I'd love to read a detailed description of their braking and recovery sequence, but I haven't found one. Do you or anyone else know of one?
You can probably find the relevant information on nasaspaceflight.com/forums . The method is actually pretty old in concept-- it's called "boost back" recovery.
The rocket lifts off from Cape Canaveral, and ascends out of the sensible atmosphere and arcs out over the Atlantic as it accelerates the upper stage and payload towards orbit. Then the first stage shuts its engines down and stages normally like any other expendable rocket.
Now, once the stages separate and the upper stage engine ignites and continues on with the payload towards orbit, the first stage continues on a ballistic trajectory identical to how our model rockets continue on a ballistic trajectory after burnout, coasting to apogee as it decelerates due to gravity and then arcs back over toward the ocean... it begins to accelerate as it's now falling back toward Earth, specifically the ocean below. Normally the stage would fall, gaining speed, but as it reenters the sensible atmosphere, aerodynamic drag rapidly slows it down to terminal velocity, usually a couple hundred miles an hour, for an ocean impact. This is how all the Saturns and Gemini's (and all other expendable rockets) flew. The only difference with the shuttle boosters is that they deployed drogue chutes to stabilize the falling booster, and then deployed recovery parachutes which then dereefed in stages to slow the booster, detonated a linear shaped charge to sever the nozzle, and then soft-landed in the ocean. The boosters also continued to fly basically along their pre-determined ballistic trajectory that the shuttle was on when the boosters burned out and separated.
Boost-back, on the other hand, changes the equation some. The first stage shuts down and the rocket stages normally, as before... the second stage departs with the payload, and now the first stage maneuvers tail-first (in the direction of its ballistic flight path trajectory) and reignites some of its rocket engines. Remember that now the stage is empty, since most of the propellants (except the residuals in the tanks and the extra propellants carried for the boost-back and landing requirements) has been burnt off on ascent... so the stage weighs a small fraction of its mass at liftoff... The engine thrust basically performs a "retro burn" to kill the forward velocity, so the rocket begins to drop essentially "straight down" due to gravity... The rocket then accelerates back toward the landing site (in this case, the launch site). Basically, the rocket puts itself back on a ballistic arc trajectory that would drop it near the desired landing site. The engines, having imparted the necessary velocity to put the rocket on this ballistic arc to the landing site, then shut down. The rocket "coasts" toward the landing site, arcing up through apogee and then gaining speed as it falls back toward the landing site.
Once the rocket is approaching the landing site, it's falling forward at whatever velocity, and of course falling back down toward Earth. The landing engine is then ignited at the proper time, to decelerate the rocket and slow it down for a soft landing. The rocket decelerates as it gets closer to the ground, acquires the landing site location beacons that guide it to the soft landing in the proper spot, and then it enters a hover (or near hover) under the thrust of the rocket engine and gently descends at the proper descent rate, steering itself down under terminal guidance to the landing pad, and touches down, shutting down its rocket engine.
This type of recovery has been proposed a few times for different vehicles, even back during the proposals for shuttle, like SERV...
Now, the PROBLEM is, at staging, the rocket is going very fast... part of that velocity vector is UP as the rocket is climbing toward space, but a significant part of it is FORWARD MOTION (downrange) AWAY from the launch site. The most IDEAL way to do a vertical landing on land recovery of the stage would be to have the rocket reignite its engines, decelerate, begin falling toward the landing site, and then enter the hover under rocket engine power at the proper time, and soft land. Ideally, the landing site would be almost in the same spot that the stage would ballistically impact were it to land totally unpowered after staging, like an expendable stage. This requires the least amount of propellant and least amount of maneuvering and guidance accuracy from the stage's recovery systems. Returning to the launch site for recovery means you have to burn the rocket engines long enough with sufficient thrust to brake the stage velocity from liftoff to burnout back basically to zero forward (downrange) velocity, at which point it's falling more or less straight down, then accelerate BACK toward the landing site with sufficient velocity to put it on a trajectory that will intersect the landing site. The rocket basically goes back into an unpowered coast toward the landing site, then has to reignite the engine AGAIN for deceleration, hover, and landing.
If you can land along the nominal flight path for the stage AFTER staging, you save all these fuel-consuming maneuvers requiring precision guidance. You also have the stage on a safe flight path so that if anything goes wrong, the stage simply splashes down in the ocean somewhere near the impact point if the stage were simply expendable.
With the boost-back principle, if the stage decelerates correctly and then enters a ballistic flight path back towards the launch site, and THEN has some sort of guidance failure, well, you've put a spent rocket stage still containing enough fuel for deceleration and landing on a ballistic path for INHABITED LAND... (IOW, the Florida coast). If the stage burned short or suffered an engine failure before getting on the correct landing site intersection trajectory, it will land "short"-- IOW, splash down just off the coast of Florida somewhere fairly close to the launch site. If, however, the stage suffered some sort of guidance or control (GNC) malfunction that caused it to burn too long, it would LAND "LONG", IOW *PAST* the landing site, which is to say, somewhere *inland* (west) of the Cape Canaveral landing site... If the stage then malfunctioned to the point that the engine didn't reignite for the terminal deceleration, hover, and soft landing, you could potentially have a spent rocket stage crashing into Orlando... and it'd look awfully bad to have a spent booster dropping in the middle of Disney World...
IMHO, They should keep working on the concept, but come up with some sort of concept like a floating oil platform for the stages to land on out in the Atlantic, so that IF something goes wrong, it simply splashes down out there in empty ocean instead of "somewhere in east-central Florida"...
And while I'm sure they'd have range safety systems to blow the stage up if it were malfunctioning to the point it was unsure where it'd land, that really doesn't gain you much if it malfunctions and is going to "land long"... you blow the stage up, but guess what-- now you have a million pieces of stage scrap metal on a ballistic path to impact in the same area... probably worse than just taking your chances of getting hit by the whole, intact stage, as the damage would be much more widespread... Remember the debris footprint from Columbia's breakup across several states?? Wouldn't be that bad, but bad enough... it's acknowledged that it was a miracle nobody on the ground got killed from Columbia's debris impacting the ground... the areas the debris impacted are relatively sparsely inhabited, by and in large... east-central Florida, on the other hand, is more heavily populated...
Later! OL JR
