Porous plug for Gravity Probe B

Abstract

The confinement of superfluid helium for a Dewar in space poses a unique challenge due to its propensity to minimize thermal gradients by essentially viscous-free counterflow. This poses the risk of losing liquid through a vent pipe, reducing the efficiency of the cooling process. To confine the liquid helium in the Gravity Probe B (GP-B) flight Dewar, a porous plug technique was invented at Stanford University. Here, we review the history of the porous plug and its development, and describe the physics underlying its operation. We summarize a few missions that employed porous plugs, some of which preceded the launch of GP-B. The design, manufacture and flight performance of the GP-B plug are described, and its use resulted in the successful operation of the 2441 l flight Dewar on-orbit for 17.3 months.

Quantum Physics in Space

“…Additionally, keeping liquid helium in a container in a gravity free environment is a challenge on its own because dewars (Thermos bottles) function due to gravity, enabling the liquid and gas to naturally separate. In the zero gravity of space, a different method of separation is needed, which has been solved by the Gravity Probe B using a porous plug that releases the evaporating helium while retaining the superfluid liquid helium [728,729]. The mission used a sponge mechanism inside a porous plug to prevent liquid helium sloshing and to control the pressure in the dewar [730,731].…”

Proportional helium thrusters for Gravity Probe B

“…The plug was sized for a flow rate during on-orbit science data collection of 6-7 mg s −1 . It was fabricated from 316L sintered powdered stainless steel, 6.35 mm thick, 69 mm diameter, and with a permeability of 3.8×10 −8 mm [2,8].…”

Control of fluid mass center in the Gravity Probe B space mission Dewar

“…Higher gas flow and authority was available by operation of a tank heater, although the resulting reduction in helium lifetime would degrade experiment precision. A sintered titanium porous plug [2] allowed the gas to vent while ensuring the liquid remained within.…”