Simon_Jester wrote:
Once the liquid hydrogen has boiled, it will rapidly escape from the coolant container you put it in. Alternatively, if you're trying to confine the hydrogen in a fixed container so that it is unable to escape, you will encounter huge, rapidly increasing gas pressure that causes the coolant container to burst open and explode. The large influx of energy associated with a boiling liquid is, by definition, the energy required to take that material from a compact 'liquid' form into a diffuse 'gas' form.
I was only trying to calculate the magnitude of how bad the idea was, not trying to solve it.
I'm sure there are ways to continue heating gaseous hydrogen after the first liquid-to-gas pass. I mentioned some of the problems with this, and with trying to recover or contain the liquid hydrogen, in the first time this was mentioned, so I am not ignoring to the pressurization or other issues .
Review your high school chemistry book on the meaning of "heat of vaporization."
Heat of vaporization is the enthalpy of vaporization, the energy required to boil a liquid, which is 445kJ/kg for liquid hydrogen. Why?
Seriously, when using liquid coolant, after the coolant vaporizes you're done. It will no longer function effectively as a coolant. Either it will explosively blow holes in the coolant pipes, or it will safely boil off and be lost.
I... agree? Yes?
Exactly how does heating hydrogen from 20K to 1000K absorb thirteen times more energy than heating water from 373K to 1000K? You're asserting that it does.
Yes. Liquid hydrogen absorbs 445 (some sources say 461) kJ/kg when boiling. Then, over 14.5kJ/kg/K in gaseous form. Water absorbs 2257kJ/kg when boiling, but only 1-2kJ/kg in vapor form. I calculated using these figures to give a rough estimate of how much coolant is needed.
Also, what effect will tons of extremely hot hydrogen gas have on the interior of whatever cryogenic tank you were once containing an equal quantity of liquid hydrogen? Hydrogen can be rather corrosive, you know.
Yes, which is why it is a bad idea that I did not develop on.
By definition, energy absorbed by the brakes as waste heat is NOT energy that goes into accelerating the payload.
Review basic thermodynamics.
In the basic runway version of the BOT, from the point of view of the station, the payload comes to rest relative to the tether. Therefore it has to remove kinetic energy as heat.
The problem is that the heat isn't being uniformly applied to the tether, it's being applied to the surface. The interaction between the brakes and the tether doesn't just heat the tether; it ablatively melts the outermost layer. Unless Zylon is a much, much better conductor of heat than I expect.
A cylindrical Zylon tether required to survive the braking force of a 10 ton payload at 3G is 8.33mm thick.
If we considered this a problem we had to solve, I would propose a ribbon-shaped tether, which maximizes surface area for both braking and heat transfer. For example, it can be shaped into a rectangle 0.5mm thick and 10.9cm wide.
Heat conductivity in Zylon is hard to find, but I've read it stated as twice as good as Dyneema, which has 60-100W/m-K (
http://users.mrl.illinois.edu/cahill/psu_sep13.pdf
), at least at low temperatures. It can be estimated to be 120 W/m-K for Zylon. At 673K, it should conduct heat at a rate of about 19.4 to 32.3MW/m^2 to its center. The 0.5mm ribbon absorbs at a rate of 323 to 538.6MW/m^2 to its center. 673K is so that Zylon does not lose more than 25% of its strength.
A 4.5m long brake pad on either side will have a surface area in contact with the ribbon-shaped tether of 0.981m^2. It distributes the heat load at a rate of 2.2GW/m^2. To bring it down to survivable levels, it needs brake pads to be more than 37.4m long.
[Yes, but this isn't a problem you're going to be able to duck out of entirely. The total quantities of mass being slung around here are too large; something is going to have to give.
Estimating the loads, the simple runway tether will require ridiculously complex or massive cooling systems and a slightly less ridiculous set of brake-skis. The airbag-brakes version has much lower forces and energies involved.
Like what? Furthermore, such systems are not necessarily going to be more accurate and reliable than a suborbital rocket. They might even be less so.
A railgun launch system would likely produce a very reliable, very predictable trajectory every time. Getting 10 tons to 2km/s requires 20GJ or more, however. Hypersonic launch craft are another option, as are smaller versions of a launch loop or space fountain.
When the payload is launched, the station and payload are many hundreds or thousands of kilometers apart. The payload cannot acquire the station on radar until it has cleared the atmosphere (and the payload fairing is out of the way), and has had time to deploy a radar antenna, AND until the payload and station have line of sight on each other above the curve of the Earth. Given the timescales involved, that may not give them much time to track each other.
Radar on ground stations can track the BOT using longer wavelengths until the rendezvous is closer, from behind the horizon and through 100km or more of atmosphere. If more precise radar is required, another satellite in space can provide tracking from above, using short wavelength radar and telemetry. Light-speed lag is not an issue, and currently available radar power is well adapted to the ranges involved.
q
For final approach, radar will be necessary. But for tracking and ensuring that the payload is going in the right general direction, before it gets close enough for a radar lock with the station, you need a mix of other systems like ground-based radars and GPS.
I agree.
Do you or do you not know the accuracy of these systems?
I know that 1cm wavelength radar is pretty much un-impeded by the atmosphere and two such stations below the rendezvous site can track both the payload and the BOT target to much more accuracy than will ever be needed. Millimeter radar will be needed in space for guiding the drone and hook.
The energies have been achieved, the muzzle velocities haven't. We don't know everything that's going to happen inside a light gas gun that tries to handle 7000 m/s muzzle velocities on a multi-ton payload. We can extrapolate, but that takes a more detailed analysis.
I think there is some confusion here. The railgun/voitenko/ect high speed guns are used to accelerate the 1kg or less heatshield the hook sits on while it is ploughing through the braking gas. The energies involved are very much within our reach.
If the cylinder has matched velocity with the hook, what actually does the work of accelerating the payload?
Seriously, SOMETHING has to exert massive force on the payload, somewhere, at some time. You can't design around that aspect of the system, because that is the entire point of the system in the first place.
I think there is some confusion here. The hook sits on a heatshield for 0.24 seconds, to slow it down enough for an arrestor wire to catch it and push bolts through the holes it has. Once the main tether is bolted to the hook, the main tether/hook/payload tether forms a continuous line that is released and starts the previously described flywheel+pulley train braking process.
This two step system makes sure that the only piece that suffers from 'harsh' forces is the solid lump of hook steel. None of the tether, brakes, flywheel, pulleys or payload have to suffer more than 3-9G depending on altitude.
So which part of the system is it? You're the one designing this and I'm the one trying to figure out which version of your launch system you're visualizing today. But I'm not psychic and time presses, so why don't you tell me which part is undergoing extreme stress? What accelerations are being applied to the payload? What physical object is responsible for exerting the force that imparts this acceleration?
I wrote down a list of the versions that I have come up with to solve each of the issues that were raised, in a previous post. Would you like me to keep track of them on the OP? Here it is again:
The first design was a simple braking wire. It was relatively easy to reach it, but required unavailable cooling methods.
The second design added a flywheel to pull on the tether in the opposite direction. It added the risk of flywheel exploding, but removed the requirement for propellants entirely.
The third design (not posted) had a bow-and-arrow configuration. Intercept is easy, no hypervelocity braking, but the payload's hook would collide with the tether at orbital velocity.
The fourth design is the skip-rotor design. No braking, no propellants, zero relative velocity intercept at the tip of the tether... but intercept precision is too high.
The fifth design I'm looking at now uses a tube of gas to brake only the small hook.
This sixth design considers not using a tube at all, but a series of airbags preceded by puffs of gas.
So to be clear, you expect to impart the desired acceleration by dragging a piston through a series of airbags. Do I have that correct?
Yes.
Again, sooner or later, great force must be exerted on a multi-ton payload. You cannot finesse your way around this, because your system is useless if it does not accelerate massive payloads.
That's the role of the main pulley-train and brakes system.
[Yes, but not a problem that we can solve faster than we can significantly improve the state of material science. By the time it's possible to do what must happen for your tether systems to work, there will be other, better ways to accomplish the same things.
I have been putting a lot of effort into trying to devise solutions that do not require improvements in materials technology.
This is why, for instance, you talk about having maneuvering thrusters that can accelerate the payload at 0.3g. Hint: that is not especially realistic, for a lightweight maneuvering thruster package that can be used for fine precision control. RCS thrusters tend to have very low thrust, precisely because you need to be able to use them to control your velocity to a precision of centimeters per second for high-precision docking maneuvers.
0.3g is an emergency burn that burns through all available deltaV. On the main payload, this can be achieved by a 29.4kN thruster. Six of them would mass about 352kg if use the AJ190-10 like the Space Shuttle.
ASAT weapons also collide with the target at high speeds rather than rendezvousing gently. This is a much simpler problem.
Well, the entire objective behind the BOT is controlled collision, so I do not see why the capabilities of ASATs/ABMs cannot be transferred to larger payload. If a kinetic kill vehicle can collide with a small satellite, then a payload can be guided near the gas bags and the tether.
Oh, sure, if we could predict in advance exactly what the deviations from the ideal flight path were going to crop up, and build a custom engine ahead of time to cancel those deviations out...
...Really? 
I don't see how you arrived to that conclusion. If the miniature solid rockets provide precisely 0.1m/s each, the only custom-building is tailoring the propellant in each successive rocket to be smaller to account for the mass lost after each burn. For example, the first solid rocket would be 10 times longer than the final rocket, so that they each provide a precise amount of deltaV per burn. I believe a modern smartphone can handle the software required to control and predict the sequence of burns required to achieve a specific velocity on 2 axes..
I'm going to be honest, your idea is now sufficiently complex that I no longer believe this can be implemented. I can't trust your math because you routinely ignore important aspects of physical systems, completely by accident and without realizing that you have done so.
Is complexity a good estimate for whether something works? An example I used before is the F1 car's 80000 parts. It is complex, but racers routinely trust their lives to it at 300km/h.
If I had to break down the current design, I'd list these components:
Payload end:
-Rocket booster
-Payload
-Payload safety and abort systems
-Payload RCS
-Payload radar
-Payload tether
-Tether drone
-Payload hook
Tether end:
-Gas release tanks
-Airbags
-Arrestor wire
-Bolt insertor and release mechanism
-Segmented Tether
-Inter-segment tether connections
-Pulleys
-Pulley brakes
-Pulley suspension
-Flywheel
-Flywheel brakes
-Flywheel/tether connection
-Electric engine
-Solar panels
-Off-axis flywheels for maneuvering
-Station Radar
When real science and engineering people talk, they usually start from the assumption that they can trust each other's math. They still cross-check, but they can at least assume that there aren't glaring mistakes in literally every calculation.
I think you'll have to back that statement up. I don't believe I'm making wild mistakes in literally every calculation.
The problem is that you've overlooked enough important things and made enough errors that I can no longer make that assumption. I can't trust you to know what you're talking about, even about basic simple things, until I've run the math myself.
Many of these 'overlooked' problems seem to result from a lack of communication, such as your confusion on whether the hook or the payload was being braked by the airbags.
Which means that if you spend hours calculating something, I can't spend hours verifying it; I haven't got the time.
I am grateful for the time you have already given. For each hour of maths, I spend two hours repeating my calculations under different assumptions and three hours doing research.
This is, again, why I keep pushing you to spot more of your own mistakes. Outsourcing literally all error-checking onto others places undue demands on our time and makes us disinclined to examing your proposals in detail.
I'm going to be quite honest, the next time I see you make a fundamental mistake based out of ignorance of basic scientific facts (like "heat applied to a rope by brakes hits the outside of the rope first" or "oh I forgot I need umpty tons of propellant/coolant lol"), I'm just going to give up on you entirely. I like the idea of trying to help you learn this stuff in the abstract, but I can't take the combined responsibility of being your K-12 science teacher on every one of a dozen different topics.
I was not formally taught to remember check thermal conductivity of a material under braking forces. I don't have the instincts that tell me that a tapering length is the solution to the maximum velocity of a rotating tether. These are things I research and come up with on my own, in my free time, so please recognize that I am trying my best, and no, they are not taught in the american K-12 science classroom.
There is a certain "you must be this tall to enter" level of background knowledge required to talk intelligently about systems like this, and that's the case for a good reason.
I'm sorry you feel this way, but I would have liked this to be stated from the beginning before 'one more mistake and I'm out' ultimatums are given.
Statistics: Posted by matterbeam — 2017-02-03 08:29pm