K^2

Of course, but we need a shopping list first, and for that, we need a near-final design. I simply couldn't tell you quality/types of some of components we will need at this point. But yeah, it is something we'll have to have eventually, down to specific places where we can buy units and price for each.

Depends on what you call an effective exhaust velocity, I suppose, but it has more to do with definition of ISP. It is defined as impulse you get per weight of fuel you have to expend. If all of your reaction mass comes from expended fuel, then ISP and exhaust velocity are proportional. vp = g ISP. But if you have access to external reaction mass, relationship becomes more complicated, and you can increase ISP even as exhaust velocity goes down. And yeah, so long as you aren't traveling too fast, progression KerikBalm describes applies. The more air you push, the slower you are going to push it, and the more fuel-efficient your engine will be. Unfortunately, at high speeds, energy losses on collecting the air tend to outweigh benefits.

Methyl mercaptan. You know, for safety reasons.

Second profile is, in fact, the optimal policy if you allow for infinite thrust. (It is a bit of work to derive.) Furthermore, if we now vary height as a parameter, we see that the first-order optimal condition has no real solution. That means that the optimal altitude for the orbit is at one of the boundaries, either surface elevation or +inf. Simple algebraic check confirms that you want your orbit as low as possible to minimize delta-V requirement. QED.

Lets say that atmosphere is also water vapor, for simplicity, so that molecular weight is exactly the same. Burning hydrogen with oxygen produces enough energy to accelerate the exhaust to about 4km/s. (It's not vacuum ISP, after all.) Now, imagine that for each kg of fuel/ox, we pull in 3kg of atmosphere. So now the same energy is pushing 4kg. What will the exhaust velocity be? That's right. 2km/s. What happened to the thrust? It doubled. Consequently, your ISP has doubled as well. It is true that if you are carrying all of the propellant with you, you want to have as much energy going into it as possible. All you care about is getting maximum exhaust velocity. But augmented rocket is way more efficient for the same reason that the jet engine is. You simply don't carry all of the propellant with you. You use atmosphere for extra reaction mass. That gives you way better efficiency.

Erm... I could. Are you guys using some specific software/service, or just grab image, edit in image editor, upload it back?

Modern sensors don't seem to have that problem. At least at reasonable exposure times. Of course you can. Trails are just a convolution, so you can deconvolute data with an educated guess of the kernel and a sensible filter. I wrote my own. It's a fairly trivial matter, if you understand convolution theorem and optimal filters or MEM theory. Alternatively, a lot of kernels can be deconvoluted using SVD methods. Though, I've only used that to reverse gaussian blurs. I've never tried SVD on motion trails.

That is absolutely not true. Air augmented rocket relies on atmosphere primarily for added reaction mass. It can be completely inert. You'll just have to inject a bit more oxidizer into combustion chamber. I think, you might be confusing air-augmented rocket with scramjet. The low pressure of Martian atmosphere is another matter, though. At high enough speeds, you'll still get a decent flow through the engine, but you'll also have heating losses at that point. Add to that extra weight of an air-augmented rocket over conventional biprop, and I'm not sure it'd be worth the trouble. Venus, on the other hand... (Edit: Or Titan. There, you might not need to bring the fuel. Just oxidizer. That should be fun.)

That's 20th century thinking. Deconvolution can increase that threshold to several minutes, easily. That introduces a new source of noise, but one you have much more control over. Specifically, once you can capture images of some stars, sharp enough to ensure good localization, you can just keep stacking these images, and noise will go down as inverse square. But that's an aside. The important bit is that even with exposure you can get without doing any numerical tricks, you can capture brighter stars with a decent cellphone camera. And then you can skip the deconvolution and go straight to stacking.

Yes, good catch. Thank you.

Real rockets are a bad example of it, because they tend not to be very efficient away from full throttle. So it ends up being more efficient to lift up balls to the wall, rather than throttle down for optimal TWR. TWR = 2 is optimal under two assumptions. Your ascent is perfectly vertical, and drag is quadratic. Neither of these are true. Quadratic drag model does not apply anywhere in the vicinity of transonic region, which is basically all of early ascent. Once the rocket gets into mach-independence region, you are well into gravity turn, and density starts to play a role as well. At any rate, claiming that TWR should be exactly 2, or certainly over 2, for a real rocket is a bit reaching. But there will certainly be times when you need TWR in that vicinity during ascent. TWR of 1.2 is not going to cut it for a good ascent. Some rockets might be built to have a 1.2 from the pad, simply because they will also be running balls to the wall and will get up to higher TWR further in flight, but no rocket will ever be built to intentionally maintain 1.2 or anything like it.

I've updated the front page. Please, let me know if I'm missing something.

Sensor size has nothing to do with signal-to-noise ratio. Twice the area of the sensor simple means it needs half the time to get the same exposure. You can totally compensate for small sensor size in phone cameras with longer exposure times. On the other hand, some phones have just crappy sensors. Sony's been pretty good about it, however. Xperias usually have quite decent night capture, for phones, at any rate. And like I said above, with the right settings and a steady shot, you can get a picture with some stars visible. And that you can use with stacker to bring out some of the dimmer stars as well. People on this forum have done that. They have posted pictures of stars taken with things like iPhone. I'd go outside and do a proof of concept, but we happen to be under dense cover right now, and doesn't look like it's going to clear out tonight.

If your hands are steady enough, or better yet, if you have a camera stand, you should be able to take a picture of the stars. You'll get much better results with a camera with a large lens, but even on most modern camera phones, you can get a faint picture where you can see some stars. You might have to play with exposure settings a bit, though. It's unlikely that you'll get anything on auto settings. Once you can do that, the next step is to try to take a whole bunch of pictures like that and then use stacking software to put them together into a single image. Several members of this forum have shared pictures they've made this way.

If isotope composition is about the same, then sure, there shouldn't be any problems with using Thorium for NTR fuel. In fact, haven't there been some discussions of Thorium-based NTR projects on this forum? Oxygen is pretty bad for propellant, though. You'll get about 1/4 of the ISP compared to hydrogen. All other things being the same, it drops as inverse square root of molecular mass of the propellant. Oxygen being 16 times heavier gives you that factor of 4 drop. That's still well over 200s, though, so if it's abundant enough, it might still be a cheaper option.

That seems entirely reasonable. I was kind of expecting that we'll need a vacuum chamber test. Since vacuum quality shouldn't be a major issue, I'm sure we can find someone that'd let us use one without too much cost.

Hm. Might work. TWR of NERVA isn't as bad as I thought it'd be. I still think you'll get much better milage out of having a proper chemical first stage, but you might be able to make it into an SSTO with a chemical afterburner instead if you really wanted to. It'd be one big SSTO, though. If it's just a matter of making it recyclable, I'd rather make first stage an entirely chemical space-plane. That stage wouldn't quite make it to orbit with payload, but it'd get the NERVA stage onto high enough trajectory to take over, and then return safely to the ground.

Injecting oxygen is a bad idea. It looks good until you ask yourself why they used hydrogen for propellant and not something that's much easier to store and handle. And the reason is that you get higher ISP at the same chamber temperatures with lighter propellant. To get the same ISP from water, which is what you'll get in exhaust if you start burning hydrogen, you need 3x higher chamber temperatures. And NERVA is already running close to the limit. Even if you were to inject both oxygen and hydrogen from a NERVA-like system, and you happen to have some sort of an impossible to melt chamber to do the afterburn in, you'd still end up with worse performance than NERVA alone.

If Russia's actually getting it funded, you can bet your behind that primary mission is anti-satellite or anti-missile. Though, to be fair, USSR and Russia have been pretty good about dual-purposing their space-tech. So maybe something useful will come out of it.

Yeah, that's probably the best way to go about it. If we are still ok with rad-hard version of 8051 CPU, that draws about 25mA at 5V. That's 125mW. RAM, receiver, and other circuitry will increase the minimum, but we should be able to keep basic "not dead" state in under 1W. If our average power supply drops bellow that, we're screwed. But that can be achieved with just one 5x10 panel at a weird angle to the Sun. So overall, it's pretty safe. This will be enough for the sat to remain in the stand-by mode, waiting for either a signal or timed operation. So what we need overhead for is to reduce time between operations. Transmitter might require 200W+, for example, which will be the biggest drain. But taking pictures and even getting sensor data will also require extra power. And the baud rate will be pretty bad. I'm thinking 50k-100k, which means that even a single JPEG might take a minute to send, which is about a full pass. If we wish to catch it again on the next pass, which will be about 100min later, it will need at least 2W of average overhead to recharge. If we wish to have it try and communicate with several stations along the way, that's even more power. So 20W I'm confident about with fixed panels is enough, but it's a bit less breathing room than I'd like to have. If we don't end up having any other power-hungry systems, I would probably lean to call it sufficient in the name of simplicity. If something else comes up, we'll have to think it through. I don't think I have a working version of that link? I'll probably find it as I catch up, but if somebody can drop it here, it might save some time. Agreed. There is no reason to waste money on cube-specific frames. We can get custom ones machined to tolerances for much less. So far, the only must-have expensive thing we need is solar panels, with rad-hard CPU and RAM being on the short list if we can get the funds. Almost everything else can be either off-the-shelf consumer grade or custom built.

Cube rides up to orbit folded up, and once in orbit, whether it's fragile or not won't matter. And if there is a mechanical failure, worst case scenario, the cube defaults to the same configuration you'd use if you didn't have deployable panels. There is added complexity, but no actual increase in failure modes. On the contrary, you end up with more options to fix any problems that do develop.

Aerospike has nothing to do with fuel you're using. It's just a way to get rid of the nozzle bell. You can use the principle with a conventional rocket, an air augmented one, a ram jet, or even a turbojet. Though, you start getting diminishing returns as your exhaust velocity drops. That said, modern scramjets already have aerospike-like geometry. I don't think one can much improve on that. Now, an air augmented rocket could probably make better use of a proper toroidal aerospike.

Well, if you don't want to risk opening panels, you can at most expose 3 faces at 1/sqrt(3) efficiency, which gives you effective area of 170cm², or 0.017m². At 1.3kW/m² and 20% efficiency, we're looking at 40W maximum. Realistically, it will probably be less, and only available for half of the time. Average power consumption needs to be kept under 10W with this setup. Fortunately, the only seriously power-hungry system is transmitter, and we can burst that from battery. But I still think we should be at least considering fold out panels. We'll need to spin the sat for attitude control either way, so we might as well make panels deployable. That will give us effective area of up to 500cm², more than tripling available power. I'd be much more comfortable with this overhead and added redundancy. Though, I admit, lack of mechanical parts does appeal to me as well.

Ideally, neither. Solid rocket is best built with uniform grains of fuel and oxidizer bound in a polymer matrix. But that's tricky to achieve with home made components. If your components are KNO3 and sugar, your options are, indeed, limited to these two. Candy can give you better performance in theory, but if you happen to have a crack, it will explode. That's why real rockets tend to have polymer matrix. For home-built stuff, powder is much easier to pack without cracks, so you might have more consistency with it. In practice, both will give you pretty low performance and will occasionally explode, so it's really whatever works for you. Keep in mind that there are extra safety concerns for both methods, too. If you are packing powder, read some info on packing black powder for rockets. This should be pretty similar. I would definitely avoid being too close to it. Certainly, don't hold the rocket while packing powder. There's always a chance of explosion. If you are going with candy method, it's even more likely that you'll accidentally set the stuff on fire too early, but at least it's not confined, so you'll avoid explosion. Doesn't mean hot, burning stuff won't fly all over the place. Gloves, goggles, etc. are a good idea in both cases. Just common sense stuff, really.

That isn't strictly wrong, though. An ellipse can be drawn with epicycles, so an epicycles model, with sufficiently complex settings, is perfectly consistent with a non-interacting Solar system. You can even add corrections for perturbations and GR. You simply might need to go to an infinite order in epicycles. But this is just really messy, and perhaps, philosophically unpleasant, but it isn't strictly wrong. There is no contradiction. Energy/momentum not being conserved is a hard contradiction. No matter how small violation, it breaks the foundation. By the way, I should probably clarify, since we've been talking about it long enough. What's actually conserved is the stress energy tensor. It is the conserved charge of the Poincare symmetry group. Energy and momentum conservation for a particular coordinate system follow from that via Killing field.