K^2

Indeed. Latin, etc.

Purely historical reasons. Rocket science started out in Germany and spread primarily to USSR and United States after WWII. Soviets continued to use metric system. So if you take any Soviet textbook on rockets, you'll see specific impulse defined per unit mass and the units are going to be m/s. Americans have always favored using Imperial units in engineering, and rocketry was no exception. So instead of taking impulse per kilogram, they took impulse per pound. Now, it is what they call "pound of mass", which is property of material, not of the actual weight. So 1lb of mass on Earth is the same as 1lb of mass on the Moon. But the problem is that this value is defined as "Mass of an object that has weight of 1lb on Earth". And that introduces that factor of g into all of the equations that deal with ISP using U.S. convention. Furthermore, the unit is still the pound. So even though it's called "pound of mass", it's still formally a weight. Just that instead of current weight of object in whatever gravity, it's always the weight that object would have if it was placed on Earth's surface. Hopefully that makes some sense. It is a bit confusing. I still mentally convert all specific impulse to m/s, because it makes more sense to me that way. But I'm going to arm myself with Nuke's method now as well.

That is actually a very good way of putting it. I've never thought of it that way. Just so long as it's a unit of fuel by weight. It makes the most sense in Imperial units, actually. If you have an engine with 300s of ISP, then at 1lb of thrust, 1lb of fuel will last you 300 seconds. (And, of course, this is where we get such silly units from, anyways.) But this works in metric as well. If you have 1N of thrust, then 1N of fuel by weight will last you 300s. Oh, and it's always going to be Earth/Kerbin weight. (Both have g0 = 9.8m/s²) Just the way these units are defined.

The amount of impulse a rocket receives per weight of the fuel. The reason it works out to be in seconds is that impulse has units of force multiplied by time, and you divide it by weight, which is a force, leaving you with units of time. P.S. Impulse is the change in momentum, which for a classical rocket is it's mass times velocity.

The difference here is that we already have prototypes flying scrams at Mach 5. So we evidently have means of building these with current materials. SABRE's yet to be built. As for the costs, construction of the SABRE involves machining turbines, compressors, pump, cooling units, combustion cameras, and various cowlings that need to be regularly replaced. An air-augmented rocket has combustion cameras, pumps, and cowlings. All of which are simpler than SABRE equivalents. Even if the one-time cost of building the body of the scram jet ends up being higher, which would be very surprising even at that, the operating costs of the scram are going to offset it by far.

In air, larger one will fall a little bit faster. It has larger area, but also more mass. Area increases as square of the size, but mass is going to increase as a cube. So a weight-to-drag ratio is higher for larger cannonball, giving it faster fall speed.

Does FAR account for the fact that CoP of the heat shield moves as the lander tilts? With static CoP, I can imagine a lot of ships being unstable on re-entry that shouldn't be.

We need to rename that planet. It's bad enough by itself, but when you start talking about probes and moons in the same context... You're wondering why we don't have more missions to study it? It's because even if NASA can propose it to congress with a straight face, there is no way congress is going to be mature about it. I bet ESA has the same problem.

Your claim was, "We don't know." That's nonsense. We know what it takes to store all relevant information, and such storage is available, and we have very good estimates on what it would take to simulate the human brain, as well as what outcomes of such simulation can be like. I've addressed that in my earlier post. So the parts I've quoted remain to be nonsense. As for your additional nonsense on nanosystems and complexity, the bottle neck right now is speed of memory access and total processing power, both of which can easily be addressed with an optical system. We don't have these built, and it might be a few decades before we do, but this isn't a conceptual problem. Just an engineering one. As for the size of the system, I've never claimed that it'd be something that can fit in human cranium. We are discussing capabilities of clusters which consist of entire rooms of computer parts. But yes, if we want to build something of the same size capable of simulating human brain, that will require some conceptually different approaches to computation which we don't have a start on yet. Nothing impossible there either, but I'd agree with you on lots of unknowns there.

It helps to read the thread. I've addressed most of this. Running as a scramjet, air-augmented rocket can pull ISP closer to 1,500s. On Jet-A. If we make it impulse per $$$, instead of weight, a scramjet beats SABRE hands down. And this is atmospheric flight. It's not all about ISP. TWR is a very important factor as well, which is way, way better for a scramjet. And then there is the question of the craft's size. Hydrogen is very, very low density. For same mass of fuel, the tanks have to be 10x larger. So even if ISP is 2x better, for the same operation time at the same thrust, you need 5x larger tanks on SABRE. Did I mention that they are cryo tanks, which need to be extra thick and extra heavy? Oh, and also, they vent H2 gas while on the ground, which is about as flamable a gas as you are likely to find. As for complexity, SABRE also includes a rocket engine. And turbines. And compressors. And coolers. It's a way more complex engine with all of these components having to run 100% of the time. The rocket engine on air-augmented rocket is needed only for the takeoff and getting past sound barrier, where extra thrust is needed. There, you'll be down to something like 700s - 800s of ISP on a Jet-A/LOX engine, but it's a small fraction of the operation time. It's simpler overall, and the most complex and expensive parts don't get their resource used up nearly as fast. It's just not even a competition when you consider all factors.

Nonsense; nonsense. More nonsense! It's a finite state machine with quite a manageable state vector. As I've pointed out, we're already well capable of storing all required information to describe the brain's state. Simulation is a bigger hurdle, but nothing like what you're saying.

Regular home computer? No. Not enough storage. A server or cluster with huge storage capacity? Yes, in principle. As far as running it, that's a more complicated question. On one hand, there is a lot that we still don't know about the exact way messages are sent between the neurons. There is just so much going on. But on the other hand, human brain keeps mostly functioning across a range of moods, illnesses, chemical imbalances, and under influences of poisons and drugs. We can certainly simulate neuron-to-neuron communication with enough precision to fall somewhere within that range. The other factor is that there is no computer in the world that can simulate neural activity in human brain at anywhere remotely close to real time. Human brain has hundreds of billions of neurons, each connected to hundreds or even thousands of its neighbors. Each neuron refreshes at something like 100Hz, but if you want a good simulation, you'll need a much shorter time step, on the order of 10kHz or better. This dwarfs any supercomputer by itself. If you take into consideration what sort of a royal mess this does to memory mapping, it could easily take days of real time to simulate 1s of brain activity on largest clusters. But given that you are ok with slowing the time down for the simulated brain, and you don't care if simulated brain turns out to be depressed or high as a kite, yes, in principle, we can simulate a human brain with all of the thoughts in it. Of course, this assumes you have a full map of a brain of a grown human brain, which we have no chance to obtain. And we can't really start from scratch and teach a "blank" brain either. Not only would it take way too long, because we can't simulate it at real time, but learning involves changing the way the neurons are connected, and we don't understand that part of the process enough to build something that grows and develops the way human brain does. So in practice, we just don't have the data to work with to make such a simulation worthwhile.

The thread's about using this kind of an engine, specifically, the Scimitar variant, on a jet liner. We are talking 1-2 hour flights at Mach 5 or so, doing cross-Atlantic or cross-Pacific flights. Doing one two-way flight a week, you'll burn through that 50 hour resource in less than 3 months. This is good enough for Skylon, of course, especially given the competition, but you can do way better with air-augmented rocket if you plan to run an airliner. Heavy and expensive. There are many advantages that SR-71's hybrids have for a spy plane. But you got to go with something lighter, cheaper, and more reliable for an airliner.

If I was looking for a (sc)ramjet alternative to be used for something like an airliner, my first choice would probably be an air-augmented rocket. It has all of the advantages of both systems without either drawbacks. The foremost advantage is that an air-augmented rocket can run as a conventional scramejet at hypersonic speeds. In other words, you get hypercruise capability, where you only burn conventional jet fuel. This makes such an airliner very cheap to operate compared to a Scimitar alternative. Further advantage over Scimitar is safer, denser fuel. Tanks will be smaller and lighter, not only because Jet-A takes up less volume for the same energy, but also because it's not cryogenic. And safety is no small factor. Any LH2 leak is an explosion hazard. Hydrogen gas mixed with air will auto-ignite in presence of certain metals, such as platinum. Never mind real ignition sources, such as sparks or open flame. Jet-A, in contrast, is hard to light with a match. Finally, engine complexity. Scimitar is going to be a nightmare to maintain. You have coolers, compressors, turbines. Tons of feeds, valves, and various moving parts. And FAA requires you to pretty much take everything apart and check every 100 hours. Expensive. In contrast, an air-augmented rocket is just as simple as a scramjet. A duct. A few fuel and ox inlets. Maybe a few cowlings. That makes it easier to maintain, and safer to operate. Not only are there fewer things to break, but unlike any turbine engine, it can take sand and bird strikes, and just keep on going. So it's almost as simple and cheap as conventional scramjet. But it doesn't require boosters to get it off the ground. It can get rolling and up to takeoff speeds using rocket thrust. At that point, it will have enough air coming in to start running in its air-augmented mode. As an air-augmented rocket, it will have way more thrust than a conventional ramjet while passing the sound barrier, and that's where all that thrust is needed. Once well past the sound barrier, the ox supply can be cut off, and the aircraft continues as a scramjet. The only disadvantage of an air-augmented rocket used as a jet engine is the fact that you do need oxidizer. You don't need much. Most of the flight you'll be burning conventional jet fuel. So the costs aren't really an issue. But you do need an extra tank, and pretty much anything you go with for ox will be some sort of hazard. At the end of the day, I'd probably go with LOX for that. It's comparable to liquid Methane in ease of use, except that it's also corrosive. On the plus side, it's not a suffocation hazard, it's not quite as much of a fire hazard, and at any rate, you'll have way, way less of it to worry about than you'd have to with Scimitar's LH2. No, not really. Methane freezes at 90K. You wouldn't be able to get it to a low enough temperature to run precooler. That's why SABRE/Scimitar can only run with LH2 as fuel.

You can, but your energy efficiency is going to be even worse than with hydrogen. ÃŽâ€H of ammonia is -80kJ/mol. So at 800s, you'll be losing about 600W per 1N of thrust just on ammonia decomposition. H2 is going to stay as H2, so ÃŽâ€H is zero. Hydrazine, on the other hand, gives you ÃŽâ€H of +50kJ/mol. So at 600s, which you get with a hydrazine arcjet, you'll be saving about 270W per 1N of thrust. Since an arcjet can be up to about 50% energy-efficient to begin with, we're looking at about 5.6kW/N with hydrazine, and about 8.4kW/N with ammonia. In most practical applications, this increase in power requirement is going to negate any benefits of higher ISP. Finally, one more advantage of hydrazine is having a backup. An arcjet with no power can be built to function as monoprop on hydrazine. That won't work on hydrogen or ammonia.

A turbocharger is designed to force and compress an airflow. Turbopump pumps liquid fuel. The pump and the charger are going to be very different, because they are designed around very different densities. Now, the turbine stage of the turbocharger you might be able to reuse for a turbopump. It's going to be a matter of torque and RPMs you need for both, but you could probably find a pump that will work with what you have on the turbocharger's turbine output. Fortunately, finding a centrifugal pump that would work is much easier than finding a turbine. There are a number of centrifugal water pumps used for irrigation and what not designed to be driven directly by an electric motor. That means high RPM and low torque, which is what you have from the turbine. I'd check gardening and home improvement stores, then compare the specs you have on the turbocharger to these of the pump and try to find the best match. It won't make a great turbopump, but as far as building something at home, that's probably your best bet. Of course, there's still the issue of driving the turbine, but that's a separate problem. Honestly, for an early project, a turbopump is way overkill. If you don't have rocket-building experience, this is several levels of over your head. Start simple. I would recommend getting basics of solid fuel rocketry first. It's the sort of stuff that will explode but probably won't injure you unless you do something really stupid. Failures there will teach you how likely failures are and how to protect yourself from them. If and when you are ready to graduate to liquid fuels, again, start way simple. Build a pulse jet. Then try biprop rocket, but still just pressure-feeding the fuel and ox. Trust me, there are a lot of challenges just getting to such a stage. When you feel that you need more thrust than you can get with pressure-fed system, your first pump should be electric. Way, way safer and easier to scale. Alternatively, if you really can't wait to make use of the turbines, you can try building a very basic turbojet out of a turbocharger using skills you learned building a pulse jet. Even here, I must stress, this shouldn't be your first project in propulsion. Hopefully, this gives you some sort of idea of just how much you're jumping the gun here by trying to figure out turbopump first.

Depends on what you mean by delta-V. What you get from the rocket, yes, you just use ISP for atmospheric operation. But then your rocket is also going to lose a lot of speed to drag, and that will depend on ascent profile. Either way, however, your total delta will be smaller due to drag.

Asteroids might be far apart, but resources are really cheap to move around if you don't care how long it takes for you to get them. So as long as you have a steady stream of resources from various asteroids or asteroid groups, you're good. Vacuum proofing has two parts to it. Pressure - that's actually not so bad. It's much easier to build in vacuum than in high pressure environment. If your station is built as a rotating self-suspended bridge, this is going to take the bulk of structural strength, with pressurization being handled by internal structure without any problems. And then there are leaks. These you'll have to deal with with any hostile environment. So overall, it's not so bad. Same deal with possibility of damage. If one of your modules depressurizes high in the Venusian clouds, anyone inside is going to be just as dead as if this happens in vacuum. (P.S. On asteroid impacts. Micro asteroids can be handled easily enough with shields, and larger ones are just as likely to hit you on world as off-world.) There are challenges in building in space, and in short term, that might be the more expensive option. But the moment you need to move people between habitats in different parts of the Solar system, the added cost of getting someone off world will make expense of building in space seem like nothing in comparison. Once you are in space, you have the whole of Solar system in your disposal, and that by far outweighs any minor disadvantages that we'll learn to overcome with better techniques and materials.

Keep in mind that this is a measure of performance, but not necessarily the only one to consider. Also, even a small difference in ISP adds up over a long mission to considerable savings, thanks to that log function in the rocket equation. Ultimately, of course, if you are sending an interplanetary probe, what you are trying to minimize is the weight to LEO/LKO that gets the mission done. But all else being the same, one with higher effective ISP will be the ligher probe. This is why I recommend it as a measure of efficiency. As for how much it adds up, look at your own example. You have total of 760kg to LKO or 1,158kg to LKO to accomplish the same mission. The difference in specific impulse isn't large, but it's a big enough fraction of the total impulse your probe builds up, so it makes a considerable impact.

I can definitely see benefits in building a research statoin like that, but I don't know if it makes sense from colonization perspective. In fact, I'm not entirely sure why people are so inclined towards planetside colonization when there is so much habitable space, well, in space. Artificial gravity and pressure are easy to maintain if you are building something populated by thousands of people. And you can find everything from organic building blocks to heavy metals in asteroids.

Energy efficiency is a bit different from what you are asking, but also a very interesting subject for rocketry. Some day, we should have a topic about it. What you are asking about is still ultimatley about mass efficiency, but including a lot of dead weight you have to carry around. One way to sort of work around this issue is define effective ISP. The whole point of specific impulse is impulse per mass of propellant. But it doesn't have to be just propellant. You can include any additional dead weight. So for example, normally you say that your rocket's mass started out at some m0 and decreased to m1. Rearranging the delta-V formula, we can get ISP as the following. ISP = ÃŽâ€V / (g0 ln(m0/m1)) What you can do is instead of taking m1 to be the total mass of the rocket after it has exhausted the fuel to be the just the mass of the useful load. In other words, you'd subtract mass of any batteries, generators, solar pannels, and anything else you don't actually need to reach that delta-V. The resulting effective ISP will be significantly lower than actual, because it reflects mass efficiency of your propulsion system. Naturally, the above is going to depend very much on how much delta-V you actually plan to achieve. If your power plant has a fixed weight, the more delta-V you achieve, the less of an impact the dead weight makes. Which is one of the reasons why ion drives make more sense on long journeys than short ones. But this can apply to more conventional engines as well. For example, we are happy with low ISP of the monoprop for the RCS, because we don't need much dleta-V out of it, and a full bi-prop engine is just too much dead weight to carry around. There are some other advantages there as well, but it's a big part of it. P.S. Just for future reference if anyone's searching for energy efficiency. For conventional rocket following Tsiolkovsky Rocket Equation, the specific work done on propellant reducing mass of the rocket from m0 to m1 to achieve ÃŽâ€V is given by the following equation. ÃŽâ€E = (m0/m1 - 1)ÃŽâ€V² / (2 ln(m0/m1)) This has units of energy/kg. The above achieves absolute minimum, regardless of ÃŽâ€V, at the following mass ratio. m0/m1 = -2/W(2/ln(2)) Where W(z) is Lambert W Function, also known as product logarithm, defined as solution of z = W(x)Exp(W(z)). Numerically, this evaluates to approximately m0/m1 = 4.29155. Given this optimal mass ratio, efficiency of conventional rocket, not counting thermal or other losses of the engine itself, is approximately 64.76%, which is way more than you might have thought. Especially, when you consider that base efficiency of an ion drive can be over 90%. This does lead to an interesting conclusion. Since mass ratio here is fixed for any delta-V, the optimal ISP depends on desired delta-V. (One can use the equation for ISP mentioned above.) More specifically, exhaust velocity should exit at about 62.75% of delta-V, which gives ISP of about 0.064s for every 1m/s of delta-V desired. Of course, all of this assumes that the limiting factor is energy, rather than mass. However, with conventional rockets, amount of energy itself is limited by chemical energy in the available mass of the fuel, and that tends to fall short of the mark. For 9km/s of delta-V, which is what you'd typically want to enter LEO, energy-optimal ISP would have been about 576s, with real chemical fuels making it as high as 450s. On the other hand, with conventional rocket, we really don't care about energy use, so much as we care about the mass. So if we had fuels with ISP well over the 576s above, we'd probably use them anyways despite it being a bit wasteful because it would make for smaller, lighter rockets. This theory does, however, directly apply to ion drives. Not anything that would use on-board battery, but anything that would use sollar or RTG power, certainly. Assuming a long duration mission, minimizing energy requirement for mission delta-V minimizes the size of the generator or solar array. Whether that's more important than reducing total payload is a separate question, since for energy-optimal operation the probe would have to cary more than four times it's own weight in propellant.

I've seen properly restrained passengers with facial bruises that say otherwise. Crash test data concurs. Because we all know that real life crashes never exceed any safety limits. That's why everyone always walks away from them perfectly fine. Your argument is absolutely absurd. What matters is that we take a range of collision speeds from a bump in the parking lot to a head-on collision with oncoming at highway speeds. And we look at range of injuries and survivability with and without air bags. That's what matters in real life situation. That's what crash ratings look at. And yes, they go past what the car is meant to withstand, because guess what, that's the scenario that's going to kill you. If your crash was mild enough that your seat belt was enough and you never hit airbag, good for you. But if your crash was severe enough for you to plummet face first into the airbag, it might have just saved your life. Airliner nver does anything to reduce lift. And it's not how spoilers work either. And both are fatality free in majority situations. So they are kind of irrelevant. How many racing crashes do you know that are head-on either into an on-coming car or into a fixed barrier? (Not one of these soft crash barriers.) A typical racing incident either has the car brake into a specially designed barrier, or is a glancing collision, which sends the car spinning or rolling. (Yes, "flying" was an exaggeration.) You don't see extreme forward deceleration as much as lateral in racing accidents. Road accidents are by far dominated with linear deceleration. That's kind of the point, yes. Just so long as we keep in mind that direction of acceleration in racing and road accidents tends to be very different, moving on... This is true only while you can guarantee that the acceleration of the safety cell proper is within survivable envelope. I can strap you to the inside of a solid steel structure and use it as a wrecking ball. You aren't going to make it, simply because the very harness to which you are attached is going to undergo accelerations that will brake your rib cage and ruin your internal organs. This is a limitation for real cars as well. Which results in there being severe limitations on how strongly the restraints can hold you in case of rapid, linear deceleration. These are the cases in which we rely on the air bag. The safety cell proper decelerates too fast for you to survive with restraints that can hold you. A typical belt will simply break your ribs if it's strong enough to hold you to the seat. This is why such a thing as airbag exists. You reduce the holding power of the seat belt, you allow passenger to travel through the cell under extreme deceleration, and you mitigate the problem with an almost-instantly deployed airbag. And the reason this isn't a factor in racing is Good. Now we're learning something. Now, I'll give you a few minutes to compare what you just said here, focusing on difference in design of passenger car belts and racing harnesses, with everything going on above. I agree that it's a major factor. Thank you for spelling out the differences for people who aren't aware of them. But you have to understand that it's not only a matter of convenience. There are factors here that have to do with differences between typical road and racing collision. And this goes for everything from the durability of the cell, to amount of crumple space allowed by a vehicle, and by far not least, the way the collisions actually happen. Some of these limitations are due to cost, yes. You can make a car that's half less likely to kill you in a catastrophic collision, but if it costs twice as much for all the same features, nobody is going to buy it. And that's a fact you have to deal with. Saying that this is why airbags are useless is absolutely wrong. They provide similar levels of protection at a significantly lower costs not just to the restraint system, but to the entire vehicle. From frame, to engine design, to the seats themselves. You seriously need to look at some of the actual reports, both from crash tests, and actual accidents. Airbag dramatically increases risk of neck injuries to unrestrained passengers in low speed collisions. In fact, there have been cases of airbags killing an unrestrained passenger. You seem to have zero understanding of how these systems work or timing involved. A properly restrained passenger strikes airbag after it has deployed. An unrestrained passenger is hit by deploying airbag, typically, at an upward angle, which causes the head to be pushed backwards while torso continues down and forward. This is how people brake their necks.

Of course. But for monoprop, if we are looking at an arcjet, I'd definitely go with hydrazine. (You can't use H2 at all as conventional monoprop.) While you lose quite a bit of ISP, what you gain is a lot of extra TWR and power efficiency. Now, TWR probably isn't a huge deal in a lot of cases, but power efficiency is going to be important even on longer missions. In my mind, H2 only makes sense with LOX as a biprop, or as propellant for an NTR.

Well, sure. But the guidance code for this can be written by a good grad student. I understand that even such basic tests are necessary, but I don't quite understand the reason behind posting the video here in its own topic, I guess. I'm kind of expecting some sort of innovation behind the project, even if it's not being demonstrated yet. (Early grasshopper tests weren't all that impressive either.) That's why I'm asking.

Any specific goals behind this test? It all seems very straight forward. Are they working on a vehicle, engines, or guidance systems?