Explainable Simulation in EMail

Here is a hypothetical e-mail message between engineers who are collaborating on a design. The message is about the Reaction Control System (RCS), the network of tanks, valves, and thrusters that are used to steer the space shuttle. Using hypermedia links (shown as highlighted text), the author refers to a "scenario" and to specific events in it (the drop in pressure, the operator response, the result on the system, an alternative scenario). A scenario is a description of the dynamic behavior of a simulated system (the RCS) under specific initial conditions and environmental events (operator actions).

In this example, the engineer has tested an operator procedure on the RCS, and is sharing the results with colleagues. When the reader traverses one of the highlighted hypermedia links in the email message, DME generates the corresponding explanation. Each explanation offers a set of follow-up questions, leading to further explanation.

Ralph Johnson <johnson@rcs.ss.nasa.gov>
Date: Thu, 21 Oct 93  17:29:19 PDT
From: Ralph Johnson <johnson@rcs.ss.nasa.gov>
To: RCS design team <design-team@rcs.ss.nasa.gov>
Subject: Operator response to RCS system failure

Here is a scenario in which the RCS has a leak in one of the legs near a thruster.

In the training session, the operator sees the drop in pressure at the oxygen tank and then closes the valves in the order prescribed by procedure RCS-OP-24.5. As shown in the scenario at State 4, the operator closed the valves on the thrusters first. Then the operator closed the isolation valves at the thruster manifolds, working toward the tank (see State 6).

After closing the other valve using this breadth-first, upstream strategy, you can see that the pressure is stablized.

If the operator had closed the valves in a different order, allowing the pressure to drop too quickly in the thruster connected to the leaking leg, then that thruster would have started to cavitate. This scenario is shown in RCS-scenario-38.