sevenperforce

This is not limited to metallic hydrogen. Any modern orbital-class liquid engine would melt instantly if you tried to operate it without a coolant, which is why virtually all1 modern liquid rocket engines pump their fuel around the combustion chamber and nozzle to act as a coolant. However, if metastable metallic hydrogen existed, its properties would preclude it from being used as a coolant. So you would need an auxiliary propellant, like liquid hydrogen, to provide that coolant effect. 1. Some engines use ablative cooling, where the "coolant" is a solid layer coating the inside of the chamber and nozzle that burns off gradually. But that's not true. There are well-defined limits to everything. What you've done (and what you tend to do) is ask questions like, "If there wasn't a limit, where would the limit be?" And that's why these threads can be frustrating. I will point out that it is fine to go the hard-science route and get into the nitty gritty details. The Martian did a great job with this. But the problem arises when you try to mix nitty gritty hard-science details with handwavium. Because that breaks the reader's ability to maintain suspension of disbelief. See, this is exactly what I'm talking about. An anti-inertia drive is fine. Does it break physics fundamentally? Yes. Does it make any logical sense at all? No. But can you make one and put it into your story? Sure. Of course, what you're describing is WILDLY overpowered, but you are the god of your story, so you can fix that. If you don't your anti-inertia drive to be abused, then just say that it releases "graviton radiation" whenever it is used in a way that would make it too overpowered. You don't have to explain it. Your reader will accept the suspension of disbelief. But it makes no sense to talk about the nitty gritty details of material thermal properties in a metallic hydrogen engine when you've also got a universe with an anti-inertia drive. It's like trying to rewrite the story of Apollo 13, but with Moon Vampires. One of these things is not like the other. Agreed. It helps to know what you're talking about. And I agree. That's why I often show up in these threads. I remember when I was just learning all of this stuff. I will say, however, that I wish he'd just make one thread and stick with it. Otherwise it gets hard to see the rest of the threads I want.

No, the melting point of materials is not a limiting factor for any modern rocket engine. Like I said before, there is really no realistic limit to the capacity of regenerative cooling. Modern orbital-class rocket engines run at temperatures far, far greater than the melting point of their constituent materials. For example, the RS-25 Space Shuttle Main Engine (that is going on SLS, someday) has a chamber combustion temperature of 3,300°C. Not only is that higher than the melting point of most metals, it is higher than the boiling point of most metals. The RS-25 combustion chamber is made of Inconel-718, which melts at 1,430°C, and it is lined with a copper-silver-zirconium alloy which melts at only 500°C. And yet, it persists. That is the power of regenerative cooling. Temperature is never, ever a problem.

Again, Orion and NSWR are not remotely comparable. If you have access to an energy source like pure fusion, antimatter, or metallic hydrogen, it is TRIVIAL to build a very simple, bulletproof, impossible-to-mess-up engine. And you won't have to bother with making it "advanced" because your energy source is so OP that you don't need it to operate at the limits of its capabilities. If you want to have your science fiction spaceship powered by a pusher-plate spacecraft, then just propose a future where humans either (a) evolve immunity to radiation poisoning, or (b) invent super-lightweight radioactive shielding. And propose that massive uranium deposits are discovered, making nuclear charge propulsion cheaper than even a solid-fueled rocket. Then, boom (no pun intended), you've got your reason for a pusher-plate SSTO.

OP tech that just works too well tends to break settings so much that arbritrary limits must be imposed to 'unbreak' them. Or your handwavium can be invoked on both ends: it can provide the "overpowered" characteristics and also provide their limits. For example, consider the beautiful Firefly-Class transport, the Serenity. Its main powerplant is the big thing in the back. It has extremely high specific impulse and boasts a respectable amount of thrust, enough to enable the ship to traverse the Verse quite quickly in quasi-Brachistochrone trajectories. It is, in essence, a torchship. Now, if such a drive were to exist, it would absolutely remove any need for the ship's twin VTOL engines, because everything in the series would be an instant tailsitter. But the writers wanted it to have cool twin VTOL engines anyway. So they avoid this problem by postulating that the engine really only works in a vacuum. When fired in space, the engine produces a pretty, ethereal glow: But firing it in the atmosphere? Well, that's a different story. Specifically, a story with a very big KABOOM. In the show, firing the main powerplant in-atmo produces a catalytic reaction with atmospheric oxygen which results in a tremendous, sky-wrenching, city-flattening firestorm-like explosion: And of course this also seriously damages the engine compressor coil, so you wouldn't ordinarily do this unless you were trying to escape a bunch of cannibalistic space pirates (which is what they were doing in this situation). What is a compressor coil? No one knows. What is a catalytic reaction with oxygen? Again, no one knows. It's just handwavium, just like the engine itself. But that's fine. The authors wanted it to look a certain way, and so that's how they made it look. Then by all means, combine rocket engines and a pusher plate. The writers of Firefly wanted to make a spaceship that looked like a lightning bug, and so they did. You can do the same. Just make it work the way you want it to, but don't expect hard science to fill in the gaps. That is not remotely true. Any vehicle which uses pulsed blasts for propulsion (let alone pulsed blasts with a ridiculously heavy and inefficient pusher plate) will have inferior dV and inferior TW in comparison to a vehicle which uses a continuously-burning engine with the same fuel. Let me say that again just so there is no confusion. Given any energy source, a controlled continuous-thrust engine will have far greater thrust and impulse than a pulsed-blast engine. This is a fundamental fact of reaction physics. It's just how things works. A nuclear saltwater rocket harnesses thermal-neutron criticality. Orion harnesses prompt-neutron criticality. The two have virtually nothing in common. It is like comparing a fuel-air explosive with a coal-burning stove and saying "well they're both the same because they're both burning hydrocarbons with air." (Technically a NSWR harnesses water as a reaction mass, boiled by thermal-neutron criticality, while Orion harnesses tungsten as a reaction mass, vaporized by a thermonuclear explosions that is initiated by prompt-neutron criticality. But that's beside the point.) This is a perfect example of why you are incorrect. An antimatter orion would be horribly wasteful and useless in comparison to an antimatter rocket. Given any energy source, a pusher-plate arrangement will always have less thrust and less impulse. Now if you want a dual-thrust-axis spacecraft with VTOL rockets and a pusher plate, then by all means create one, and handwave the reason why. But don't expect science to back you up. The "orion system" is a massive accident waiting to happen. It is a horrible, horrible idea. It is incredibly hazardous and incredibly wasteful. There is absolutely no reason to use a system like orion unless you are stuck with current-tech thermonuclear weapons as your only propellant choice.

This has to do with power cycles, which are probably beyond the scope of what you are trying to figure out here. The predominant challenge for any orbital-class rocket engine is not the combustion or the choice of propellant or the cooling system, but the question of how to force propellant out of the tanks and into the thrust chamber at a high enough pressure that the pressure in the thrust chamber doesn't push back out into the tanks. Unless you have immensely high pressure in your tanks, you will need a turbopump of some kind. For expander-cycle engines, where the heat generated by the engine is used to directly drive the turbopump, the straightforward approach is to run a little cryogenic propellant into the coolant loop and allow it to emerge in an auxiliary chamber where the heated propellant can expand into a gas. That escaping gas can then turn a turbine, which turns the turbopump, which pumps the propellant. You can do regenerative cooling with any propellant, of course, but an expander-cycle engine will require a cryogenic propellant, which you probably don't want for your SSTO.

No, that is not true. No orbital-class engine can "survive the heat" of active combustion, even with low-energy chemical combustion like kerolox or hypergols. That's why all orbital-class liquid-fueled engines are regeneratively cooled. A metallic hydrogen engine would require auxiliary propellant, not to "water it down", but to provide remass. The release of energy does not provide impulse without a means of transferring momentum. The same is true for pulsed-charge Orion designs; they require reaction mass in the form of powdered tungsten. A metallic hydrogen Orion would require the same, but there would be no reason to make a metallic hydrogen bomb Orion, both because (a) metallic hydrogen is not nearly as energetic as a thermonuclear warhead, and (b) if you have access to metallic hydrogen, you simply use it in an ordinary engine with a liquid remass like anhydrous ammonia. The pusher-plate design has nothing to do with surviving heat. It has to do with an inability to control nuclear explosions. That is nothing at all like what a pulse jet is. A pulse jet is simply a jet engine which uses pulsed combustion rather than continuous combustion. It was first used in combat in World War II with the V-1 flying bomb and it has never been used since because turbojets are more efficient. All of this worrying over launching "an orion" and nozzle size is stuff and nonsense. The nozzle can be as big or as small as you want it to be. No, that is not true. The heat is carried away by the escaping propellant, and any heat that is transferred to the nozzle is carried back into the propellant via regenerative cooling. Making a nozzle thicker won't keep it from melting. Making it thicker will, in fact, make the heat management problem worse. Nope, you do not need to do that. You simply use regenerative cooling, like every modern liquid-fueled orbital-class rocket engine in existence today. A rocket engine operates in a steady-state mode with a constant chamber temperature. If you have a super-material with a melting point higher than the temperature of combustion, then it doesn't matter how long you operate it for; it won't melt and you won't need to stop and let it cool off. However, you will still need regenerative cooling somewhere in the cycle or the rocket engine will transfer that heat to other parts of your ship and melt them, too. There is no limit to the heat management capabilities of regenerative cooling, and there is no reason to think that future technology with the capacity to build SSTOs would somehow forget what regenerative cooling is. If you want to broil everyone in the ship, sure.

RGVAerial isn't connected to SpaceX.

I believe more than once. An MH bomb is a metallic hydrogen bomb... Read the emphasized portion of what you quoted. A society which can produce a metallic hydrogen bomb will also be able to produce a metallic hydrogen engine. Or, at the very least, a metallic hydrogen pulsejet, which will do just as well. The only reason to ever pursue a pusher-plate pulse propulsion design is if you have access to a large energy source (like nuclear weapons) but are incapable of harnessing it other than in large explosions. The pistons can handle proximity to a detonating nuke. They'll be fine. This would (or wouldn't) be the case whether it's a tailsitter or a dual-thrust-axis vehicle. If you don't want ladders than put a bloody crane on it with an elevator. Dude, if you want a dual-thrust-axis vehicle for whatever reason, fine. Just do it. Rule of cool, whatever. But don't try to add new parameters just to scientifically justify something that isn't scientifically justifiable. Write your fiction however you want it.

Any of these ideas are, in principle, the attempt to use the Earth as part of the remass. Just like an airbreather is an attempt to use the atmosphere as part of the remass.

Thermobaric bombs are two-stage weapons which use a small primary explosion to mix fuel (usually some kind of liquid hydrocarbon) with air to use as both oxidizer and remass. They will not work if the atmosphere is moving relative to the weapon when it is detonated. Therefore they cannot be used for a pusher-plate design. And thermobaric bombs by their design cannot produced a shaped blast. Not that it matters. If you wish to use the air as oxidizer and reaction mass, then do this: Stretch your pusher plate out into a cone, and extend the hole in the middle to create a tube that opens at the front of the ship. Place a propeller inside the tube to pull air in from the front of the ship. Remove the liquid hydrocarbon fuel from your thermobaric weapon and store it in tanks with pipes that allow it to drip into the tube just downstream of the propeller. Shrink the primary from one of your thermobaric weapons into an igniter. Congratulations, you have invented a turbojet engine. What is an MH bomb? Any society with technology to harness greater destructive force than a thermonuclear weapon (such as pure fusion bombs, antimatter bombs, and the like) will have the technology to put it into an ordinary engine. Pusher-plate designs are extremely inefficient and horrible at literally everything. The only reason to use them is if your society is incapable of harnessing supercritical nuclear reactions without blowing things up. That makes no sense. If your vehicle is attached by springs to a giant flat metal plate, then congratulations -- you have the most safe and stable and effective landing gear possible. Just use your VTOL rockets for Vertical Take Off and Landing.

It will soon be expressed by everyone else here, but for the record -- no, none of this would work in any way, shape, or form. You cannot use thermobaric explosives with a pusher plate. They will not work. There is no reason to go through the additional mass penalty of strengthening your vehicle to allow for dual-axis thrust. If you have VTOL-capable engines, just make the whole thing a tailsitter. The crew can use a ladder. If your society has the tech to produce basketball-sized pure fusion bombs, they have the tech to produce a pure-fusion engine that needs no pusher plate at all.

It has already been expressed by everyone else here, but for the record -- no, none of this would work in any way, shape, or form. Medieval ballistae didn't even actually work on spring-constant-force cords; rather, they worked on tension via torsion. You design a ballista such that pulling back on the projectile twists the ropes holding it all together. The energy is stored in the twisted ropes which want to untwist, which is easier than trying to store energy in something stretchy.

It looks like RocketLab made the mistake. Today, their website says the fairing is only 5 meters. But this was the screenshot I took yesterday:

Yeah, something's tricky here. Also, there are some baked-in assumptions with the mass of the second stage, too. There is volumetric margin for the second stage to be larger and heavier.

An extensible tethered hab (a la Stowaway) is the lowest-mass and most readily-realizable solution for artificial gravity during the transfer to Mars. It would need to be tested extensively in LEO and possibly cislunar space as well, probably using a modified Dragon or Starliner to start and then upgrading. The transfer coast will, of course, require a larger hab, built up with successive launches in LEO like the ISS, containing several rigid pressurized modules and several expandable modules. That hab would subsequently be rigorously tested with the tethered artificial gravity solution. They'd need the hab for both directions of the journey, and it would be far too large to aerocapture, so it would need to bring propellant for a Martian capture burn. It would also need to bring propellant for the return. That's 4.6 km/s round-trip, so if we are using storables then you're looking at a transfer hab that is 77% propellant after trans-martian injection (slightly less if you use drop tanks, but not by much). Orion is capable of handling entry from an interplanetary transfer so they'd have an Orion docked underneath it for the final entry at Earth, but the hab would probably end up being single-use. A shame. So you use orbital assembly for the hab, orbital assembly for the upgraded tether system, orbital assembly to add hypergolic propellants for the trip, a single Falcon Heavy launch to add an unmanned Orion, a Falcon 9 Crew Dragon launch to send up the crew, and then an SLS launch of a naked, stretched EUS to perform the TMI burn. But you'd need to stretch it a good bit. And that gets you on the way to Mars, but that's not enough. Unless you were looking at a truly minimalist flags-and-footprints mission, you'd want a pre-emplaced hab on the surface. So that would need to be done well in advance, using a series of Falcon Heavy or even SLS launches. You also need a way of getting down to the surface and back up. They would probably use an Orion-based framework for an EDL vehicle that would be launched on Falcon Heavy and aerocapture with a heat shield into low Martian orbit for rendezvous with the transfer hab. Of course they would need to test it unmanned at least once. For the ascent vehicle, they would have to design something completely from scratch, with an expandable heat shield and enough fuel for the landing and ascent but only enough oxidizer for the landing, and a disposable oxygen separator that would use solar or RTG power to crack and liquefy oxygen out of the Martian atmosphere to refill the oxidizer tanks for the ascent. That would also need to be tested.

The RD-0124 produces 359 seconds of specific impulse thanks to its chamber pressure of 2280 psi. The Rutherford vacuum engine, at 1400 psi, can only manage 343 seconds of specific impulse. So even though electric turbopumps are more efficient from a chemical perspective, chamber pressure is going to drive specific impulse much more aggressively. I will also note that the Merlin 1D vacuum engine is able to push 348 seconds of specific impulse despite having a chamber pressure only slightly higher than the Rutherford, and with a much less efficient power cycle.

If only there was an eccentric billionaire with spaceflight aspirations who owned the boring company that could used to bore the bore of that cannon into the side of a mountain… A light gas gun can get some terrific muzzle velocities but that might be overkill.

Curiouser and curiouser. The Rutherford has a big whopping advantage over the RD-180, of course, because 100% of its propellant goes directly to thrust, while the ORSC cycle of the RD-180 uses up a significant amount of chemical energy in the preburner. That is why it has a fairly low TWR: the electric motor and batteries are hella heavy. But the RD-180 has a chamber pressure more than double that of the Rutherford, too. Generally, what is the relationship between chamber pressure and specific impulse for a given propellant? Is it quadratic? Exponential?

I did more math and I’m not so sure that makes sense. Or at least, there’s something fishy with the thrust numbers given on RocketLab’s site. The seven Archimedes engines together are supposed to have 5960 kN at liftoff and 7530 kN at burnout. The vacuum Archimedes produces 1110 kN. But if that difference in liftoff thrust and burnout thrust is attributable to SL vs vacuum performance, and the vacuum isp of the SL engines is 320 seconds, then that would put the sea level isp at only 253 seconds, which is impossibly low. It would also suggest that the specific impulse of the vacuum Archimedes is a paltry 330 seconds, far lower than the Merlin 1D Vacuum. So that can’t be right, either.

Perhaps it’s phatt enough that the Mach number will change really really rapidly in the high-density regime, rapidly enough that the flow around such sharp protrusions will remain chaotic. I know he just means that CF is 4X lighter than SS in terms of weight per unit area, not that the mass ratio will be 4X better than Centaur, but still…wow. I’m sure the upper stage engine will have to have decent throttling. You need that for precision orbital insertion, anyway. Unthrottleable or limited-throttle liquid engines are mostly the regime of disposable first-stage engines. And throttling isn’t too hard to do with a GG engine.

I built it in stock, with appropriate balancing: It works all right. One of the issues is that since you can't close the fairing (and you really kinda need it for the aerodynamics), you have to use the "decouple inside the fairing and warp out" trick, which only works outside of the atmosphere, which means you have to wait until you're outside of the atmosphere to separate and boostback. I may try to do something with a telescoping piston as a way to separate it inside the atmosphere -- push it through the fairing, decouple, then retract the piston. I also had issues with it wanting to go nose-down on re-entry. Maybe clipping some extra tanks near the bottom will help.

Wow. Just wow.

Visually, it’s pretty cool. Makes me think of some of the early speculative BFR designs. I wonder if they are going with lined or unlined carbon composite.

The Merlin 1D clocks in at just under 10 MPa; Raptor is 30+ MPa. What is typical thermodynamic efficiency in gas generators?

It's easy enough to do a drop tank for any propellant type. However, the upper stage engine nozzle is almost certainly radiatively cooled, so it needs to be open to space to work properly.