sevenperforce

Sure! If I had my druthers (which I don't, because no one is going to listen to me when it comes to planet definition schemes), I would start by hammering down the definition of "moon" and "natural satellite". A "natural satellite" (you could also call it a "dwarf moon" or a "moonlet" but I don't much like those terms) is a self-gravitationally-bound object orbiting a larger body around a barycenter inside that primary. Being self-gravitationally-bound is important; a random chunk of rock or a speck of space dust is a meteoroid, not a natural satellite. Thus the word "natural" serves not only to differentiate it from artificial satellites, but also to identify its formation as natural, i.e., by gravitational accretion. A "moon" is a natural satellite which is large enough to be gravitationally-rounded. What, then, is a planet? A planet is an astronomical body in orbit around the sun which is large enough to have a moon. This may seem too minimalistic, but it's actually quite specified. Remember that a moon is a type of natural satellite, and a natural satellite must orbit a barycentre inside its primary. Even though Mercury has no satellites and Mars has no "moons" in the sense of a gravitationally-rounded satellite, both are large enough to hold an object like Ceres or Charon or Rhea in an orbit with the barycentre inside it. Pluto, however, is not large enough to maintain a gravitationally-rounded moon (its barycentre with Charon is well out between the two of them) and so it is not a planet. The Pluto-Charon system is a dwarf planet binary. By comparison, an "asteroid" is a self-gravitationally-bound object in orbit around the sun, and a "dwarf planet" is a gravitationally-rounded object in orbit around the sun which is too small to be a planet. I think this sorts objects into much more easily recognizable and intuitive classes without depending on concepts like "clearing one's orbit" which depend on many other often-arbitary factors.

An inner-tapered cylinder maintains gas reflection inside and converts those reflections into compression, which are then converted back into expansion after the narrowest point is passed. Obviously, a truly frictionless object is impossible, but that doesn't mean we can't model one. Perhaps something approximating frictionless behavior could be achieved by certain types of boundary layers or bleeds or electromagnetic fields, and in such an instance modeling the object as frictionless would be desirable.

Clearing one's orbit is accepted to include maintaining nearby objects in resonant orbits. "Dominant mass" would, I suppose, be less clear. I don't much like the IAU definitions...not because I think Pluto should still be a planet, but because objects end up being defined based on their local neighborhood rather than their relationship to their parent star.

Indeed. But the compression/expansion cycle can be made arbitrarily close to adiabatic by choosing a particularly optimized form factor, so it is still valid to consider conservative forces only. Sure, but the OP specified a frictionless object.

Oh, it definitely has cleared its orbit, if it exists. "Cleared its orbit" includes being the dominant mass in its neighborhood, like Jupiter to its Trojans. The reason we know anything about planet 9 is precisely because of its gravitational influence on Sedna and the other objects in its neighborhood which don't line up with Neptune.

Typically, forces which can be made arbitrarily close to zero are set aside for the purposes of analysis. Anyway, it isn't true that motion through the atmosphere must necessarily be non-conservative. Consider a frictionless cylinder which tapers internally like a scramjet engine. The air which is displaced is compressed as it enters and can then expand at the exit for no loss.

The biggest factor is probably drag. The air is very thin at this point, but the vehicle is traveling very fast, and drag forces are still very much active. The much longer first stage has considerably more drag than the second stage, and falls away rapidly after the pneumatic pushers give it that first shove.

Well, crap. I didn't think about the possibility of ditching it in the ocean. I guess I should have specified vertical landing...the main idea was to return a solid-fueled craft from space and land it on its tail using only solid fuel. Requiring some pretty careful calculations, great flying skills, and very skillfully timed decouplings. I probably should have said "return to propulsive landing on landing legs on land" in the OP.

I am sure that they already were considering reuse. After all, they did put GoPro cameras on them.

I am pretty darn sure that the man-hours required to construct nine Merlin 1Ds plus a whole booster are VASTLY higher than the man-hours required to scrub off a Falcon 9 booster, inspect its frame and tanks, and test-fire its engine cluster ten times. Engine recertification simply requires refiring. If it doesn't blow up during the first ten test-fires, it won't blow up during launch.

The very reason we know of the existence of natural fission reactors is because fissile fuel is depleted in the affected deposit. Radioactive decay is a well-understood gradual geothermal heat source; fission chain reactions burn through fissile material FAST.

But a clickbait article headline told me that Blue Origin was going to take me to the moon in four years!

The SSMEs couldn't be restarted in-flight. Upon landing, they were immediately removed from the orbiter and essentially rebuilt individually before being reinstalled. In contrast, the landing of the Falcon 9 first stage already requires multiple mid-flight restarts. They have demonstrated this by successfully re-entering almost a dozen boosters so far. So just like Elon said, they can literally strap the stage down, test fire it a dozen times, and agree that it is certified for relaunch. The F9 first stage uses nitrogen cold gas thrusters for attitude control; the Dragon uses hypergolics. Of course the Shuttle used hypergolics for both OMS and RCS. The Merlins run very LOX-rich both to increase thrust and to prevent coking. They don't coke at all. The SSMEs ran fuel-rich because, yay diatomic hydrogen and its marvelous influence on exhaust velocity, but virtually all other fuel combinations run oxy-rich. The repeated test-firing is intended to ensure that no damage was sustained during launch. These are engines which are test-fired repeatedly without refurbishment as part of their preflight sequence. They are designed not to sustain significant wear from normal operation (start, burn, throttling, cutoff). So we have no reason to doubt that they can simply test fire and refuel+relaunch.

Not quite. A climber on a space elevator can move at an arbitrarily low speed. Since the force required to displace air depends on velocity, the force can be made arbitrarily close to zero, meaning that the work done to move through the atmosphere can also be arbitrarily low. There actually is a fairly significant contribution to the energy budget of the system that might not be readily apparent: buoyancy. At any given point in the atmosphere, the air pressure on the bottom of an object is just slightly greater than the air pressure on the top of the object. This force differential is what allows sufficiently lightweight objects like helium balloons to rise. An object moving up through the atmosphere will have work done to it; an object falling down will do work to the atmosphere. But the total energy will be conserved.

The ship is already roughly the size of a football field.

Oh, you can use electricity. The demo includes rechargeable batteries and solar panels. I just didn't list them because the miniature inline reaction wheel has so little pitch authority. I assumed an RCS thruster array and an octet of roundified monoprop tanks would be plenty.

Easily. Now, components will probably be reused in the same configuration at first, simply because that can be done at the launch site. But once a booster has been reused several times and needs to be shipped back to the factory for more extensive refurb, we will surely see reconfig.

A gradual decrease in gravity, even over the course of only a few weeks, should at least give the astronauts a better shot at getting used to moving around. Anyway this would be in the future, when we have high energy brachistochrone-capable engines.

Nuclear packages are not weapons unless they are intended for offensive use. Hydrazine could be an extremely effective biological WMD but that doesn't mean using it in an engine constitutes "putting a WMD in space". Likewise, the atmospheric test ban treaty does not prohibit peaceful use of nuclear explosives.

In practice, yes, but not in principle. For example, if you used a space elevator to lower something from orbit to the surface and back again, the total work done to it by the atmosphere would be effectively zero.

Yeah, total bunk. A significant portion of geothermal heat does come from radioactive decay, but not from chain reactions as in a nuclear reactor. You need a moderator for that.

Frictionless doesn't imply a lack of force interaction; it just specifies that all force interaction between the projectile and the surrounding medium is via conservative forces. Nothing physics-breaking about that, technically. You would still have compression and internal heating in the medium per the ideal gas law.

My ability to like posts was utterly trashed by the landing yesterday.

It will be a high-speed, low-margin barge landing attempt coming out of a GTO trajectory. And it will use a new booster; this one isn't slated for reuse until May at the earliest...June more likely. SES has expressed interest in purchasing the first launch on a reused booster. But SpaceX would probably want to use it on a lower-velocity mission so they can RTLS. First-attempt repeat reuse will be quite nice. Elon seemed pretty blunt about the recert process: wash it off, do ten test fires to make sure aerodynamic stresses didn't damage anything, then refuel and refly. All from the launch site. Not much more than what Blue Origin did.

I don't think they have any plans for developing second-stage reuse...at least, not for the Merlin engine class. I foresee them testing the Raptor engine as a BLEO Falcon 9 or Falcon Heavy upper stage, but it's anybody's guess whether they'll explore direct reuse on that stage. Red Dragon was replaced with Dragon V2. The V2 platform is supposed to be customizable enough to serve as a lander for basically any destination in the solar system.