What's a technology you think is overhyped?Stephenson: I'm going to go with an oldie: rockets. It's just a historical accident that chemical rockets became our only way of putting stuff into space, and if we had started at a different time we would have ended up doing something that works better.One alternative would be beaming energy from the ground to vehicles, using lasers or microwaves. That seems like a doable project right now. There's nuclear propulsion, which I think is probably never going to happen at scale, because it's politically impossible, but even something as simple as constructing a very tall building or a tall tower and using that as a launch platform, or as a way to accelerate things up upward, could really change the economics of spaceflight.
I like Stephenson, but hard disagree on this. We have tried other methods. They all suck. The specific power of a rocket engine is just absurdly higher than virtually everything else (especially lasers & microwaves), at orders of magnitude lower cost. The efficiency is good, too.And you have physics fighting against you given the elements we have. You want to use beamed lasers or nuclear to make a higher Isp thermal rocket? Well you have to use hydrogen but effectively as inert gas, & the input energy to make hydrogen is 10 times methalox per kg!That’s not even counting the nuclear or laser energy!Let’s say you use lasers to power a space elevator climber. well, there’s a 50% efficient conversion of electricity to photons then at best another 50% from photons to electricity, plus the powertrain and drivetrain and friction losses, plus half your mass is crawler mass. 12.5%?And chemical rocket engines are REMARKABLY efficient thermodynamically as they operate at VERY high pressure and temperature and exhaust to a cold vacuum. They’re (nearly) single-stage heat engines that can get 50-60% chemical-to-mechanical energy efficiency.And the efficiency loss of having to carry propellant with you is MUCH smaller than you might think and for difficult to visualize reasons, but basically by accelerating the propellant inside the rocket tanks, you’re increasing its “stored” kinetic energy.So that energy is not really lost (except gravity losses, which themselves are not always purely loss).In fact, if you look at a rocket stage traveling at 9km/s but that does a burn with a very efficient engine, the kinetic energy added to the rest is greater than the chemical energy in the fuel! Because the propellant taking advantage of its own momentum.Anyway, if you had an infinitely variable Isp rocket that is zero dry mass and outside of a gravity well (to simplify), then (other than the brief very small impulse to get started) you can get 100% efficient conversion of jet energy into payload kinetic energy……EVEN THO the rocket had to carry its own propellant. And you do this by setting the exhaust velocity equal to your current spacecraft velocity so your exhaust is effectively at a standstill with respect to your starting point. All kinetic energy is in the payload, NOT exhaust.But even if you don’t do that, if you have to use a fixed Isp (exhaust velocity), then the optimal Isp to set it at is 62.75% of the total needed delta-v, and your jet-to-payload efficiency will be 64.75%. (After doing some math.)And so combined with the previous thermal efficiency, you could make a rocket that has up to, say, total chemical-to payload 35% efficiency. But you still have finite dry mass that eats into that.But rockets are SUPER good at reducing dry mass because they don’t need bulky wings, they use pumps to reduce tank pressure, and their thrust-to-weight ratios are INSANE compared to virtually any other propulsion system. Also, chemical rockets stage, often multiple times.If you ensure that your empty stage dry mass is much less than the mass of whatever is “above” it in the stack, then you’re ensuring that as much kinetic energy as possible ends up in the payload. As long as this ratio is 1:4 or better, you are doing very well.Overall efficiency, then, can approach like 20-25%, ideally, even with dry masses. This can beat space elevators and many other alt-launch concepts. And we haven’t even mentioned the second-worst part of space elevators: they take like 2 weeks to get to orbit & back, vs hours.Also, rockets are REALLY good at minimizing all the loss sources. They typically launch straight up to get out of the atmosphere, and sometimes throttling down to reduce drag/load, then punch it at high altitude horizontally, minimizing gravity losses.That means they’re never moving hypersonically through the atmosphere, and never for long at any speed, unlike gun-launch/spinlaunch or airbreathing concepts. And they have low gravity losses due to high T/W ratio, so they don’t need heavy wings.They need a lot less ground infrastructure than many of these other concepts as well. A space elevator would never pay itself off.(Couple notes: you might notice that 62.75% of the ~8-9km/s needed to get to LEO is 500-600s Isp, which is a bit outside the reach of chemical, and that’s true, but you’re close enough to the optimum it doesn’t make much difference, plus you often DO adjust Isp during flight……also, I said never going hypersonic through the atmosphere, and that depends on the rocket. I mean the thick part of the atmosphere. Unlike gun/accelerator launch that goes its peak speed (before rocket booster starts) at sea level.)…I will say, ultimately, skyhooks could be better than rockets. They can kind of cheat by losslessly transferring momentum back and forth between launching and entering mass. & they work/can be reused quickly (within minutes, like rockets) unlike elevatorAlso, as an example calculation, the energy of kerosene required by the F9 to get 1kg to low earth orbit is about 308MJ in full expendable mode, vs 32MJ/kg theoretical limit given orbital kinetic & potential energy. 10.4% efficient. 7kg of fuel per kg to orbit. That’s pretty goodThis sounds bad, right? Kerosene is about $1/kg right now, elevated due to war. Given typical air freight efficiency, it takes about 8kg of kerosene per kg of payload flown around the world (assuming regular refueling!), so fuel costs to orbit are comparable to air freight!Newer aircraft are more efficient, partially reusable version of F9 takes 10kg fuel per kg of payload, and there is some energy (less than 1MJ/kgO2) needed to make oxygen. HOWEVER, Higher pressure rocket engines (esp. that run oxygen-rich) can be significantly more fuel-efficient
Stephenson: .........; That seems like a doable project right now.............There's nuclear propulsion.............something as simple as constructing a very tall building or a tall tower and using that as a launch platform,
Quote from: su27k on 08/29/2022 02:28 pmStephenson: .........; That seems like a doable project right now.............There's nuclear propulsion.............something as simple as constructing a very tall building or a tall tower and using that as a launch platform,He's a writer and geographer and the physics minor didn't stick.These points show he isn't a knowledgable person on the subject, so why does it matter what he think?
He’s pretty smart, but wrong in this topic.
He's a writer and geographer and the physics minor didn't stick.These points show he isn't a knowledgable person on the subject, so why does it matter what he think?
Quote from: Robotbeat on 08/29/2022 04:24 pmHe’s pretty smart, but wrong in this topic.This is the bane of modern journalism getting experts in one field to comment on stuff well outside their area of expertise and end up with some dumb replies.
What didn't get mentioned earlier in this thread is that Neal Stephenson is actually the first employee of Blue Origin, and his main responsibility at Blue back then is research on non-rocket space launch concepts... So yeah, he sort of is an expert on this, but I suspect his knowledge is mainly theoretical side instead of practical side.
Quote from: su27k on 01/23/2023 11:42 amWhat didn't get mentioned earlier in this thread is that Neal Stephenson is actually the first employee of Blue Origin, and his main responsibility at Blue back then is research on non-rocket space launch concepts... So yeah, he sort of is an expert on this, but I suspect his knowledge is mainly theoretical side instead of practical side.An expert on putting things in space without rockets?I don't think that word expert means what you think it means.
