Steel
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Posts posted by Steel
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8 hours ago, 0111narwhalz said:
What are the scaling laws for rocket nozzles? Particularly, to which variable to they scale? Only sensible candidates I can think of are ṁ, F, or maybe Fp.
You'll have to define your variables to stop everyone guessing. I'm assuming mass flow rate, thrust and something to do with pressure?
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3 minutes ago, Brotoro said:
I suspect that the Moon would hold an atmosphere for several thousands of years.
Without a magnetosphere it would be blasted away pretty quickly, though since it's likely that it never really had one then it's very difficult to say. Technically, the moon still has a very tenuous atmosphere, so I guess it also depends on where you draw the line.
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15 minutes ago, raxo2222 said:
Well for both gasses at standard conditions average velocity is few hundreds m/s and Moons escape velocity is 2400 m/s
I got atmosphere height (altitide where atmosphere has half of pressure) at 64 km for moon.
Ok, I was wrong on that one then. The average velocity would be below escape, but a significant proportion of the high velocity tail of the energy distribution would still be above it. There's also the non-thermal mechanisms such as the solar wind to contend with too.
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As @Green Baron mentioned, the average molecular velocity would be way higher than escape velocity, so it would go pretty rapidly (though how the atmosphere would get there in the first place is an interesting question)
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34 minutes ago, sevenperforce said:
Lorentz factor is great if you are trying to quantify how much quantum-mechanical weirdness is going on as a result of your velocity. It works well in all reference frames.
Rapidity is great if you are trying to quantify how the observer in the reference frame of interest is going to experience the rate of their transit. It works best for the reference frame that has been accelerated to a high velocity.
As long as by "quantum-mechanical" you mean "relativistic", then yeah I agree
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5 hours ago, cratercracker said:
Sorry, wanted to update the OP and post a new mission to Venus, but my image sharin websites are going crazy for absolutly no reason.
If it's a completely different mission architecture and not just an update of the original, I'd start a new thread anyway to avoid confusion
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52 minutes ago, Northstar1989 said:
They could reinforce the nozzle if the stresses on it are the only conceen. Maybe shorten it a bit (and slightly enlarge the fuel-tanks) if the structural reinforcement makes the extra expansion no longer worth the extra mass...
Stresses are the main concern, but since the nozzle extension alone for the M1DVac is about the same size as an entire M1D system- and has an exit area about 7 times larger - drag and aerodynamic effects are also a big concern. (see image below for reference)
Spoiler
Here you have a man with an M1D to his right (our left) and a M1DVac without extension (pictured below) to his left (our right)
Shortening it doesn't really work too well, you'll lose so much ISP that you'll end up having to carry a lot more fuel. Obviously the worst case is if you get rid of the whole extension, which adds around 40 s of ISP, so you'd need to carry around 15% more fuel to compensate. This subsequently means the first stage won't push the second stage as far or fast, which in turn means you have to carry even more fuel e.t.c.
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Isn't this argument a bit cyclical?
For this to be worthwhile, the recovery spacecraft and it's launcher must be fully reusable (otherwise you need a recovery spacecraft to recover the second stage of the recovery spacecraftcraft and so on forever). Surely though, if we have the capability to create a fully reusable recovery craft, then you could just made slight changes to it and turn it into a fully recoverable launcher and then eliminate the need for a recovery craft in the first place?
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20 minutes ago, NSEP said:
Im not dure if its 100% precise though.
It's pretty good, since all imperial units are now defined as an exact number of metric units
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1 hour ago, ChrisSpace said:
In Isaac Arthur's "Dead Aliens" video, he described a scenario in which multiple alien individuals intentionally froze themselves in a crater of permanent darkness so their bodies could be brought back to life in the distant future. In the scenario, humans showed up 2-4 million years later, but given the circumstances described in the video, what's the longest the bodies could've stayed sufficiently intact?
Also, I once heard that one possible solution to the Fermi Paradox is that aliens could be contacting Earth, but their thoughts and actions are so slow that it takes months for them to say a single word, and as such it's indistinguishable from background noise. While I highly doubt this is common enough to be relevant to the FP, or that it would take MONTHS to hold a single conversation, I'm still wondering: What's the slowest speed at which thoughts and actions can occur in a complex biological organism? One-quarter of a human's? One-tenth? One-thousandth?
1) Hard to say, since these are alien bodies (and thus could/will work completely differently to our bodies). If they are very close to humans then they're stuffed (for want of a better phrase), since we currently have no way of re-animating frozen human bodies unless they have been incredibly carefully looked after with life-support. If they're nothing like humans then ther'es no way to answer the question, since we've never seen any advanced beings that are not like humans.
2) There's no biological limit that we're aware of. The sort of life you're talking about here is so unlike anything we've ever encountered that we simply have no data to answer that question I'm afraid.
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4 hours ago, RoadRunnerAerospace said:
Engine blend done!
Wow, this thread really took off too!
Have you done the calculations to get the throat area, chamber volume e.t.c to roughly the correct size?
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8 minutes ago, Grand Ship Builder said:
In the long run, yes. On the short term, no.
There's absolutely no guarantee that it will ever be cheaper (and equally no guarantee that it won't be cheaper). There's no way to predict it because nothing like this has ever been done before.
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2 hours ago, sevenperforce said:
Do you mean to pump in compressed air? I would look at NO2 or H2O2 if I were you.
How are you going to pump in the fuel?
If you plan to use air (probably not the best plan, since its only 21% oxygen) you'd need a compressor set up to actually get the required flow rate into the chamber.
Gaseous oxygen is not too hard to get hold of, and would serve the purposes of a rocket engine far better. Also, there are existing examples of people who have done DIY GOX rockets to help you along.
Also, if you haven't seen this thread, I'd highly recommend checking it out, it's pretty extensive on calculations and such.
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7 hours ago, Spaced Out said:
What are the equations for drag loss and gravity loss? Plus, does the weight of a rocket on wikipedia include the upper stages?
Unfortunately there aren't any nice analytical equations for those things. You end up with differential equations, which you (or a computer) would have to integrate at small intervals along your ascent trajectory in order to get the answer.
I'm assuming you're talking about the rocket equation? In that case there are two masses, the full mass (including any additional stages) and the empty mass (i.e the mass without propellant)
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5 hours ago, NSEP said:
If a something explodes in a vacuum like a spaceship, could you still hear the gasses released from the explosion?
Unless the amount of gas is large enough that it can effectively transmit sound to you, then no.
21 minutes ago, Spaced Out said:Do spent boosters re-entering the atmosphere burn up completely?
No. Depending on the amount of potentially explosive stuff onboard it may reduce into to very small pieces, but most stuff that boosters are made of do not combust, so there is no way that they can burn up completely. Most boosters will break up into small pieces.
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14 minutes ago, WildLynx said:
It's how it probably works:
Amateurs -> ATC : Can we have please a safe launch window next week?
ATC-> Aircraft transport companies : Could you bump / cancel a few flights so Amateurs can launch their rocket?
Aircraft transport companies -> ATC : No, no, it's millions of dollars, here, have those few thousands $$$ as gift.
ATC->Amateurs: No, you cannot launch your rocket next week, next month or whatever!!!
Amateurs need to "befriend" few politicians to push a law that gives Amateurs right to demand a launch window few times a year.
You are implying that ATC and air transport companies are involved in widespread bribery and corruption, so I'd be slightly careful
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29 minutes ago, Spaced Out said:
That answer is good enough for me. Next question. Is it possible that any parts of the rocket equation are wrong?
There aren't many "parts" to the rocket equation, it is very simple as far as rocket science goes and you can derive it directly from conservation of momentum. Unless we have the laws of physics completely wrong, it should be pretty correct!
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5 minutes ago, Brotoro said:
I continue to be unsatisfied. But thank you for taking the time to discuss it. I enjoyed that interaction. Have a Like.
I don't blame you. After all, Einstein wasn't particularity satisfied with quantum mechanics! It's unintuitive, complicated and difficult to explain; even with a physics degree I've barely scratched the surface.
No problem, I enjoy talking about his stuff with people. Also, by trying to answer your questions, you've helped me clear up some things in my own head that I didn't completely get before, so thanks!
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Just now, Brotoro said:
If the fermions are not interacting... how does the second fermion know the first one is alreay in that primo parking location?
It doesn't. The point i'm trying to make is that in quantum systems, the particles don't behave as individual particles, the system acts as a whole.
The whole system has a possible set of outcomes based on its current set up, external forces e.t.c, and each outcome has a certain probability of occurring. When you leave the system alone and come back to observe it, it will be in one of these outcomes. There are no interactions between members of the system in the same way as you get in classical systems. The PEP essentially means that in these systems, it will never be possible (i.e the probability of it occurring is exactly zero) for there to be a system outcome where two fermions are in the same quantum state.
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8 minutes ago, Brotoro said:
I don't see how you can say there us no interaction going on between particles. If just two fermions try to inhabit the same location with all of their other quantum states being the same, it will not happen because the rules of quantum mechanics prohibit it. One electron could sit there just fine. The other electron couls sit there just fine. But try to put bith there at the same time and it will not happen. How does this not involve an interaction between these two electrons?
Believe me, I wish this were simpler!
Essentially, the second electron cannot occupy that filled state because the wavefunction of the system (basically a bit of maths that determines the probabilities of each possible outcome of the system, given its current conditions) literally does not allow another electron to exist there. You may have heard that quantum systems behave probabilistically, well the PEP basically means that there is zero chance (that is exactly zero, not negligible, not incomprehensibly small, literally zero) that the second electron will be found in that same state, it can only ever be found in another state. The particles are not interacting to tell each other anything, its just that the system as a whole has a zero chance (a statistical impossibility if you will) that you will see the second electron in that state.
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11 minutes ago, Brotoro said:
You see, it's the "That effect that is counterbalancing the force of gravity is not a 'force'..." song and dance that seems to be sidestepping the issue. Particles are trying to move inward under the force of gravity. Interactions with other particles are preventing that from happening. Just saying "it's not a force' doesn't change the fact that particles are interacting with measurable effects without it being caused by any of the four fundemental interactions that can occur between particles.
Unsatisfying.
It's not sidestepping the issue, I'm just trying to explain it to you without lying (a la popular science) about what is actually happening (or at least what we think is happening).
What you're saying about this effect being caused by interactions of particles with other particles is what is wrong with your statement. In a quantum system, you do not have distinct particle interactions. The system behaves as a whole, with the outcome of events determined by the state of each particle within it. It's a subtle difference, and one that is near impossible to get across to someone who hasn't studied the maths behind quantum mechanics which determines how the whole thing behaves.
The long and the short of it is that degeneracy pressure is not caused by a "force" due to interactions between the particles, its is caused by the intrinsic behavior of the degenerate system as a whole. Maybe think of a white dwarf or neutron star not as a collection of electrons or neutrons, instead try to see it a one huge quantum blob. This "blob" has intrinsic behaviors that are totally different to that of a collection of normal matter and largely unrelated to the classical behavior of the stuff its made up of.
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31 minutes ago, Brotoro said:
Of course, atoms repel each other because of the electromagnetic force between their electrons. But I think the reason mikegarrison says that it involves the Pauli Exclusion Principle is because you then have to answer the question "but what is keeping the electrons in place?". Or, skip atoms completely and go to the question of degenerate electron pressure in a white dwarf or degenerate neutron pressure in a neutron star. The force of gravity is acting inward to try to collapse these objects, but they are not collapsing...so some outward force must be acting to counterbalance gravity. This force cannot be the result of the electromagnetic force because it operates for any fermions, even uncharged fermions. That quite real outward force is NOT the result of any of the four fundemental 'forces', but instead results from the PEP.
It only appears to be a force from a layman perspective, it is not actually a force.
In objects with densities as high as in white dwarves, the collection of matter that makes up the object basically ceases to behave as a group of distinct particles in continuum and becomes one large quantum system, which we call degenerate matter. In a quantum system particles may only have certain energies corresponding to certain quantum states. One property of fermions in a quantum system is that they cannot share these quantum states, thus to add another fermion into the same volume, it must occupy a different state with a higher energy. This means that compressing degenerate matter requires energy, and this behavior manifests as degeneracy pressure that opposes gravity, but isn't a force.
This is one of those things that makes you suddenly realise just how weird quantum mechanics is, when a quantum system can resist gravity without an opposing force.
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9 minutes ago, Physics Student said:
The question I have is: Does Dark Matter actually exist or do we just have to figure out gravity a little better? Has anyone ever seen dark matter anyways?
There is reasonably good evidence that it exists in some form [1], if we assume that GR is somewhere close to full description of gravity. Otherwise GR is wrong (or at least incomplete) and we have very little understanding of how gravity truely works.
Assuming one of the current theories of it is correct, no one has ever seen it and no one ever will. This is because, by it's definition, it does not interact with light and so cannot be seen or detected using electromagnetic radiation.
[1] https://en.wikipedia.org/wiki/Dark_matter#Observational_evidence (I know it's wikipedia, but it's good enough for the purposes of this discussion)
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3 hours ago, mikegarrison said:
Gravity pulls us toward the center of the Earth. What (obviously stronger) force keeps us from falling there? What creates the resistance that keeps one solid object from moving through another? The standard answer seems to be "the Pauli Exclusion Principle", but if everything comes down to four forces, which one is involved in that? Is that electromagnetism?
16 minutes ago, Brotoro said:Now THAT is an interesting question, for which I’ve never received a satisfying answer (although I asked about it in the context of degenerate pressure in white dwarfs and neutron stars).
The force that keeps us from falling into the centre of the Earth is just the mutual electromagnetic repulsion of the electrons in the atoms, not the PEP. The PEP only kicks in for super dense objects like white dwarves.
The PEP itself is not a force and nor does it act like one, it's just the slightly hand-wavy popular-science analogies that make us think it is a force. The PEP is actually a fundamental behavior of fermions in quantum systems. This thread explains quite well.
For Questions That Don't Merit Their Own Thread
in Science & Spaceflight
Posted · Edited by Steel
Thanks! Rocket nozzles aren't quite that simple unfortunately.
Generally you can get most of the potential thrust from a short nozzle with a well designed bell contour, however increasing the length can also improve your thrust slightly. The size of this increase is dependent on how good the nozzle is, so generally you'll see a larger effect for simple nozzle (i.e conical) than for more complex ones.
However, the biggest thing that changes with nozzle length is ISP, since a longer nozzle has a larger exit area, thus a larger throat-to-exit area ratio and thus a faster exhaust velocity.
Mass flow rate is fixed for any given combination of chamber pressure, throat area and outside pressure, so the nozzle has no effect.
Basically, it scales with exhaust velocity, which is related to thrust and ISP. It also differs depending on the design of the nozzle, but that's another story.