On a Roll: One Small Step for Reusable Rockets

 

Updated information at the bottom regarding thrusters and fuel!!

SpaceX’s Falcon 9 rocket is on a roll.  Not literally of course, that would be bad.  More importantly, SpaceX noted on their Twitter feed that roll rate of the Falcon 9 first stage booster was close to zero.  Why is that so important that SpaceX felt the need to highlight it?

“Last known state for rocket boost stage is 360 m/s, Mach 1.1, 8.5 km altitude and roll rate close to zero (very important!)”

Let’s clear that up.  The roll rate is the amount of “spinning” a body does, in this case the spinning of the Falcon 9 first stage rocket body.  And the rotation must happen around the “longitudinal axis” or the spine of the booster, running from its tail to its top.  The picture below, of an airplane rolling, gives perhaps a better idea?

So, why is roll bad?  It’s not, if it’s controlled.  But when roll becomes uncontrolled, when something spins so quickly that people and equipment can’t compensate, that’s when roll is a problem.  It can affect direction.  Uncontrolled roll can damage equipment–just by spinning so fast, some equipment flies off.

As you might imagine, then, keeping Falcon 9 from rolling uncontrollably is important to landing it safely.  There are thrusters mounted around the rocket body, which can be ignited in a particular sequence to help with a controlled reentry.  Falcon 9’s central rocket engine is designed to turn on (reignite) and off to also help control the rocket’s descent to the Earth.  And allowing both thrusters and central engine to start and stop while not spinning around makes controlled descent of the body easier.  How did SpaceX get uncontrolled roll to zero?  First, a short history of SpaceX’s Falcon 9 roll problem and solution.

According to an earlier blurb on SpaceX’s website, the landing legs that were added to this particular Falcon 9 were key to reducing first stage roll.  As noted in a previous post, SpaceX say their efforts to reignite the booster in the Sept. 29, 2013 launch was hampered because they “exceeded the roll control authority of the attitude thrusters.”  Essentially, there is a sort of “default” setting for the thrusters that control the movement of the booster.

Within that setting is a limit or two, designed to keep the thrusters from damaging themselves, or the rocket body.  Other limits, probably to do with the rate of the roll (the speed of the spin), might tell the thrusters to not even bother turning on, because they can’t compensate if the rocket body is spinning too quickly.  Those limits were exceeded.

In that same paragraph about the roll control authority, SpaceX also say the roll was excessive because the stage didn’t have the landing gear.  So the roll made it difficult to control the booster’s reentry with thrusters.  However, SpaceX had successfully reignited the first stage’s central engine.  But with no way to control roll, the rocket body couldn’t be safely controlled.  In the Air Force we have a saying “All thrust, no vector.”  That saying literally applies to SpaceX’s Sept. 29 reentry and reignition attempts.

“This particular stage was not equipped with landing gear which could have helped stabilize the stage like fins would on an aircraft.” 

Obviously, then, that changed for the April 18, 2014 launch.  On 25 February, 2014, Elon Musk twittered that landing legs had been added to the Falcon 9 for this launch.  In theory, at least according to SpaceX, these legs should help against uncontrolled vehicle roll.  And the landing legs worked as fin stabilizers, keeping the booster from rolling uncontrollably.  For the latest resupply launch yesterday, SpaceX again tried the reignition of the Falcon 9 first stage booster.  And this time they seem to think things went pretty well.

The booster did land in the ocean, as SpaceX always had said was their goal (this time), but it did so in a controlled way, with no roll.  I am not privy to SpaceX’s timeline, but it seems they are much closer to being able to fly a booster back to a launch pad to be used again.  That would be the other reason for the landing legs.  But the fact the landing legs help control roll too is very cool!!

Just one weird thing to note.  Nowhere, in SpaceX’s mission literature, does it say anything about their attempts to do these reignitions.  Perhaps because their customers, like NASA, don’t view these attempts as part of the “real” mission?  It’s weird to me, because, well, these are real, historic attempts at doing something different and meaningful for space launch.

Update 2248MST 19 Apr 2014:  I just read Spaceflightnow’s piece posted not too long ago.  They have added information that stronger thrusters were mounted to the Falcon 9 launched on 18 Apr 2014.  And there was extra nitrogen fuel to expend on the thrusters to help keep roll under control.