Spica
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Posts posted by Spica
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On 11/6/2021 at 8:40 PM, Lisias said:
In fact, the Germans had developed a kind of cannon where the projectile was kicked out from the huge barrel by a series os smaller explosions, each one adding up from the last. Pretty interesting, pretty impressive, and this idea can be useful on our thesis:
Unfortunately this trick doesn't actually let you gain significant speed beyond what you'd get from a normal gun.
Once the projectile reaches speeds close to the practical limit of a single chambered gun, any additional chambers provide little if any further velocity increase.
Think of it like this: before the gas from the additional chambers can push on the projectile it must first catch up with the projectile. Once it has accelerated to match the speed of the base of the projectile little energy remains to continue to accelerate the projectile along.
A more technical description is found here: https://www.researchgate.net/publication/268456817_A_comparison_of_distributed_injection_hypervelocity_accelerators
There are other tricks that can work though, which are described in more detail in the paper I linked here. One of which involved injecting gas perpendicular to the barrel and expanding it along a tapered projectile.
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2 hours ago, king of nowhere said:
I mean, you take a fuel and an oxidizer, you mix them together, put in a spark, and they burn. You orient the exhaust gases backward with the nozzle, and it will provide propulsion. Basically, you should need a pipe for the fuel, a pipe for the oxidizer, a combustion chamber and a nozzle.
Well this is basically the textbook case of "the devil is in the details". In order to build a working rocket engine you need to figure out exactly how to get the propellants into the engine, mix them, ignite them, and send them down the nozzle.
Here are just a few of the problems you need to solve to make a working rocket engine:
The pressure in the tanks is lower than the pressure in the engine, how do you make sure the propellants only flow "uphill" from the tank to the combustion chamber?
The gases in the combustion chamber are burning at temperatures of over 3000K, how do you stop the engine (this includes any fuel injectors or ignition system too) from melting or otherwise falling apart?
You need to control it, how? With what?
Remember, the precise shape of every part much be chosen to make it as light at possible, what material is strictly necessary? Where isn't it necessary?
Here's an unofficial cutaway diagram of the raptor engine's systems, each one of these solves some problem mentioned above, along with countless others that aren't mentioned.
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8 hours ago, Beccab said:
Reddit spacex insider:
"S21 nosecone will get the change [of putting the forward flaps 120 degrees apart]. It's an easy iteration. Fwd flaps for S21 already made on S20 design, but that will change too for S22."
Wording is a bit weird, but this means that S21 will have already the flaps moved 120 degrees apart but of the same size as S20; S22+ instead will have flaps of different dimensions too
Edit: for reference, this is how S21 will look like now:
I remember there was some lively discussion about the new flap configuration when it was first announced, but I think it makes the ship look so much more put together.
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There's one big snag with reusable nuclear rockets that I don't seem to see mentioned often enough, especially in discussions of reusable nuclear first stages:
A nuclear reactor that has never fissioned is not going to be radioactive to any worrying extent, but that all changes after it's first turned on.
The more energy it has generated throughout its history, the more radioactive it is. This will make each successive re-use more difficult and dangerous than the last, since a reactor light enough to launch a rocket with will not be heavily shielded. If the reactor is not submerged in the ocean during the recovery process don't expect crews to access the ship without exceeding their legal yearly radiation limits in a matter of seconds.
Here's a diagram of a NTR powered reusable nuclear shuttle concept from the '70s, showing the radiation levels after engine shutdown.
High flight rates are not going to be possible without highly if not fully automated reuse procedures. -
Triton gets about as close to zero as you'll find in the solar system, with an orbital eccentricity around Neptune of 0.000016
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2 hours ago, Spacescifi said:
because my instincts tell me that it should just be steam!
This right here is 96% steam by mass. The thing you see in daily life and call steam is nothing compared to "thermodynamically interesting" steam.
The transparent part of the plume is around 1200 K, and the pale blue Mach disk is around 3,200 K.
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42 minutes ago, YNM said:
Wonder if we can do the opposite way (going upwind with nothing but windpower)...
You absolutely can, all you need to do is attach a rotor that's twisted the other way to the same vehicle.
In that case, the rotor will act like a turbine that slows the air in the car-fixed frame, and uses that power to drive the wheels.
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15 hours ago, Silavite said:
Today I discovered the wonderous program which is CEA (Chemical Equilibrium with Applications),
If your goal is to try out all kinds of propellant combinations, you should also give RPA a look: https://www.rocket-propulsion.com/RPA/download.htm.
The 1.2.9 lite edition is free to download and use.
I've also found it a good bit easier to use than CEA for rocket engine analysis.
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A not completely unreasonable guess would be to assume a burnup threshold beyond which the reactor can not be started, due to a buildup of fission products.
I really have no idea what that usable fraction of fuel energy is, and it will surely depend on the reactor design. Let's use 5% as a value for now.
1 kg of fissile material contains around 80 TJ of nuclear energy within, and with a 5% usable burnup that gives us 4 TJ/kg.
Hydrogen takes around 44 MJ/kg to be heated to 3,000K, a warm but not unreasonable NTR core temperature.
This means that 1 kg of fissile material provides the energy to expel 90,000 kg of hydrogen propellant, that's a lot.This leads me to think that the limit is not one of time, but one of "propellant expended", and it will be fairly long.
But beware the safety hazard that large amounts of long lived fission products create, I wouldn't want to be caught near one unshielded, and the shielding will be thick and heavy (and usually just a puck above the engine, since only the ship that carries it needs to be shielded). -
Conservation of angular momentum prevents you from strictly stopping your own rotation purely with internal processes.
You can redistribute any angular momentum within your own body by whirling your arms or using a gyroscope, but you can't ever get rid of it by yourself.
Of course if you can throw things away from you or react against some external object or field you have more options, like RCS or a magnetorquer.
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Just to give you an idea of the energy scales required for this task:
Accelerating 2,000 tons up to 8,500 km/s with a rocket requires at minimum 1.11*10^20 Joules, or around 25 Gigatons TNT equivalent. This is with a propellant mass ratio of 4.922, and negligible structural (non payload) mass.
The power required to perform this task is also on the order of 140 kilotons per second.
This task can be performed with Uranium as the energy source, but it will require about 1300 of your 1 ton modules, and that's just for the energy requirement, you'll need even more to account for the propellant you need to do this.
If instead for example DT fusion powers this, the "radius of instant death" for unshielded people is on the order of hundreds of kilometers.
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1 hour ago, magnemoe said:
What is I missing.
Starship is not designed to ascend at supersonic speeds under it's own power. I assume it is both unstable and insufficiently controllable in that flight regime without being attached to the superheavy booster.
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I see three issues with silane as a choice of propellant for ISRU on the moon:
1) I assume any engine intended for this role ought to be reusable, and this seems to conflict with the fact that the exhaust of any silane fueled rocket is largely quartz, some of which will be left behind in the engine after shutdown. Solid silicon deposits may also build up in parts of the engine plumbing wherever high temperatures are experienced. The large amount of solids and liquids in the nozzle will also reduce Isp.
2) Hypersonic quartz smoke in the exhaust will be damaging to any nearby structures on the ground and in orbit.
3) You still need large quantities of hydrogen in order to make large quantities of silane.
I'm not sure how important each of these problems would actually be in reality, but they certainly need to be either solved or shown to be insignificant in order to make silane a useful propellant.
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2 hours ago, Spacescifi said:
the front is repelled and therefore flies forward while the rear keeps up the chase.
Try applying Newton's third law to this problem, you'll find that this is impossible for real matter.
Newton's third law and momentum conservation come hand-in-hand, and there's no way you'll be able to violate either, in any situation.
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11 hours ago, wumpus said:
You could adjust the spring profile to get a smoother acceleration profile, but expect it to always want to revert to closer to a sawtooth. Don't forget that on an Orion, some of the efficiency (energy to momentum conversion) scales up as the pusher plate mass increases. Orion can have a reasonably constant acceleration, but I'm sure you'd notice the difference. Might wind up being similar to being at sea (and get more and more stable as larger and larger Orions are built).
I know it runs counter to the "low tech starship" mythos around Orion, but I'm sure modern technology would let you use linear electric motors to smooth out the acceleration far better than any purely mechanical system would let you. I'd imagine the best case wouldn't be all that different from how it feels to drive on a bridge with multiple expansion joints, or ride a train over old rails.
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It would sound basically the same as a normal chemical rocket of the same thrust and dimensions, since the loudest part of a rocket engine is the supersonic exhaust gas jet.
So it would be just as deafening.
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In my opinion at least it seems hard to beat liquid chemical fuels for the role of "extension cord" between a power plant on the ground and an aircraft, at least for high speed or long range use. The power conversion method aboard the aircraft could be a heat engine and generator, fuel cell, or really anything that meets the specific power and efficiency requirements you need.
Although if you're already making hydrogen, methane, or a heavier hydrocarbon from mass and energy on the ground, it seems like a pretty good idea to just burn the fuel in a traditional aero engine rather than converting all its energy into electricity first. Then you can have most of the "environmental cleanliness" (this depends on the source of the electricity of course) of an electric aircraft while still making use of the high energy content and easy handling of hydrocarbon fuels.
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1 hour ago, Lo.M said:
We have to consider the use of batteries of
- Metal-Air
- Silicon–air
Any sort of air-breathing battery has the unfortunate tendency to get heavier as its energy is depleted. This is one sticking point that makes them less useful for aviation than you would immediately suspect given their very high specific energy content.
1 hour ago, sevenperforce said:Electric engines are not limited by thermal efficiency. That's the critical distinction here.
Yes of course, my point was that even with theoretically ideal battery chemistries the product of battery specific energy and electric motor efficiency barely if at all beats current hydrocarbon fueled engines.
Not including the mass of the oxygen picked up by the battery during operation, a perfect Lithium-Air battery can store 40 MJ/kg, on par with hydrocarbons. When you then consider the conversion efficiency from stored energy into propulsive energy the perfect battery wins handily.
The story changes when you include the mass of the oxygen picked up by the battery, which drops the battery specific energy to at most 18 MJ/kg, which is competitive with current jet engines after accounting for conversion losses, and that's for a theoretically perfect battery.
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And unfortunately for electric engines, there simply isn't room for an order of magnitude increase in efficiency over current hydrocarbon fueled jet engines.
It's a fascinating technology, but I'm not sure it fits into any niche not already filled by another type of engine.
In the far future something like this could conceivably be used in a fusion powered jet engine, possibly with magnetohydrodynamic compressors and "turbines".
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And not only are the aerodynamic loads distributed in space, they are distributed in time as well.
The grid fins may be designed to take large loads, but that does not mean they're designed for impact loads. The closest thing to an impact a grid fin should feel in flight is the unstart that happens around Mach 1, which itself is distributed over the fin.
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While I can believe the range numbers stated by Otto aviation are possible, I doubt they will ever be relied on in practice.
Long natural laminar flow runs are really quite delicate. Raindrops, icing, or even a few bugs smashed onto the forward fuselage or wing leading edges could trip the boundary layer, negating any benefits NLF would get you over a large region downstream of the "disturbance".
Other than that, I sure hope there's a toilet on board for longer flights, if the maximum possible range is ever used.
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16 minutes ago, kerbiloid said:
Actually he was doing things which every pupil does in school, just three centuries before, and they call him genius.
Inventing Calculus at age 22 during a plague outbreak that sent him home from school is at least a little more noteworthy than "what every pupil does in school".
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Even better, if you are lagging behind another vessel in your orbit and you want to catch up with it, you need to slow down. The reverse is true as well. If you are leading another vessel and want to "slow down" to rendezvous with it, you need to speed up.
Admittedly any KSP player figures this out after his/her first attempt at orbital rendezvous, but that doesn't change how wrong it feels at first. In fact, this is the very topic that Buzz Aldrin wrote his thesis about.
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Modern IRST (InfraRed Search and Track) systems operate in the mid-infrared. That makes active camouflage very difficult if not impossible to pull off.
Engine exhaust, onboard electronics, and adiabatic compression of the oncoming air will necessarily heat aircraft surfaces well above ambient temperatures, making the aircraft literally glow in the mid-IR.
Counter-illumination can't possibly make an object darker.
1 hour ago, Boyster said:Its a secret technology in testing right in front of our eyes camouflaged in weird conspiracies and deniers, making the perfect smoke screen.
Claiming the DoD "spreads" conspiracy theories to hide technology is itself a conspiracy theory, I don't see how this claim holds any water on its own.
Jules Verne Cannon Payload To Orbit.... Feasible?
in Science & Spaceflight
Posted
Yes, each successive chamber needs to contain significantly hotter gases than the preceding one, but at that point why not make the first chamber larger and hotter. The trouble here is bridging the gap between chemical (thousands of kelvin) and nuclear (hundreds of millions of kelvin) explosions. You could use an electrical discharge instead, but then you ought to check whether an electric light gas gun is more or less efficient on an energy basis than a laser rocket like this:
Such a vehicle would also be FAR gentler to any payload it carries than a gun-launched vehicle would.