shynung

If we're talking about an orbit-based surface shuttle for tactical drops, I have an idea that just might work: drop tanks. Essentially, the ship has an internal tank and detachable external tanks. The external tanks carry the propellant for EDL, while the internal tank carry the propellant for STO flight. The ship would land on nearly-empty external tanks, deploy/pickup payload as necessary, and drop the empty external tanks before liftoff. If the EDL is aborted for any reason (ex: hot LZ), then the ship can just drop the external tanks in-flight, and still has enough dV to boost itself back to orbit right away.

I think "craft" would make sense as a common word. We have such words as "watercraft", "aircraft", and "spacecraft". Not a linguist, so please don't quote me on that.

For the actual chemists in this forum. Are common automobile gasoline suitable for engine regenerative cooling? I'm talking about piston engines, not rocket combustion chambers.

One thing I'd always thought about cargo airships is that they can carry large pieces of cargo that can't be broken down into smaller things, and can also act as stable aerial platforms. This enables us, for instance, to carry fully-assembled wind turbines slung underneath right to their installation point, and lower it into position using airship-mounted winches, essentially using it as a gigantic aircrane. The other thing I can think about is that a cargo airship can be incredibly useful if we have a launch pad on a mountain. This vehicle can lift entire rocket stages, fully-assembled, right on top of the pad, and stack them into position.

Kind of a nitpick here. Aviation turbofans derive most of their thrust from the fan, not the engine. On most common current-gen aviation turbojets, about 1/6 of the air entering the engine ever encounters the combustion chamber, while the rest goes straight to the exhaust nozzle; a bypass ratio of 5:1. Some newer engines such as some models of the PW1000G has bypass ratios that goes up to 12.5:1. That's how they get their efficiency: only a small amount of air entering the engine actually gets combusted, the rest is simply pushed backwards by a big fan. Power generating turbines differ from aviation ones, mostly because they don't have to bother with generating aerodynamic thrust and size/mass limitations, except maybe for ships. This is a steam turbine from power plant. High-pressure steam would enter from the middle, where the red-shirt person is standing, and exit through both axial ends. This reduces thrust loads on the bearings.

LH2 for nozzle and chamber cooling. Hot LO2/GO2 in small cooling channels sounds like a disaster to me - it'd corrode everything frighteningly fast.

I was thinking about highly localized gusts. Though, as you said, chances are small; this is the kind of thing you'd encounter while flying into really bad weather. Hopefully, SC isn't going to act like Russians and launch in blizzards. Also, I missed that point. It'll definitely be stiffer when the tailplane and the main wings I think the tail section is supposed to be able to withstand its own weight while clinging to the rest of the airframe. From what I see, the payload rocket only attaches to the main wing, and doesn't apply any forces on the tail region. Any lift there would be helpful, but might induce a pitch-down tendency. Will answer via PM.

I'm not just talking about swept wings, though. Any design which requires a major rework of the fuselage is basically off budget at this point. Bigger wings, though, I think is still feasible. Merging the tailplanes would need quite some rework on the tail section, as you'd expect. The tail of each fuselage, if using a merging tailplane, now applies a torque on each other whenever there is a slight imbalance in lift, since they're now connected. Original 747 fuselage design is likely to not consider this effect; P-38's certainly does, as it was designed to have a merged tailplane from the start. The obvious solution would be to stiffen the tail torsionally, but this would add weight. Even then, we still have to consider if the merged tailplane gets tilted due to lift imbalances, the roll effect this would induce, and whether the main wing can counteract this induced roll. There's also the question of how much torque stress the tail section can withstand, and for how long, before it eventually wears enough of the airframe to render it unsafe to fly.

I believe we've offered quite a few answers already. Avoiding overstressing the wing mounts, design requirements calling for altitude rather than speed, ability to treat a straight wing as a single piece that the rest of the plane hangs off of. I'm sure there are plenty of others. There you have it. Scaled Composites is experienced in designing efficient high altitude flyers. Building the Stratolaunch as another efficient high altitude flyer allows them to tap to their expertise and save on design costs. Their decision to buy old 747 is also due to cost, so that they don't have to design a new fuselage, avionics, pressurized cabin, engines, and so on. Northstar, you have to understand, their engineers have a budget constraint. They can't just design the best-performing plane without considering costs. They're working with a lot of compromises to create something Scaled Composites are able to afford that is able to do the job, and little more. The changes of the wing you proposed would cost them either a major rework of the 747 fuselages they have, or 2 brand-new fuselages.

No way. This is more fun than my actual job. Oh, and it's Shynung. How much higher will that extra Mach .2 allow the launching aircraft fly? Wiki says the SC design can go up to 35K feet. More importantly, does this extra altitude a higher-speed design allows justify the additional R&D costs? Remember, they're stuck with the 747's original wing mounts; building a swept wing is likely to mean a redesign of the mounts, which include gutting the structural parts of the original fuselage. If they are willing to do that, they might as well scrap the 747 they bought and built new fuselages from scratch. Also, as everybody else pointed out, this is Burt Rutan's company we're talking about. They have experience in building high-endurance subsonic aircraft. Wouldn't building a swept wing design to squeeze out some more speed be outside their current specialty? Sure, they can do that, but that'd meant they have to throw away almost all their experience and learn entirely new things.

Rocket's final speed is about Mach 24. Starting from Mach .86 instead of Mach .66 doesn't reduce enough chunk of the 2nd stage rocket's deltaV requirement to justify building two entirely new fuselages and wings just to take advantages of the PW4056's maximum speed.

Bingo. The original Boeing 747 wings are meant to support one fuselage. The engineers at Boeing designed those wings' construction parameters with that in mind, and came up with the swept wings the 747 came with from the factory. The Stratolaunch's wings are meant to support not one, but three fuselages. Designing a swept wing big enough to handle the load would necessitate rebuilding the wing mounting points, which means reshaping the fuselages. At that point, the costs would be so high, they might as well design a brand new fuselage. So the guys at SC have to come up with a wing design that can support the weight of 2 additional fuselages, yet fits into 747's original wing mounts and won't overstress it. And they came up with the straight wing design, to minimize torsional loads on the mounts, and get maximum lift out of a limited wing surface area.

I don't think we're on the same page here. I'm under the assumption that the metallic hydrogen is to be treated as propellant, injected into a combustion chamber, somehow make it release its stored energy, increasing pressure in the chamber, and then converting that pressure into velocity via a nozzle. How did you come to the conclusion that "pure" Hmetal has 100ks Isp? Atomic Rocket's page on Hmetal listed 1700 s Isp.

@munlander1 It's termed range safety, or flight termination. Usually, the charges are standard explosive compounds like C4.

Scott Manley's assumption is more-or-less the most efficient way to do it. Rocket nozzles are designed to convert pressure in a chamber into directed linear velocity. Having the propellant mass receive/release energy when it's well past the nozzle throat would lower specific impulse, because pressure generated after the throat is not as efficiently converted to thrust as when the same pressure is generated before the throat. Diagram of typical rocket nozzle. Combustion chamber to the left.

Is Far Future Tech still being worked on, or held off for the time being?

I can imagine a plausible ion-thruster MMU setup that could have comparable levels of dV reserves to the KSP EVA suits. It'd look much different, though, and have a much weaker thrust.

@DDE Note that the Rocketdyne test crew depicted in Ignition! has to keep all of the propellants (lithium, fluorine, hydrogen) in liquid form. That means dealing with liquids that have wildly different boiling/freezing points (lithium melts at 453.7 K, hydrogen boils at 20.23 K). Add that to the problems of dealing with liquid fluorine (!), and difficulties of liquid hydrogen long-term storage, I wouldn't expect it to be a practical choice. Also, there's a fusion rocket being worked on right now. 5000 s ISP. It basically uses electricity to induce fusion, and using that fusion energy to heat propellant (lithium) and blast it out the nozzle. Bear in mind that this isn't a reactor, so it needs electrical energy rather than producing it. The fusion was needed to obtain high exhaust temperatures, boosting ISP.

I'm happy to report that I have recently graduated as a B.Sc. in Computer Science from Binus University in 2016.

Only if you have someone else constantly scanning the rig, the experimenters, and the surrounding areas. Basically, you don't want to be in a situation where a fire can start without you knowing about it. That means good ventilation, minimizing electrical sparks including statics, avoiding using flammable tools and materials, and good safety procedures above all else. That includes what to do when disaster strikes.

As an addition, liquefying hydrogen requires cooling it to 20 K or -253 oC. This is difficult; liquid nitrogen's boiling point is 77 K, which is much warmer than that of hydrogen. The liquefaction equipment won't be cheap. Also worth mentioning is the fact that liquid hydrogen can and will leak out no matter how tight the plumbing, because the hydrogen molecules are small enough to slip in between the molecules comprising the tank. On top of all that, hydrogen burns without a visible flame. One can walk into a hydrogen fire without noticing it until they start to feel the heat. @munlander1, however you choose to store hydrogen gas, keep an IR fire sensor handy.

@todofwar Definitely. Rocket engines work by using de Laval nozzles, which are designed to convert gas pressure to velocity. This design works by making sure gas pressure and flow rate makes the gas go supersonic at the throat/choke point of the assembly. If the reaction happened after the propellants go past the throat, pressure generated there won't be converted to velocity as efficiently as if it happened before the throat. This figure shows the cross-section of a typical de Laval nozzle. Exhaust nozzle to the right, combustion chamber to the left.

You can work it out, at least in theory. Look at the propellants used, determine the reaction products and energy liberated, and assume this energy goes wholly to accelerating the reaction products. Then, knowing the reaction product's mass and kinetic energy, you can work out the 'short-hand' theoretical exhaust velocity, which becomes specific impulse after divided by standard gravity. I read somewhere that if you do this with the hydrogen-oxygen mix, you'd get a specific impulse well above 500 s. I say short-hand, because even with a perfect engine design that's 100% efficient at converting chemical energy to kinetic, the exact chemical composition of the exhaust need not necessarily be the same as what a chemical textbook would tell you; burning 2 molecules of hydrogen with 1 of oxygen does not produce 100% water, even at stoichiometric ratios. Some of the exhaust could be stuff like HO, H, O, or other rarer combinations. These reduce the chemical efficiency, as some of the reaction products have not fully liberated their chemical energy by the time they leave the nozzle. Then there are the physical issues. Exhaust particles impart momentum directly opposite of their direction of travel onto the engine. Excluding ion thrusters, rocket engine exhaust doesn't always travel towards the same direction as the nozzle points. These tangentially-leaving exhaust particles impart less forward momentum, reducing effective specific impulse. All this, before considering the mechanical efficiency of the engine design itself. Older engines use what's called a gas generator cycle, where part of the propellant is used to run the propellant pump, then thrown away. Newer designs attempt to improve specific impulse by routing this propellant back into the main chamber, at the cost of reduced thrust, and needing to run the turbine at uncomfortably high temperatures, requiring either heat-resistant materials, extensive cooling systems, or both.

#2 fuel saving tip: use engines with higher ISP.

What are the biggest hurdles in designing a working fusion power reactor today, in ELI5 terms?