sevenperforce

What's the tradeoff between using railgun-launched kinetic-ignition fusion bullets (where the fusion fuel is fired at the target alone and compressed to fusion on impact) and dual-bullet railgun-initiated fusion shots?

Tradeoffs?

Familiar with both, but I don't believe railgun-initiated nuclear fusion bullets have been proposed before.

Depends on the type of nuclear reaction. One cannot use a chain of thermonuclear bombs as a fusion reactor; even though Q>1 fusion is taking place, the reaction is so energetic that it would be impossible to contain the heat and pressure enough to produce a sustained reaction. Same thing here, though on a much smaller scale. Which prompts the question: would it be possible to use a ship-mounted railgun pair to fire a nuclear bullet at a target in outer space?

Right; Q >>>> 1, but just like with thermonuclear bombs, one cannot actually recirculate enough of the heat to sustain the reaction.

Reactionless drives are primarily prohibited by conservation of momentum, not by general or special relativity. I mean, general and special relativity factor into conservation of momentum, but that's neither here nor there. And the setup you describe does not violate conservation of momentum. Each time a photon is released by the bulb, the total mass-energy of the spacecraft decreases ever so slightly. The loss of mass-energy is the energy of the photon; the change in momentum is the momentum of the photon.

Layout in KSP, Tweakscaled, showing gimbal action (8 degree limit): A major advantage of this layout is that every engine has pairwise symmetry, meaning that if you have a single engine-out, you need only shut down one opposite engine to balance thrust, rather than shutting down two as with triplicate symmetry.

The Raptor engine bell appears to be about 1.8 meters long. I don't know if the gimbal is at the throat or at the chamber, but it's not going to make much of a difference; it'll need about 24 cm of clearance in every direction. However, the engines will gimbal together for pitch and yaw and in opposite directions for roll control, so the core, gimballed engines can be packed closely, just as shown here: This, I believe, is the ideal layout for fitting 1.5-meter engines in a 9-meter cross-section with optimal packing and over 8 degrees of gimbal space for the core: If seven core engines are enough to land a booster that uses 42 engines, four should be plenty for a 22-engine booster. The limiting factor for the dimensions of the original ITS is diameter, not length. F9 has a much higher fineness ratio than the ITS model.

Ooh, right. Correction: air-turborocket. If you ran monopropellant through the edge/tip of a ducted turbine, the monopropellant would both spin the turbine and produce thrust, with the spinning turbine pulling through air to augment the thrust. You don't need any heavy motor to drive the turbine. And no, I wouldn't jump off a cliff; I'd want to jump off the ground.

Not true. A net-negative fusion engine does not break even in terms of the power loop, but it more than breaks even in terms of kinetic energy. "Not breaking even" simply means that you cannot recover enough excess energy from the fusion to continue powering it, so it can't be used as a reactor, but it still makes an awesome engine.

I can't take jetpacks seriously either, but it was 90%+ HTP. Anything over 85% HTP would be enough for a static fire. Speaking of which, why on earth has no one made an air-turboramrocket monopropellant jetpack? Waaaay more thrust and way more efficient. Unless you're using the highest grades of HTP, ordinary silver will do the trick. Not hard to get.

After glancing at a few landing videos, this is about the hardest single-engine gimbal I saw: It's roughly 8 degrees. I'll take a look at Raptor's design and see what kind of planar displacement clearance would be required.

Does anyone know roughly what gimbal range the Raptor will have? Or, if not, what gimbal range the Merlin 1D currently has?

On the subject of the 9-meter BFR... Presumably SpaceX will attempt to pack as many engines as possible into those nine meters. This presents a few possibilities: Interestingly, the 22-engine one has only bilateral symmetry, not triangular symmetry, so SpaceX may go back to 4-symmetry rather than 3-symmetry for legs and grid fins. It may be possible to pack more than 22 engines using advanced circle packing, though you run into rather aggressive engine asymmetry. ITS(y) uses biconic entry, so its engines are not pointed retrograde on entry anyway. Its turbopump can also throttle aggressively, so no need to cosine out for throttling losses. And even if it pointed its engines out, this wouldn't help with plume impingement leading to engine bell damage. Its vacuum engines don't gimbal at all.

But they're a pain if you wanna use the same engines to take off and head to orbit.

The inescapable problem with landing with downward-facing engines, assuming you intend to use those engines again, is regolith damage. Rocks and debris kicked up by the engine plume are almost guaranteed to damage your engine bells, and then you're boned. The LM (and, if I read the diagrams correctly, the Mars Lander shown above) had its ascent engines shielded during descent, so this wasn't a problem. Red Dragon would have had its SuperDracos tucked up above the heat shield, albeit with some cosine losses. But ITS and anything with a similar profile is going to have to land on its engines, and that seems really problematic. Having long landing legs helps, I suppose, but it still seems like unacceptable risk.

Would still have some tendency to tunnel through. A very thin, flat plastic ring set in guide grooves would be forced down by the pressurant gas and ensure that pressure was delivered evenly across the upper face of the napalm.

I'm still holding out for propulsive splashdowns on cargo returns. No reason they can't at least test it out.

Absolutely.

Previously-flown Dragon 1 spacecraft will continue to be used for CRS cargo missions as Dragon 2 development and testing continues. Once Dragon 2 comes online, new Dragon 2s will be used for both commercial crew and cargo missions alongside reused Dragon 1s until Dragon 1 end of life. Elon hinted at using cargo missions to test Dragon 2 more aggressively than would be possible with crew. We may yet see propulsive landing, if only to splashdown. If I have some time tomorrow, I'll work up the numbers on a real nine-meter ITS.

Any word on when Dragon 2 Cargo will start flying?

Napalm removes roughly half the issues of pressurizing a fuel (leaving only the HTP pressurization) and effectively serves as the combustion chamber. I'd check* to see if it is lacking protection from boiling too fast compared to normal "rubber" hybrids, but it certainly greatly reduces the problem set. I'm hesitant to let HTP+napalm go just yet. My earlier designs were an attempt to model a close-to-steady-state chamber design, but a more conventional hybrid design is possible as well. The gas-tunneling problem worries me to some degree, but one positive thing is that we're dealing with more than just a pressure balance; we're also dealing with the weight of the fuel column. The rocket's acceleration will force the mass of the fuel down toward the nozzle against the combustion pressure. Another option would be to use a lightweight annular disc wrapping around the HTP tank to push the fuel column down. Other people have certainly obtained HTP. For jetpacks: https://www.theguardian.com/technology/2008/oct/30/aeronautics-gadgets-rocketman Copenhagen Suborbitals investigated using HTP to drive turbopumps: https://www.wired.com/2013/03/diy-rocket-grade-hydrogen-peroxide/ Small monoprop peroxide rocket engines: http://www.tecaeromex.com/ingles/peroxidoi.html Larger monoprop peroxide rocket: http://www.rrs.org/a-hydrogen-peroxide-rocket/

Here's what a theoretical 9-meter Raptor-based launch vehicle might look like (4-meter Raptor Vacs and 1.5-meter SL Raptors):

The black hole would be a good target from a marketing sense: having a specific target with some meaningful destination. Obviously the signal is never going to be received. I did not mean slingshot in the Oberth sense; rather, I meant that photons tangent to the photon sphere will be flung out in an infinite number of directions.

It sounds like he's doing some kind of marketing stunt...in which case sending something toward a "black hole lens" such that it will be sent out into the rest of the galaxy is kinda cool, even though obviously it's not truly able to be received in any meaningful way. It will be "amplified" in a very loose sense for an observer directly on the other side of the black hole via gravitational lensing. I doubt our intrepid OP has the funds for an interstellar probe.