High Force Thermal Latches

Presented byKaren Hawes ASEN 5519 Fall 1999karen.e.hawes@lmco.com

Designed ByWilliam Nygren

Built ByStarsys Research Center

High Force Thermal Latches (HFTL)

HFTLs Presentation

❚ Problem Statement❚ Trade Study Summary❚ Design Features❚ HFTL Operation (Steps 1-10)❚ Design Devlopment❚ Summary❚ References

Problem Statement

Need a release device that,❚ provides support for a high load path;❚ is small in size;❚ is light in weight;❚ is simple in operation & maintenance;❚ is a non-pyro activated device;❚ induces low-to-no loads into the two sides

of the mated interface.

Trade Study Summary

“Old” device a.k.a “Claw” mechanism:❚ Cons of “Old” device:

It was larger, heavier, more complex, and provided a lower pre-load.

❚ Pros of “Old” device:It was capable of re-mating the two assemblies on orbit.

Design Features

BODY-SIDE Features❚ Assist Spring: Crest-to-Crest Wave spring provides a higher force with the

desired (long) stroke of up to 3/4 inch, versus other spring designs (Belleville or Coil springs).

❚ Cylinder: Beryllium Copper (http://www.be-cu.com/beryllium_copper.html) provides adequate strength for the static pressure loads, once the Slug has melted; it also expands adequately to provide a flow path.

❚ Slug: A low-Temp Fusible Alloy Lead-Bismuth (http://www.welcocastings.com/products/fusible-print.htm).

❚ Limit Switch: “Implies” separation, position of Cylinder is such that the likelihood of the Latch Bolt bieng in the Body-Side are unlikely.

❚ Heating Element: High W/area (125-200 W/sq-in), custom made by Memco, heating the Cylinder and Slug materials from -10 C to 71 C in no more than 10 Minutes.

Design Features, con’t

DEPLOYED-SIDE Features❚ Latch Bolt: Multi-Phase Nickel Cobalt; supports high loads, up to 8000 lbf.❚ Retractor Spring: Low spring constant allows for easy installation.❚ Super Nut/Jack Screws: Sequential torquing of eight screws, at low loads,

provides the high load through the Latch Bolt.

Design Features, con’t

Design Features, con’t

BODY-SIDE OF HFTL DEPLOYED-SIDE OF HFTL

HFTL Cross-Sectional View

HFTL Operation

INSTALLATION:❚ Prepare Body-Side of device by pre-loading the assist spring after first

melting the slug and compressing the Assist Spring; allow the slug material to solidify in the pre-load position.

❚ Install two halves of the device into the two sides of mated assembly.❚ On the Body-Side of the device, move the Slide Gate away from the mated

interface, to allow room for the Latch Bolt.❚ Insert the Latch Bolt into the Body-Side cavity and lock into place by

moving the Slide Gate back towards the mated interface.❚ Pre-load the Latch Bolt to desired, high load, by sequentially torquing the

eight Jack Screws. Possible loading up to 8000 lbf (+/- 500 lbf).

HFTL Operation, con’t

OPERATION:❚ Apply current to the device, heating the thermal element which expands

the Cylinder (around the carrier and Piston) and melts the Slug material.❚ The Cylinder expands around 120 F, Slug melts around 124 F.❚ The expansion of the cylinder provides the path for fluid flow, once the

Slug melts; the Assist Spring provides the force to move the Piston/Cylinder assembly which slides the Latch Bolt past the Slide Gate.

❚ Once the Latch Bolt passes the outer edge of the Slide Gate, the two halves are demated. The Assist Spring and Retractor Spring apply a load against the loose parts of each half, to fully contain them.

❚ Once the Slug material solidifies, this will also lock the parts of the Body-Side assembly.

❚ A Limit Switch provides the telemetry which indicates the Carrier has moved past a certain position, implying separation.

HFTL Operation, Steps 1-5Body-Side of HFTLInstalled on Body

Carrier & Assist Springin Pre-Loaded Position

Slug Materialin Solid State

Slide Gate“down”

Slide Gate“up”

Deployed-Side of HFTLInstalled on Separation Item

Retractor Springin Un-Loaded Position

Cover Not Installed

Deployed-SideRetractor Spring

in Pre-Loaded

Position

Latch Boltin Positionwith Carrier

Interface Line

Slug

Carrier

Latch Bolt

STEP 1 STEP 2 STEP 3 STEP 4 STEP 5

HFTL Operation, Steps 6-10

Slide Gate“down”

Latch Bolt isnow locked

Torque Jack Screwsto Pre-Load the

Latch Bolt

STEP 6 STEP 7 STEP 8 STEP 9 STEP 10

Cover Installed

Interface Line

Actuationstarts onceSlug melts

Carriermoves down

Latch Boltis released

Lower side now deployed

Assist Springcompresses Carrierand holds Body-Side

assembly

Retractor Springholds Latch Bolt

and Deployed-Sideassembly

Design Development

The Cylinder was made aluminum but cracked during testing. Results found that the Slug material, which was Tin Bismuth, reacted with the aluminum through small cracks in the anodized surface, causing metal embrittlement and eventual failure.

The Slug material was changed to Lead Bismuth and the Cylinder to Beryllium Copper. The added strength of the Beryllium Copper allowed the walls to be thinner, increasing the surface-area to volume ratio - which also reduced the overall static pressure.

Design Development, con’t

Cracked inner (anodized) wall of aluminum cylinder

Design Development, con’t

Cracked outer wall of aluminum cylinder

Design Development, con’t

The Retractor Spring induces forces that are relatively too high for certain applications. Methods to reduce this are being pursued currently.

The pre-load of the spring is approximately 20 lbf at 1.2 inches of compression. For F=-kx, 20=-k(1.2), so k=16.66 lbf/in.

The potential energy is P.E. = 1/2kx2, so P.E. = 11.99 in-lbf (fairly low)Possible alternative design is the FASSN, also designed by William

Nygren, which dissipates the energy with a mini flywheel operation. Additional design research is a possibility for those interested in the FASSN device.

Summary

❚ This device is small yet it still provides a high-force load path.❚ It is simple to install and can be re-set for multiple operations

on the ground; only one operation is available on orbit.❚ Do not need motors to operate on orbit; it is simple to

operate and requires minimal telemetry to initiate and confirm operation.

❚ Quick deployment without pyro-devices - safer to handle.❚ Low loads are induced into the two halves of the interface.❚ Reliability of operation is shown to be high.

References

❚ Starsys Research Center: Doug MonickCritical Design Review package 5/28/97

❚ Lockheed Martin-Denver: Karl Fortney, Rod Sumpter, and Roger HirschmanLMA Drawing PD9500056, rev C, “HFTL”Trade Summary Memo, dated 1/30/95Failure Analysis Lab, Report # SPL13342DS Procedure 6ME69-B

❚ Also, see websites in text for supplemental “FYI” information:www.welcocastings.com/products/fusible-print.htmwww.be-cu.com/beryllium_copper.htmlwww.inco.net/inco/nispanc902.htm

❚ Additional thanks to William Nygren for the original design of the HFTL and FASSN.

Blue Skies!