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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.

Comment Re:Stupid article (Score 0) 226

And one of limited real world value. LOX is dirt cheap, and launchers have to leave the atmosphere after a short amount of time in order to reach orbital velocity. They add on all sorts of complexity and losses to enable use of atmospheric oxygen. They are accepting increased vehicle and operational costs in order to reduce the tiny fraction of launch costs that come from the propellant. This simply isn't going to result in inexpensive spaceflight.

Of course, the rest of the vehicle is a fantasy structure of a carbon fiber spaceframe with gigantic liquid hydrogen tanks on the inside and an eggshell-thin ceramic heat shield suspended around it on wires, with insulation and active cooling systems in between. It might be physically possible for a structure that fulfills their requirements to exist, but can it actually be manufactured, let alone maintained and flown? (And of course, if they do manage to work these things out, the same techniques will become available to conventional rockets, along with some things like partially pressure-supported tanks that aren't usable on Skylon due to the bending loads from its horizontal launch...)

Air breathing engines are better suited for cruise than for acceleration to anything approaching orbital velocity, and with the need for LH2, it's probably not aimed at anything commercial...instead, things like hypersonic drone bombers. Skylon is just PR.

Comment Re:For future reference, (Score 1) 221

Whether it slows it down or not as an intermediate step is entirely irrelevant. Kinetic energy is proportional to the square of velocity. Applying a given increment in velocity (as a jet engine of any sort needs to do to produce thrust) costs more energy at higher initial velocities.

Comment Re:For future reference, (Score 1) 221

These issues arise because the engine takes in air as reaction mass and ejects it at higher speed to produce thrust...they apply to anything that breathes air. No fancy internal thermodynamic cycle can get around this. Ultimately, accelerating that flow of reaction mass by a given amount takes more power as the initial velocity of that reaction mass with respect to the craft increases. The only way around this is to not breathe air.

The fact that their engine is absolutely dependent on large quantities of liquid hydrogen is also not promising when it comes to economics...

Comment Re:Only good for "Near Space", not orbital re-entr (Score 1) 62

It reminds me of Branson crowing about how environmentally friendly and efficient their hybrid motor is. "We have reduced the [carbon emission] cost of somebody going into space from something like two weeks of New York’s electricity supply to less than the cost of an economy round-trip from Singapore to London."

Never mind that spaceflight would have to scale up many orders of magnitude to be a meaningful contributor to carbon emissions, that there are few rockets that emit as much carbon for their performance as their hybrid, or that they produce fewer emissions simply because they aren't doing anywhere near as much.

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