If anyone sees one, I would be very interested in an chart, graph, or infographic that shows how this landing was different (higher, faster, harder, etc.) than the last landing. A while back there were some nice pictures that showed how the SpaceX landing was different/harder than the Blue Origin landing. Something like that would be useful in explaining to my non-geek friends why this is a big deal.
First of all, you need to understand the normal landing burns. After the second stage has separate,d the first stage does a turnaround and the uses three engines for Boostback.
If the payload was light enough , there can be enough fuel left for the boost back burn to push it BACK to the launch site (well, ballistically a few miles shy of the coast). Which is what was done for the Orbcomm-2 flight, which did a successful RTLS landing back at the Cape in December.
But for heavier payloads, there is not enough fuel to boost it all the way BACK, but it can still boost part of the way back, so that for an ASDS landing, the barge does not have to be as far out in the ocean.
In any case, there has always been a boost back burn, with three engines (center and two sides). Then when the booster begins to re-enter the atmosphere,e there is a reentry burn (3 engines again) which helps to slow the rocket down some so that the re-entry heating is not as severe. Also the rocket exhaust produces a bow wave effect that reduces re-entry heating. The re-entry burn also helps with targeting/steering , in addition to the Grid Fins which aerodynamically steer the rocket from some portion of re-entry to landing.
Finally, the landing burn, using ONE engine. It usually ignites about 30 seconds before touchdown. It can be throttled somewhat, but the MINIMUM throttle is greater thrust than the weight of the rocket! So, it can never hover. The landing has to be very accurate in descent rate, so that for example, the rocket would reach ZERO velocity at say 8 feet below the ground / deck. So before it would reach zero velocity asa few feet below the landing surface
it has already touched down, softly (if things went well). So, here is a drawing that shows boost back and re-entry and landing burns in a somewhat simplistic way, the boost back for ASDS landings does not result in a purely vertical drop for reentry and landing, but its good to understand what the burns do.
So, what was different for SES-9 and JCST-14, you asked? Those satellites are very heavy, and need to be put into a Geostationary Transfer Orbit which requires more fuel than to say fly into the same orbit as ISS. Until SpaceX upgraded the Falcon-9 last fall, they would have flown SES-9 and this flight as expendable flights. Indeed once they started to try landings in early 2015, there were some expendable launches, not enough fuel left to try for landing.
With the current Falcon
. it is JUST barely possible to maybe land, but sacrifices have to be made. The biggest is the boost back burn, there is no boostback burn. So when the Falcon booster flips around, the only maneuvers it can do at first is
.. nothing
.. because it high above the atmosphere,e so the grid fins have no air. Oh, also on these flights, IIRC, they were lofted a bit higher, so the re-entry is more severe.
So, they are coming in hot, no boost back burn. The ASDS has to be near to where the booster would splash down ballistically. Also, the re-entry burn apparently is not done with as much fuel as usual, so it is not quite as effective. So one of the biggest issues was whether the boosters could even survive the re-entry and still be controllable.
And then, the really incredible landing burn. I said the burn lasts about 30 seconds. Well, as it is being slowed by the thrust,t there are also gravity losses, the rocket wants to fall faster at 32 feet a second every second. So, the faster it can LAND, the less fuel it needs. So for SES-9, and for this one, they let it fall to a LOT LOWER altitude than it would normally be when the center engine ignites for the 30 second landing burn. And then, falling WAY faster than normal for a given altitude after a normal landing burn wild have begun, it ignited THREE engines! So it had three times the potential thrust, and could stop quicker. Of course three engines use three times as much fuel, but because the gravity losses are greatly reduced by landing much sooner after ignition, it does not need as much fuel to land on 3 engines as it needs to land on one engine.
But many risks with that, controllability, incredibly more precision needed, and so forth. Reportedly, the SES-9 fight was coming down OK, and under control, but it ran out of fuel. This one, they did some tweaks, and perhaps the payload was not quite as heavy or it was otherwise able to have little bit more fuel left in the booster to try this kind of landing again.
When I first hear of this a few moths ago, for SES-9, it sounded crazy. Never mind the re-entry heating issues, they had never done any testing to support such a high-speed triple thrust level landing. I really didnt think it would work. It was incredible to see, after all that bright light and then nothing, along with some loss of signal, to see the Falcon on the deck, almost like Scotty beamed it there.
- George Gassaway
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