Why SSDs are Problematic in Space

When the switch is made from HDDs to SSDs in most applications there are usually very strong benefits, but in the case of spacecraft, SSDs are causing no end of trouble.

Sure, SSDs provide certain extraordinary performance and other technical benefits.  Of particular importance in space applications are the facts that they consume less power, they are more resilient to the high g-forces endured during  launch, and, most importantly, they weigh less than HDDs, which is extraordinarily valuable when it costs tens of dollars per gram to put satellites into orbit.  The big drawback is that SSDs have no moving parts, and that’s the root cause of immense issues for companies whose business is spacecraft.  The SSD Guy first learned of this issue at the beginning of last April, and has spent the entire past year researching the issue.  This blog post will share what I have been able to learn, despite the field’s extreme level of secrecy.

Rotation is the Root Cause

Hard disk drives have two basic mechanisms: the rotating disk, and the motion of the head mechanism as it seeks tracks to read and write on the disk’s surface.  The head assembly, or “Actuator” has been carefully designed to be as lightweight as possible to optimize track access and settling times.  For earthbound applications, the disk’s inertia is much less of an issue, so it receives little attention.  Because of this there is a lot of angular momentum in an HDD, and since there is neither gravity nor friction in space, any type of momentum must be understood and offset.

The disk’s rotation causes a lot of trouble.  Those who studied gyroscopes in their physics class know the gyroscope effect well, and how to use right-hand rule to explain how torque (τ) changes angular momentum (L).  In brief, an applied force parallel to the axis of rotation on a rotating object produces a resultant force, also parallel to the axis of rotation, offset from the applied force in the direction of rotation by π/2.  This is illustrated in the basic diagram below.

The direction of the resulting force’s vector is offset, but its magnitude is the same as the applied force’s vector, so it’s a known quantity that can be accommodated by a satellite’s guidance system, which will make any required adjustments by activating its retro-rockets.  This has been carefully applied to all guidance software for the past several decades, to the point that it’s so integral to the program that it would be nearly impossible to remove without a total rewrite of the code.

There are other less-significant effects that have also been compensated for in the code, including the HDD’s rotational kinetic energy, which might otherwise cause a satellite to rotate in the opposite direction to the HDD, or for a rocket to miss its trajectory by a degree or two, and the centripetal forces on the spinning disk, which causes turbulence in the gas sealed within the HDD (including helium, He).  Any well-experienced air traveler knows how much of an issue turbulence can be.

Add to this the fact that the HDD may be turned on and off to save energy, which is in scarce supply in most spacecraft, so measures are taken to minimize energy consumption.  The guidance software must then compensate for the HDD’s behavior in an on-again, off-again manner.

How SSDs Make this Problematic

HDDs’ tendency to behave as gyroscopes caused enormous problems when they were first put into space some decades ago, and a great and very successful effort was made to compensate for any issues they might cause.  But why would this be problematic today?  It all boils down to issues of legacy guidance systems and software qualification.

To say that space-bound software has been thoroughly tested would be a phenomenal understatement.  Today’s software has been put through an extraordinary number of test suites, field-proven for an enormous amount of time, and deployed across the thousands of satellites that today orbit the earth.  It has been brought to a point where it works so well that any effort to revise the code meets with mission-wide resistance, and may even require approval from the head of the very agency.

Space agencies are reluctant even to discuss the possibility of making the slightest change.  Removing the system’s compensation for gyroscopic and other rotational effects would cause these systems to be changed at their most fundamental level.  Such changes would require extensive and prohibitively costly requalification efforts, and they would most likely introduce new bugs in unexpected places.  For this reason, code changes are virtually ruled out altogether.

But such changes are required for SSD-based spacecraft, because the SSD has no moving parts.  If the software were not updated, then all the compensation built into time-tested guidance systems would cause satellites and rockets to misbehave in the exact opposite way than they had when HDDs were first brought into space.

A Short-Term Solution

Since mission schedules are very unforgiving, and time is not available for changing the software and then putting it through extensive testing, the space community has elected to postpone dealing with the issue by continuing to use HDDs for the foreseeable future.  This works because space mission budgets are less cost-sensitive than most earthly HDD applications.  If HDDs someday become unavailable, the space community will simply increase the price it is willing to pay for an HDD to the point that someone will produce custom-made HDDs based on obsolete designs that were abandoned by mainstream suppliers.

From the scant information I have been able to learn over the past year, this situation is likely to remain in place through next April 1, and for tens of April firsts over the foreseeable future.