD-LL-0006

-   **Lesson Date:** 26-jul-2000

-   **Submitting Organization:** Swales Aerospace, Inc. (GSFC 491)

-   **Submitted by:** Michael J. Cully

**Subject/Title/Topic(s):**

EO-1 Inertial Reference Unit Hemispherical Resonator Gyros (HRG)
Exposure to Helium at GSFC.

**Description of Driving Event:**

HRG quartz crystal becomes permeated with helium. This increases gaseous
damping and therefore requires larger drive voltage (Parametric Drive
Voltage or PDV). Further degradation will result in the need to replace
the gyros. The impacts to the program would be significant.

The PDV has been monitored on the spacecraft since initial integration
at Swales Aerospace. Since arrival at GSFC for final integration and
system level testing a significant degradation of the PDV has occurred
due to large amounts of helium release in building 7. Samples in early
January indicated levels on the order of 50 to 60 times typical ambient
environments (5-6 PPM).

**Lesson(s) Learned:**

Based on these developments an extensive effort was made to address this
issue over the last several months and the following actions have been
taken:

1)  Switched to using liquid nitrogen boil-off as purge source in lieu
    of research-grade nitrogen. It was determined after some
    investigation that the research grade nitrogen may contain helium
    molecules depending on the source and bottle usage by the vendor.

2)  Closed off 5 of the 6 venting chevrons on the IRU with aluminized
    tape. This will maximize the nitrogen purge environment internal to
    the IRU and blocks possible entry paths for helium. Litton G&C
    analyses indicate that one chevron is sufficient for
    de-pressurization profile of launch.

3)  Increased the GN2 flow rate to 16cf/hr (16 times original rate).

4)  Wrapped the teflon purge line with aluminum tape on both ends and
    inserted remaining section in copper tubing. This will eliminate the
    potential of helium penetrating the purge line. System was later
    upgraded to use a stainless steel tubing from the purge cart
    directly to the purge valve on the S/C external valve connect.

5)  Maintained the IRU heaters on whenever the S/C and ACE are powered.
    This increases molecular activity and increases the rate at which
    the helium atoms move out of the HRG.

6)  Provided automated daily capture of PDV trend data, including
    information related to temperature since the drive voltage is a
    function of temperature

7)  Eliminated use of Battery AC while at GSFC and mounted a battery
    chiller plate. This reduces the intake of a helium rich air into the
    spacecraft.

8)  Removed the IRU to another facility in late February until the
    threat in building 7 was mitigated.

9)  Implemented a continuous helium monitoring system in building 7.
    This allowed for the determination of helium sources within the
    building and the subsequent mitigation of helium threat during
    critical operations.

10) Worked with the other programs that use helium in the I&T activity
    area (MAP & IRAC) and Building 7 facilities personnel on controlled
    venting of helium from the chamber resulted in new procedures for
    dewar change out and storage.

Based on the above and particularly item #9, Swales felt confident that
it was safe to re-integrate the IRU. Of specific note, it was determined
that change-out of dewars in the cryo lab in the basement of building #7
had a significant impact on helium levels in building #7. It was
determined that this venting was exhausted out via dedicated duct work,
however a portion of the vented helium would re-enter the building due
to the fact this venting duct was in the proximity of the buildings
intake duct. GSFC facilities is currently evaluating means of modifying
the vent duct to minimize residual intake of helium back in to building
7.

Further precautions are required at the launch site since there is
extensive use of helium on the pad and very little performance margin
remains on the IRU. Swales has been working with Boeing on replacing the
purge line in the fairing. Swales has also procured a stainless steel
purge line for the supply to the spacecraft while it is on the pad.

**Recommendation(s):**

The following is recommended to mitigate the risk associated with HRG
helium intrusion.

1)  Implement a continuous purge system on the IRU at the beginning of
    the I&T flow. This is regardless of the controls on release of
    helium in the facility.

2)  Develop a continuous helium monitoring system (EO-1 has implemented
    the LEYBOLD UL 200 Helium Monitoring unit) within Building 7 or in
    any facility that uses large quantities of helium that may expose an
    HRG.

3)  Develop procedures and controls for helium Dewar changeout in the
    Building 7.

4)  Do not allow unused Helium bottles or dewars to be stored in area
    that an HRG will be integrated.

5)  Plan in advance the helium controls during launch site activities
    since this could be a major threat to the HRG.

**Evidence of Recurrence Control Effectiveness:**

Since the implementation of the continuous purge (Phase II) and helium
monitoring system in Building 7 there has been no further measurable
degradation of the HRG. In fact a slight improvement has occurred in the
PDV.

**Applicable NASA Enterprise(s):**

**Applicable Crosscutting Process(es):**

**Additional Key Phrase(s):**

Facilities

Flight Equipment

Launch Process

Spacecraft

Test Facility

**Can this lesson be included in the International Safety Lessons
Learned (ISLL) Database?** No

**Submitted by:** Michael J. Cully

**Mail Stop/Code:** Swales Aerospace, Inc.

**Lesson Author (if different):** N/A

**Phone Number:** 301-902-4356

**E-Mail Address:** mcully@swales.com

**Lesson Date:** 07 / 24 / 2000

**Reference:**

Red Team Review 3/28/00 Special Topic