sevenperforce

Aerodynamic drag on the spinning part within the pressurized chamber will slow the spin rapidly.

If it's carrying both a Dragon 1 full of supplies plus the Bigelow Expandable module, it may need extra umph to get the second stage up to speed, burning too much fuel for a boostback and necessitating a droneship landing. If it sticks the landing, when do we think that rocket might fly again?

By what definition of efficiency? By the practical definition of efficiency, air is free, and it's available enroute so you don't need to carry it around. LOX is not free and you have to carry it with you the whole way up until you burn with it. There is no freaking way it's going to result in a lower cost to move payload from one place to another. Lower cost, no. But a jet turbine engine is taking in poorly-compressed, hot nitrogen and oxygen in a less-than-ideal ratio and burning it with fuel, while a rocket uses pure liquid oxygen. As a result the reaction is fairly inefficient and only a small amount of the chemical potential energy in your fuel is actually converted into thrust. If a highly efficient turborocket gas generator was used to accelerate the same volume of nitrogen and oxygen for the same total net thrust, the fuel and the oxidizer consumed each second would be equal to or lower than the fuel consumed each second for the jet turbine case, for certain high-bypass turbofan engines. At least, that was the claim I saw; like I said, I'd need to run the numbers myself to be sure. Yeah, you'd definitely have space. The throat would be open and the path would be straight; that's what I mean about an "inside-out" engine. The flow goes through the center of the engine rather than being deflected around a turbine or spike. The convergence point of the oblique shocks changes with respect to speed, allowing you to choke the flow at low supersonic speeds but unchoke it at hypersonic speeds, without moving parts. I'll do a mockup to show you what I'm envisioning. If you send the supersonic rocket exhaust plume across the supersonic air flow, I'm pretty sure it should mix.

Could hybrid rockets have been used in place of ICEs for earlier manned controlled HTA flight?

For the upcoming ISS resupply mission at the end of this month, does anyone know whether they'll attempt landing at the pad or on a boat again? The ISS is not a high-velocity target so I'd imagine they'd have plenty of propellant for the boostback burn...unless, of course, it's a particularly heavy resupply and they need extra fuel to lift it.

Such a moon would quickly lose energy and become tidally locked. All moons do. Not if it was fairly dense, with an axis of rotation in line with its orbital plane. That's a lot of angular momentum. I'm more unsure whether field coupling could produce strong enough magnetic gradients to use for lift. Other ideas.... A planet with a very high rate of spin would likely have an equatorial ocean, making water-based launches a necessity. Then there would be fun stuff like a smaller planet with lots of geologic activity where you could ride periodic volcanic eruptions into orbit...

Here's a thought. Triton-Neptune setup, with the primary ice giant (we'll call it Theron) having a very strong quadruple magnetic field, and the dense secondary (we'll call it Fiora) having a high spin rate and a highly inclined axis of rotation, such that each of its poles sweep through Theron each orbit. It also has a strong magnetic field due to its high spin and heavy, hot core. Its orbital plane isn't quite aligned with Theron's magnetic equator. Thus, every dozen orbits or so, one of Fiora's magnetic poles will momentarily line up perfectly with one of Theron's magnetic poles, locking together. The solar wind would flow between in a brilliant aurora, and if you were in the right place at the right time with a large electromagnetic gyrostabilized platform, you could ride the magnetic gradient straight into space.

In the vein of "beat Sputnik" and "1900s Space Race"... We all know it is much easier to get to orbit from Kerbin than it is from Earth. Kerbin has roughly the same surface gravity as Earth, but is apparently far denser and thus less massive, so the escape velocity is a lot lower. But is there any way you could get a planet or planetary system even more conducive to early space access than Kerbin? Rules: The planet has to be habitable by humans: comparable surface gravity, comparable atmospheric pressure and temperature at the typical habitation regions, comparable oxygen partial pressure. It also has to have at least a moderate chance of forming naturally; no Kardeshev-II alien geoengineering. No weird physics, either. What are the options? Perhaps you could have a world with less habitable surface space, but a higher rotation rate and bulge, so launching from the equator (even if the equator isn't as temperate as ours) gives you a nice boost. Maybe the atmosphere could be made of a collection of gases which are still okay for humans, but work better for an airbreathing engine. Perhaps you have a very low tidally locked moon with a strong common magnetic field, allowing transient magnetic gradients which can be ridden into orbit. Ideas?

I was pointing out to my kids how broadly expanded the first stage's plume is at MECO. Definitely near vacuum. The second stage has a VERY efficiently expanded exhaust plume. The purpose of the nozzle is to convert the heat and pressure into kinetic energy; the exhaust temperature is probably pretty low in comparison to the first stage at ignition.

What they said. You can just replace "moles" with "millions" if that makes it easier to grasp. It's a number. Just be sure to replace "millions" with "moles" again at the end. A mole just happens to be approximately the number of hydrogen atoms in one gram of hydrogen, and since all elemental masses are roughly multiples of hydrogen's mass, using this number allows for some simplification of the math.

Claustrophobia might do me in. I don't mind enclosed spaces...even really unpleasantly cramped ones...for moderate periods of time, but over the course of months it might drive me batty. Cooking is a big deal but I could get by.

Good question. The event horizon of a singularity is a timelike border in relativity, but not necessarily a spacelike border. And not necessarily at the levels where relativity and quantum mechanics start to bleed together.

Exactly. So...design an inside-out engine with an open-drum tapering centrifugal turbocompressor, functioning as an ordinary compressor from static to transonic speeds, then using a carefully-shaped inlet to maintain terminal shock right above the compressor surface until the speeds are too great, at which point shocks are oblique through the entire engine and you function as a scramrocket.

That requires LACE, a Liquid Air Cycle Engine. The air has to be cooled to liquification, then fractionally distilled to separate out the oxygen from the nitrogen. No way to do that fast enough to make it work. Plus, the enthalpy to liquefy air is way too high...you would have a HUGE amount of hydrogen required. Now, if we can make supercooled liquid metallic hydrogen one of these days, all bets are off. A scramjet has no terminal shock at all, right? While a conventional engine needs a sliding spike to direct the shock to the right places as the speed increases, an inside-out engine could be tuned such that its oblique shocks meet downstream at hypersonic speeds.

Helium won't rip their precooler to little bitty pieces. Hydrogen destroys everything.

At certain times in the past, the universe expanded faster than the speed of light, and so this is not possible. If you could travel faster than light you would go back in time. If you could "jump" past the observable edge of the universe, you would reach regions of the universe that have been separated from our immediate vicinity ever since inflation began.

A theoretical central/open-bypass engine wouldn't have any terminal shock at all, above the design speed. That's part of the range of ideas: turn the engine inside out, and the oblique shocks start to work with you instead of against you. Sure, friction (compression, really, but that's beside the point) is nonconservative. But there is a way to use the atmosphere as your reaction mass; it's just wildly inefficient in the ways we have been doing it so far. For reaching orbit, at least. And say, we were talking about this earlier, but I forgot... If the helium loop runs the turbopump off the temperature differential, what runs the turbopump once Skylon shuts off its intakes?

Just a remembered anecdote; I'd have to run the numbers myself to know if it was correct. When I get a chance, I'll look into whether a normal shock and a bow shock could be aligned in a speed-sensitive way so as to maintain ideal inlet flow conditions relative to speed. It annoys me that we can use the atmosphere to decelerate but not to accelerate, you know?

Oddly, neither as active nor as interest-diverse.

For a given intake diameter, maybe. IIRC, high-bypass airline turbofans would actually be more efficient if they were run as turborockets, due to the more efficient burn of liquid oxygen, but the fuel savings wouldn't make up for the extra complexity and reloading cost.

They put it through the inspection and refurb program they'll be ordinarily using, and I think it was ready for flight again in a week or two. These engines were designed for in-flight restart from the beginning so they are pretty sturdy.

EML2 was a potential design target. Aiming for the size which would be useful for the greatest number of missions, you know.

Downloaded the demo but haven't had a chance to play it yet.

Believe it or not, I haven't actually ever played KSP. This just happens to be the largest and most active group of spaceflight enthusiasts on any form I have found.

It would still be a neat relationship...like how gas mileage, distance/volume, works out to units of inverse area, and happens to be the instantaneous fuel consumption cross-section along the length of your trip.