The first stage from the March 30 Falcon 9 launch prepared to land on a drone ship at sea. Frequent reflights of Falcon 9 first, and possibly second, stages will require careful logistics. (credit: SpaceX)

All at sea about reusability

by Dick Eagleson
Monday, April 10, 2017

The March 30 relaunch of a previously-flown Falcon 9 first stage (S1) was an undeniably historic event. There were two firsts, one might say, that were firsts because they were seconds. The S1 booster was flying for the second time and it was also recovered at sea for a second time—both firsts.

The press conference that followed the successful deployment of the SES-10 communications satellite was a record setter in its own right. I can’t recall another such that included more surprise announcements.

First, was the fact that SpaceX had chosen this mission on which to attempt recovery of a payload fairing. This effort was, according to reports, at least half successful.

In surprise number two, Elon Musk further revealed that, once re-entry and precision navigation of payload fairings is established with water landings, he intends to employ what he referred to as a “bouncy castle”—presumably a floating one—to land them dry.

One suspects Musk used the term “bouncy castle” for humorous effect. The real fairing recovery system is more likely to resemble one of those “Cloud 9”-type inflatable cushions used in certain athletic events, by movie stuntpeople and by many fire departments in response to would-be suicides-by-jump from high places.

Third, and even more consequential, Musk announced that SpaceX had decided to revisit the whole problem of second stage (S2) recovery that it had put aside some time ago.

Fairly early on in the Falcon 9’s march toward reusabilty, SpaceX put out a CGI animation of a future Falcon 9-Dragon ISS mission in which S1, S2, and the Dragon capsule were all recovered via tail-first propulsive landings on land. The video is still viewable on YouTube.

S2 recovery, as portrayed in this video, involved a domed heat shield on the S2’s nose and four landing legs snuggled up around the engine bell during re-entry. S2 would re-enter nose-first, then flip over in the atmosphere to allow a tail-first propulsive landing on its legs.

Some time after release of that video, SpaceX announced they no longer believed the S2 recovery scenario portrayed in the video to be viable and, seemingly, abandoned the idea of S2 recovery. But Elon Musk, at the SES-10 post-mission press conference, has now said S2 recovery is back on the table.

It is not hard to guess what has prompted this change of heart. SpaceX has now re-entered and landed Falcon 9 first stages intact nine times.

The key to doing so has been use of so-called supersonic retrofire. This involves first flipping S1 end-over-end so that the engines point in the direction of travel as S1 begins the downward half of its parabolic trajectory. Doing this allows re-firing of one or more engines to both slow the stage but also, and more importantly, to generate a bubble of relatively low-temperature expanded exhaust which acts as a dynamic thermal protection system for the rest of S1 as it encounters significant atmosphere at hypersonic speed.

In the process of making several initially unsuccessful landing attempts, as well as its nine successes, SpaceX has gathered massive amounts of data about the effects of this procedure on S1 structure. Copious sensor data was accumulated even on missions that did not end in successful landings. The nine successful landings, of course, also provide invaluable data derived from post-flight inspection of the recovered S1 hardware.

I believe one consequence of gathering this data is that SpaceX now believes it may be able to recover Falcon 9 second stages by first re-entering them using supersonic retropropulsion as has now become routine with the S1’s. I have no insider knowledge as to exactly what path SpaceX intends to follow about S2 reusability, but it seems most likely that it will be the same stepwise, incremental path taken to achieve S1 recovery and which seems again to be in use for payload fairing recovery.

S2 is already restartable in space. So two first steps would be additional guidance, navigation, and control (GNC) software to allow S2 to do a re-entry burn analogous to those already done by S1, as well as applying, to the base of S2, the same TPS coating SpaceX already applies to the base of S1.

S2 already has hypergolic attitude control thrusters. The new GNC software would use these, as necessary, to keep S2 pointed, engine-first, at the ocean on its way in and aimed at a particular “landing” locus.

For a given propellant mass, these thrusters should have a lot more authority than the nitrogen cold gas thrusters on S1. Perhaps this would obviate entirely the need to add aerodynamic controls such as S1’s grid fins. Failing that, perhaps it would be possible to get by with aerodynamic controls that only need to operate in lower speed, less thermally challenging portions of the S2 inbound trajectory.

As with S1, I don’t expect SpaceX to try landing S2 on anything solid in the early going. Initial experiments, like those that involved S1, will feature a landing burn at minimum throttle aimed at executing a hoverslam to the ocean surface. Observers, human or mechanical, would collect data from a safe distance away.

Even if successful in its hoverslam maneuver, S2 will fall over in the water like early S1’s did after engine cutoff. But S2 is a lot shorter than S1 and seems likelier to survive the experience intact. If not, well, that’s a data point too. If S2 survives essentially intact and floating on its side, though, it could be recovered and much valuable data gathered from a detailed examination of its condition.

It might take two or three such attempts to get an essentially intact, floating, recoverable S2; perhaps even more. But every mission uses an S2 so there won’t be any shortage of hardware with which to experiment. Especially if SpaceX actually manages to get its launch cadence up to a flight ever two weeks—something I have no real doubts it can manage quite soon.

After tests of barely-modified second stages would come tests with landing legs. These would first take place into the water as was the case with S1, but would also, as with S1, quickly progress to landing attempts on autonomous spaceport drone ships (ASDSs). Also, as with S1, the first such test might not be a success. But, given SpaceX’s distance up the propulsive landing learning curve acquired in developing the technology to land S1’s, the company might very well stick its very first S2 landing.

So SpaceX has a reusability strategy. To implement that strategy, it employs tactics such as supersonic retropropulsion and landings at sea on an ASDS. However, there is an old saying to the effect that while amateurs concern themselves with tactics and strategy, professionals concern themselves with logistics. I’ve always found that old saying a tad harsh, but the point it makes is still valid. Lets examine the logistical implications of achievable S2 recovery. Later on, we can also look at the logisitics considerations that attend successful recovery of payload fairings.

Given that SpaceX has only the ship Of Course I Still Love You (OCISLY) in its Atlantic ASDS fleet at the moment, testing S2 landings on OCISLY would probably be done only a short distance offshore from Port Canaveral. This would minimize OCISLY’s ocean travel to test-program landing coordinates so as to also minimize the impact on OCISLY’s continuing its primary mission—sailing well out to sea to catch S1’s and returning them to shore.

The timing required for this—a gap of up to several days between recovery of an S1 and its corresponding S2—would only be feasible if S2 test articles expended either a bit of maneuvering thruster propellant or did very brief main engine burns after deploying their payloads to put themselves into orbits high enough to allow them to loiter in space while OCISLY returns to Port Canaveral and offloads the landed first stage before putting back out to sea to catch said S2.

If S2 recovery proves doable, SpaceX would be well-advised to, at some point, put a 2nd ASDS into service out of Port Canaveral. SpaceX could maximize its launch cadence by having OCISLY and its future sister ASDS alternate doing S1 and S2 recoveries. Let’s look a bit at how this might work.

While OCISLY heads for open ocean to catch the S1 from an upcoming launch—which we will call Mission B—the second ASDS stays close to shore to recover the S2 from a previous mission, Mission A. After catching the Mission A S2, the new ASDS quickly puts that S2 ashore, then heads back out to open ocean to await an S1 from the next mission in train, Mission C. Already out at sea, OCISLY is on its way back in with Mission B’s S1.

OCISLY reaches shore and Mission B’s S1 is put onto the dock for transport to the refurb and storage facility nearby previously owned by Spacehab. OCISLY then sails back out to a suitable close-to-shore point for recovery of Mission B’s S2 after which the whole cycle begins again. Each ASDS would catch both the S1 and S2 from a given mission, but do so several days apart.

SpaceX intends to achieve a roughly every-two-weeks launch schedule this year and do even better in coming years. OCISLY, by itself, can probably handle the catching of both S1 and S2 from a given mission at a 14-day mission cadence, but the scenario just described looks inevitable if SpaceX’s launch cadence out of Florida increases much beyond that. Even on a 14-day cycle, OCISLY would be kept hopping. That would apply to both ASDSs at any average launch interval much shorter than 14 days.

ASDS downtime would have to be very carefully planned. Unforeseen misfortunes to either ASDS would significantly affect launch cadence under this scenario.

Given such a continuous pace of operations, SpaceX might have to go to an alternating Blue Crew/Gold Crew system for its ASDS’s as the US Navy does for nuclear submarines. Alternatively, SpaceX might put a third ASDS into service. Not even nuclear-powered submarines and aircraft carriers are at sea continuously for years on end.

And none of this reckons with what SpaceX intends to do with the recovery of its payload fairing. Perhaps a pair of tenders and crews could work alternating “shifts” of up to maybe 30 days or so moving from one to another on-station point distantly downrange to wrangle the “Cloud 9” (aka “bouncy castle”) SpaceX plans to use to land PF halves.

To get payload fairings back to shore without tying up a relatively slow tender for the task, perhaps SpaceX could fit out, say, a cigarette boat with deck enclosures for the fairing halves and use this to radically cut the transit time between recovery points and Port Canaveral. Alternatively, some sort of hydrofoil or hovercraft of suitable size and speed could be used instead.

With more ships in the SpaceX “navy,” the company is almost certainly going to need to lease more long-term space to berth them at the Port Canaveral piers. The logistics required to underpin a sustained high rate of missions from a given spaceport is going to require a lot of careful planning and a lot of high-quality execution on a frequent basis by all SpaceX personnel involved.

The systems I have sketched out here are speculative, but could, I think support sustained SpaceX mission ops from Florida at an average pace of perhaps as fast as a mission every seven to ten days. By acquiring enough additional “naval forces,” to operate in parallel, SpaceX might someday even be able to do the mission-per-day that both Elon Musk and Gwynne Shotwell have referred to more than once in describing SpaceX’s long-term goals.

Dick Eagleson is a former software developer, writer, trade magazine editor and tyro machinist with a life-long interest in space-related matters who comments frequently on this site and others under his own name and the Disqus ID “duheagle.”

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