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Comment Re:Less fuel. (Score 1) 71

Their earliest recovery attempts involved parachuting stages into the ocean unpowered. All those attempts apparently resulted in the vehicle breaking up during reentry. Large supersonic parachutes are very non-trivial to design and deploy, not terribly reliable at the best of times, and are actually rather heavy, and they needed to do powered reentry and landing anyway to get the vehicle down intact. Since they're already doing that, reserving a bit more propellant is simpler than pretty much any other option, plus it gives them experience they'll need for Mars.

Comment Re:Propellant or Hydraulic Fluid (Score 2) 71

The very first ASDS landing attempt ran out of hydraulic fluid for the grid fins, the engine gimbaling barely managing to get it to the barge...not upright and not at zero velocity.

The next had a sticky valve...my understanding is it was actually for throttle. The control software would command throttle changes, but the valve wouldn't respond until the commanded change was big enough to break it loose, then it'd stick at the new position. The overall effect was that the throttle was lagging behind what the control system expected, which threw things into oscillation with the rocket always overcompensating for its previous errors, always too late to fix things.

The third failure was the Jason-3 launch, which was the last launch of the Falcon 9 v1.1 (non-Full Thrust) with the first version of the legs, and took place in particularly heavy fog. The landing looked perfect, but one leg folded up afterward.

The remaining two failures were on flights 22 and 26, both on ASDS landings from geosynchronous launches with little margin for landing. 22 wasn't expected to make it, 26 came within meters of doing so.

There's probably still things to learn, but they seem out of the "getting it to work" stage and well into "making it work better" stage.

Comment Numbers... (Score 5, Insightful) 71

The numbers in the summary are a bit ambiguous/confused:
This was the *sixth* rocket they've landed. They've landed four on drone ships and two on land. That's nowhere near half the rockets they've launched (this was the 28th Falcon 9), but means just over half of their landing attempts (11 total) have succeeded.

More importantly, of the last 7 landing attempts, there were only two failures, both due to simple lack of propellant margin due to the demands of those particular launches...there weren't any failures or control problems, they just ran out of propellant. The last actual hardware failure was flight 21, the Jason-3 launch, which actually landed fine, but had an earlier version of the legs which iced up and failed to lock in the extended position. So it's looking like reliability of future landings can be expected to be quite a bit better than 50%.

All without any nets/cables/tubes/funnels/magnets/giant catcher's mitts.

Comment Re:Glass blowed 0g habitats (Score 2) 88

Basalt fiber (more or less "mystery glass" in fiber form) turns out to have surprisingly good mechanical properties, and a composite hull would be much tougher than a blown bubble, plus you would have more flexibility in the shapes achievable. Additional layers of basalt fiber fabric and "sandbags" filled with waste rock wrapped around the outside could provide radiation and micrometeorite shielding and thermal mass. Nothing you'd want to haul around the solar system, but for setting up habitable volume near an asteroid, you'd just need to import some spinning/weaving machinery and resin and a pile of silicate-rich rock.

Comment Re: drone ship landings require a lot less fuel? (Score 1) 103

2.5 degrees is about 245 km. Without the boostback, the stage ends up coming down about twice that distance to the east of the launch site despite that motion. But that's missing the point, particularly for RTLS: the directions considered horizontal and vertical are basically identical at launch and landing, and adding to vertical velocity after staging will only send the stage further downrange (unless you posit spending utterly unrealistic amounts of delta-v on the maneuver). RTLS is not an orbital maneuver, and the reasons for those rules of thumb you quoted do not apply. And yes, of course SpaceX accounts for Earth's rotation and so on, they need to actually hit a precise landing target. However, they are irrelevant to understanding the basic maneuvers, and simply do not require the maneuvers you describe.

Another factor is that if things were done as you say, the RTLS would take far, far longer than the ASDS landings, because the stage would have to be thrown on a high enough trajectory that the landing site could come into position under it, which would take much more of a rotation. The fact that CRS-9 did the entry burn just 4 minutes after the boostback burn should be another hint that something's wrong with your analysis.

Since you apparently trust Flight Club, just look at their CRS-9 model: https://flightclub.io/results/...
The boostback burn happens from 162 to 211 s. For the first 7 seconds of that, it is completing a maneuver to a pitch angle of -2 degrees, where it remains for the remainder of the burn.

Comment Re: drone ship landings require a lot less fuel? (Score 1) 103

If you truly found someone at Reddit who told you that the stage went west by thrusting up, you found someone who doesn't have a damn clue what they're talking about, and you should find someone else. Ignoring the issues of inadequate propellant and far more severe reentry conditions, the stage would have to go far higher than a couple hundred of km, and fly for far longer to allow the launch site to catch up and pass beneath it, and then it'd end up far to the south of the actual launch and landing sites due to the non-equatorial launch site. Earth only rotates about 2.5 degrees between launch and landing, the stage has the same initial velocity as Cape Canaveral and only travels a few hundred km in any direction, none of the factors that make orbital mechanics counterintuitive are significant here. None of the people who are doing simulations (you're talking to one of them here) will tell you otherwise.

And no, those diagrams are mostly correct, and nothing I've said conflicts with them...they show a horizontal boostback burn, contrary to the rule of thumb you quoted. Your mis-application of an approximate rule only appropriate for objects in the nearly-circular coplanar orbits of Niven's smoke ring (or in a very limited sense unrelated to landing, for maneuvers around geostationary orbit), on the other hand, does. It's not even internally consistent, by that reasoning the stage should fly off to the east as it descends, and the final burn should be done horizontally, like a capsule docking with the space station from above or below. SpaceX certainly needs to take factors like the rotating reference frame into account in order to hit their landing spots, but for just understanding the maneuvers it is entirely appropriate to treat them as basically parabolic trajectories over flat ground.

Comment Re: drone ship landings require a lot less fuel? (Score 1) 103

It's not that orbital mechanics knows if you're in orbit or not, it's that it simply isn't involved if you aren't. The rules of thumb you quoted work between objects in reasonably circular orbits because objects in lower circular orbits travel faster than those in higher orbits. The launch/landing site is not in any kind of orbit and is moving at only a few percent of what orbital velocity at sea level would be, and the first stage is in an extremely eccentric orbit that intersects the ground just a hundred km or so below apogee and which can accurately be analyzed as a parabolic trajectory over flat ground.

Comment Re:drone ship landings require a lot less fuel? (Score 1) 103

Even worse: canceling out your eastbound velocity, burning westward, and then canceling out your now-westward velocity so you can land. You can manage some of that with aerodynamics rather than rocket power, fortunately.

The propellant requirements of returning to land are considerably higher, roughly doubling the payload penalty. However, it means you don't have to run a barge and its support boat for a couple weeks and can immediately get started on preparing the stage for reuse, and the propellant is dirt cheap, so it's very worth doing when they can manage it.

Comment Re: drone ship landings require a lot less fuel? (Score 1) 103

It goes up because it's already going up at stage separation. Gravity will take care of reversing the vertical component of its motion, so there's no point in burning fuel to halt the upward motion: the boostback burn only has to reverse the horizontal motion. If allowed to continue on a parabolic arc to an ASDS landing, its peak altitude would be similar, it'd just be reached much further downrange.

"Out takes you west", etc. only works for objects in orbit, relative to other objects in orbit. Objects in lower orbit move faster and travel east relative to objects in higher orbits. Earth's surface isn't moving at orbital velocity and will continue to trail behind to the west until you get up past geosynchronous orbit. The RTLS trajectory is more like a ball bouncing off a vertical wall than anything to do with orbital dynamics.

Comment Re: How are the costs adding up though? (Score 1) 150

Their intent is to "gas and go": replenish fluids and fly again. They've discussed future plans of refueling at sea and flying the stages back to land, which gets them back in a few minutes and avoids a long and costly trip with a support vessel towing an ASDS which is unavailable for landings until it gets towed back out to sea.

They've not relaunched, but they refueled and fired up the first stage they brought back. Something broke loose and was ingested by one of the turbopumps, shutting one engine down, but the stage was otherwise in full working order. Their plans are to run the second recovered stage through ten test fires and fly it again, and SES is interested in having one of their satellites be the payload.

Comment Re:Cue the flood... (Score 1) 193

Surely you can cite some of these "early attempts at production of fusion power", right?
ITER is the first experimental reactor intended to produce power. Most of the research devices don't even use real fusion fuel...they know fusion works, it's the plasma physics they are researching, and building a big power-producing reactor, handling tritium, and dealing with fusion neutrons is unnecessary for that and far beyond the budgets typically allocated to fusion experiments.

Comment Re:Stupid article (Score 1) 226

They save mass that conventional rockets expend almost immediately in their flight, pay a big price in aerodynamic drag instead, and additionally pack on lots of extra mass in air breathing equipment and various HOTOL related structure that they then need to take to orbit. They compensate for that and for the efficiency losses of SSTO by claiming a structure and thermal protection system that are basically magic, and gloss over all the additional operational expenses. And no, they don't avoid the need for payload integration.

SpaceX has the advantage of having a vehicle that actually exists, and they're not content to stop with first stage reuse. The successor to the Falcon 9 will be fully reusable, and far simpler and cheaper to develop, not to mention actually operate.

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