sevenperforce

Right. Population density is not an issue, but it would require something like four Earths worth of nothing but arable land to provide an average standard of living comparable to that of the American low middle class.

Yes, your English is entirely fine; you just seemed like you were being intentionally difficult. I could be wrong...hence the questionmark.

Trolling?

As am I. Anyone want to take a whack at the propulsive-landing version?

So? Asteroids can have natural satellites, but there are no asteroids with moons -- at least, not as I have defined "moon" above. By defining "moon" as a gravitationally-rounded natural satellite, the definition of "planet" becomes much simpler.

No, can't make enough.

As others have said, aerospace-grade propellants are already usually pretty nasty stuff, so this comes with the territory. Granted, there are certain things which exceed even rocket scientists' love for delta-v, like pentaborane, but neither HTP nor hydrazine are nearly that bad. Hydrazine is fairly stable in the absence of a catalyst or ignition source; it's just quite toxic. HTP is stable enough if stored correctly and you won't be dipping anything into the tank. Yeah, the whole concept (at least as provided in the OP) was pretty much dead in the water as soon as I realized that my energy calculations were totally hacked due to improperly stacking exhaust velocities. The simplest air-augmentation setup is to wrap a simple shroud around the exhaust nozzle, which can increase thrust by up to 15% at launch due to pure ejector effect and will ramp up considerably at higher velocities. Using a single or double monopropellant injection a la TAN (thrust-augmented nozzle) arrangement could give the launch thrust boost desired while allowing a wide range of fuel-air-oxidizer mixture ratios throughout the flight path.

Because it needed to be SSTO. Giving it drop-away-and-RTLS boosters for takeoff thrust and crossfeed would give it much better margins, enough to make up for the greater dry mass of an airbreathing system.

Re-entry heat. If you watch the landing videos it is already scorched black from re-entry before it nears the ASDS.

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.