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31
Advanced Concepts / Re: Using shaped charges as a rocket engine
« Last post by lamontagne on Today at 05:43 pm »
TNT has an energy density of 4.184 MJ / kg. Explosives apparently exist with up to 2.38 times greater energy density than TNT (https://en.wikipedia.org/wiki/TNT_equivalent), but those explosives have not been synthesized in any quantity, probably for good reasons, so I'll use 1.90, the highest value for an explosive that seems like someone actually seriously considered using. With perfect efficiency the best possible exhaust velocity is sqrt(2 * 1.90 * 4.184 MJ / kg) = 3,987 m/s. That's not bad, but that assumes 100% efficiency, i.e. that the products all leave the rocket in the desired direction at the same speed. I don't know how efficient shaped charges can be but my hunch is you'd get less than half of this, probably much less, which makes this idea not competitive with traditional rocket propellants. Two things are hurting this idea: the energy density of explosives isn't as good as for bipropellants, and the efficiency of shaped charges is probably much worse than a nozzle's efficiency.

Something similar has been suggested using nuclear bombs: https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion) .
Shaped charges typically use explosives with an energy density of 9 MJ/kg, which is comparable to methalox. But the most important thing is the gigantic pressure of hundreds of thousands of bars during the formation of a cumulative jet, which allows it to be accelerated to a speed of 10 km/s or more. For comparison, in the combustion chamber of the Raptor engine, the pressure is "only" 300 bar, and the jet velocity is 3.5 km / s, and this is an incredible achievement - it is not possible to significantly increase this value in a classic rocket engine.
A nozzle in a rocket is needed for only one task - the formation of a directed high-speed jet. In a shaped charge, this jet is formed by the charge itself, so no nozzle is needed.
To form a jet, a shaped charge is accelerating a tamper, not the charge itself*. That tamper is not part of the combustion process, so is pure dead reaction mass that reduces your effective MJ/kg (e.g. if you have 1kg of explosive accelerating a 1kg tamper into a EFP, then your energy density if halved).

Then you have the problem of harvesting that energy for propulsion. A single shaped charge floating in space may be able to fling a het of hot copper to a few km/s. but there's no vehicle that accelerates in the opposite direction in that scenario. You'd need to contain the explosion somehow and capture the gasses that are produces as a result of the explosion in order to use them to push a vehicle forwards. The problem there is that whilst a shaped charge is good at accelerating a tamper in one direction, it's no so good at accelerating the combustion products in one direction: instead, they expand mostly radially: you do not get the equivalent of a lump of gas travelling at the opposite speed to the tamper jet.
We don't need a metal liner! We are not going to penetrate the armor of the tank, all we need is a high-speed jet directed in the direction we need. In a conventional rocket engine, this jet is formed and accelerated by the Laval nozzle, in our case, this jet is formed and accelerated by the charge itself due to the conical recess.
You can compare this with an ion engine where the Laval nozzle is also missing due to the fact that the flow of ionized gas is already accelerated and directed in the right direction.
Even if the gas flow velocity is uneven (which is not a fact in the absence of a liner), then the average velocity value (and hence the specific impulse value) will still be higher than that of any chemical rocket engine.
As far as I can see you have rediscovered solid fuel boosters. 
32
Advanced Concepts / Re: Medusa concept using chemical explosives
« Last post by edzieba on Today at 05:40 pm »

For example, we have a substance with an energy density of 10 MJ per kg. We can release this energy in a second and get a temperature of 3000 degrees, or we can release this energy in an hour and not even get warm. And hypothetically, we can release this energy in a nanosecond and get millions of degrees. In all three cases, the energy density is the same, but the rate of its release is different. Speed is what matters, which is why the temperature in the center of a nuclear explosion is so high, because the detonation velocity is thousands of times higher than that of any chemical explosive, and not at all because of the energy density.

This is wrong on so many levels.

First, say you have a kg of fuel with 10 MJ of energy. Burn it in any device you wish to produce a jet of gas. The kinetic energy of that gas cannot exceed 10 MJ otherwise you have violated conservation of energy and can make a perpetual motion device. One kg of gas traveling at 4472.13 m/s has just about exactly 10 MJ of energy. It is simple math. Nothing you do can get a faster exhaust velocity.

Second, lets look at the temperature. A hot gas contains more energy than a cold gas. If you burn a kg of fuel producing 10 MJ of energy then the hot gas has exactly 10 MJ of heat more than the fuel you started with. That exactly defines the maximum temperature that gas can reach.  If you exceed that temperature then you have violated conservation of energy and can make a perpetual motion device.

Now lets look at chemicals. Say you have a kilogram of hydrogen/oxygen mix. You say that if we burn that fast enough then we could have temperatures in the millions of degrees. But a kg of water heated to millions of degrees has far more energy than you can ever get from the kg of hydrogen/oxygen. Again a perpetual motion device exactly as stated above. But worse than that at any temperature above 2182 C the water will start to disassociate back into hydrogen and oxygen absorbing heat from the gas and so cooling it. That gives us the maximum temperature we can ever get from burning hydrogen. Millions of degrees would dissociate all of the water and reduce the gas to a plasma.
Why did you decide that 1 kg of water is subject to heating to millions of degrees?
Water is the combustion product of Hydrogen and Oxygen.
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The greater the specific impulse, the lower the mass of exhaust gases for a given amount of energy spent on heating.
No. Exhaust mass will not vary (if you throw 1kg of propellant out the back 1kg of propellant goes out the back as exhaust, no matter how its combusted.
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In other words, using a limited energy source of 10 MJ/kg for fast heat transfer to the reaction mass, we will increase the specific impulse, but reduce the ejected mass. There is no violation of the laws of physics here.
No, magically losing mass is indeed an egregious violation of conservation of mass. You burn 1kg of propellant, you get 1kg of reaction products.
Fuel and reaction mass are two different things! In chemical rocket engines, this is almost the same thing, which is misleading people. But for example in ion and nuclear thermal engine these things are clearly separated.
Irrelevant for an 'explosive driven' engine: either the explosive reaction products are the remass (in which case the situation is identical to monoprop or biprop chemical rockets) or you're trying to heat some other remass (which which case you have a catastrophically inefficient thermal rocket).
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Ask why engineers are so proud of the amount of pressure in the combustion chamber? This does not have a significant effect on the TWR (the Merlin has a better indicator than the RD-180), and the smaller effect of backpressure in atmosphere harms the rocket as a whole rather than helps (since it increases gravitational losses).
False. Chamber pressure is proportional to TWR, for a given engine.
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The only reason to increase this parameter is to accelerate the processes of heat transfer in the combustion chamber from the fuel (for example, Methalox) to the reaction mass (water plus CO2), which leads to a higher temperature and, accordingly, to a higher exhaust gas velocity (specific impulse improves).
False. Chamber temperature is what is most strongly coupled to exhaust temperature (and this ISP), which is why the Raptor with it's stupendous 300 Bar combustion chamber pressure has a vacuum ISP of ~363s, whereas the RL-10's puny 24 bar chamber pressure still beats it up and down the street with an ISP of 465s.
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There is no magic here, the faster the transfer of heat from the energy source to the reaction mass, the faster the speed of the outflow of gases from the engines and the lower the mass flow.
False. 'Speed of heating' has nothing whatsoever to do with either thrust or ISP. About the only thing it affects is physical length of the combustion chamber.
33
In regards to the general build out of an ISRU plant, could you not containerise (think shipping) the equipment with a series of "basic" connectors on the outside in a standardised location/format. Then you are talking about moving a standard sized object, easy enough to solve (Look to autonomous ports). For connecting the units together, look at something like Boston Dynamics Spot and its Arm. Have hoses for connecting the units be in retractable housings like garden hose for ease of use.
Would love to see a contest or a test done on this subject here on Earth, on a non prepared surface:

-Offload containers remotely, perhaps provide them with self leveling systems.
-Place them at the required distance from one another.
-Unpack the connectors.
-Attach the connectors from one shipping container to the next.
-Force a time limit of 15 minutes between each command from 'Earth'.

-Unpack solar power systems
-Connect-up and start-up power.
-Test power and controls

It would be necessary to check all of this anyway, before hand. 

Perhaps Spacex will issue out a tender and let us watch the tests?  Or DARPA might make a equivalent field test for military equipment?

34
Advanced Concepts / Re: Using shaped charges as a rocket engine
« Last post by edzieba on Today at 05:34 pm »
TNT has an energy density of 4.184 MJ / kg. Explosives apparently exist with up to 2.38 times greater energy density than TNT (https://en.wikipedia.org/wiki/TNT_equivalent), but those explosives have not been synthesized in any quantity, probably for good reasons, so I'll use 1.90, the highest value for an explosive that seems like someone actually seriously considered using. With perfect efficiency the best possible exhaust velocity is sqrt(2 * 1.90 * 4.184 MJ / kg) = 3,987 m/s. That's not bad, but that assumes 100% efficiency, i.e. that the products all leave the rocket in the desired direction at the same speed. I don't know how efficient shaped charges can be but my hunch is you'd get less than half of this, probably much less, which makes this idea not competitive with traditional rocket propellants. Two things are hurting this idea: the energy density of explosives isn't as good as for bipropellants, and the efficiency of shaped charges is probably much worse than a nozzle's efficiency.

Something similar has been suggested using nuclear bombs: https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion) .
Shaped charges typically use explosives with an energy density of 9 MJ/kg, which is comparable to methalox. But the most important thing is the gigantic pressure of hundreds of thousands of bars during the formation of a cumulative jet, which allows it to be accelerated to a speed of 10 km/s or more. For comparison, in the combustion chamber of the Raptor engine, the pressure is "only" 300 bar, and the jet velocity is 3.5 km / s, and this is an incredible achievement - it is not possible to significantly increase this value in a classic rocket engine.
A nozzle in a rocket is needed for only one task - the formation of a directed high-speed jet. In a shaped charge, this jet is formed by the charge itself, so no nozzle is needed.
To form a jet, a shaped charge is accelerating a tamper, not the charge itself*. That tamper is not part of the combustion process, so is pure dead reaction mass that reduces your effective MJ/kg (e.g. if you have 1kg of explosive accelerating a 1kg tamper into a EFP, then your energy density if halved).

Then you have the problem of harvesting that energy for propulsion. A single shaped charge floating in space may be able to fling a het of hot copper to a few km/s. but there's no vehicle that accelerates in the opposite direction in that scenario. You'd need to contain the explosion somehow and capture the gasses that are produces as a result of the explosion in order to use them to push a vehicle forwards. The problem there is that whilst a shaped charge is good at accelerating a tamper in one direction, it's no so good at accelerating the combustion products in one direction: instead, they expand mostly radially: you do not get the equivalent of a lump of gas travelling at the opposite speed to the tamper jet.
We don't need a metal liner! We are not going to penetrate the armor of the tank, all we need is a high-speed jet directed in the direction we need.
The liner forms the jet. No liner, no jet.
35
Chinese Launchers / Re: Chinese launch schedule
« Last post by FutureSpaceTourist on Today at 05:26 pm »
https://twitter.com/cnspaceflight/status/1599830466869178368

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There's a chance we could see 3 launches from China on December 10
~0830UTC Zhuque-2 of LANDSPACE from Jiuquan
~1830UTC CZ2D from Taiyuan
~2200UTC CZ11 from Xichang
In addition:
~0110UTC DEC.07 Kuaizhou-11 from JSLC
~0500UTC DEC.09 SD-3 from Yellow Sea
NET DEC.30 CZ3B from XSLC
36
Do we know why B7 is being returned to build site?
37
Moreover, launch is the safer part and it's already well understood in general. The seats are the most needed for descent and landing, and there seats operational conditions are even more airplane-like. They'd have the easiest job in the most risky part of the flight which is good as it maximizes pLOC reduction.

I don't understand. The riskiest part of EDL is, I thought, the point near max heating which is at high altitude and hypersonic Mach number.
I think it's generally understood that there are no abort options during this regime.

Oh, okay, so the idea is that abort is needed most during landing because we simply can't do anything about entry.
There are lots of things that can be done to improve the survivability of off-nominal reentry, such as making your structure out of a high temperature metal like stainless steel instead of aluminum (or conventional composites).

If the stainless could handing a NOMINAL entry, they wouldn't need tiles, to say nothing of an off-nominal entry, which is what would take place after a burnthrough.

Nope.

There is such thing as material annealing. The steel the ship is made from was hardened (it's cold rolled -- it's a type of work hardening; 300 series stainless hardens very well and its yield strength gets increased a few times). Stainless starts to slowly lose this strength above 700K and does so practically immediately above 1200K. But it's not melting point. 304 stainless steel melts at 1570K to 1630K. Shuttle tiles were to withstand 1530K. As far as we know Starship tiles are the same.


The vehicle needs its full strength primarily on ascent when its tanks are pressurized to 6bar while it faces ~35kPa max-q loads or 3.5g late booster burn load, all the while filled with 1200t of ascent propellant (so for example its skirt has to handle about 5000t load; 3.5g * ~1400t). But during EDL the vehicle is an order of magnitude lighter, dynamic loads are ~20kPa, tanks don't need high pressurization and likely are pressurized as little as possible because ullage gas has non-trivial mass. Suddenly skin doesn't have to survive stress of a 6bar, when the pressure is 2bar. Your skin structural margin increased from 40% to over 400%. The thing could become 4x weaker and it would still hold.


But after such overheating the vehicle would be a write-off (unless the overheating affected only a small patch, then a repair is an option). 300 series remains annealed after it's annealed, it doesn't heal (it doesn't age harden appreciably). You need to work harden it again (this is one of the reasons SpaceX had some initial trouble with popping tanks: weld's anneal the base material; they implemented a better controlled welding process and they also planish many welds which restores some of the strength, and they use weld doublers where fixing up seams is not feasible).


I don't buy this.

The RCC was for entry temperatures *above* 1530K.  You're equating that with a *melting* temperature in the same range.  These are NOT the same thing.  A material just below its melting point has lost most of its strength and will fail soon, if not immediately.  At 1000C 304 is 8 times weaker (lower yield stress) than at 600C.

We're talking about what to do in an off-nominal entry.  Off-nominal likely means damage (tiles or other) or loss of control.  I suspect in either case the assumption that there's nothing you can do is actually correct.  Damage will likely lead to burn through from loss of strength or full blown melting and that's worse on this vehicle than on Shuttle simply because burn through is on a pressurized tank you have to have to land safely.  Loss of control is probably worse.  So I seriously doubt that the intrinsic design of this vehicle makes it more robust against off-nominal entry conditions, and I certainly don't know of an abort option for that situation.
The idea is that when the heat shield is partly compromised, an underlying steel structure will last a lot longer than an underlying aluminum one.

The temperature at the structure will not be that of the outer skin, so the ability to withstand elevated temperatures really matters.

Similarly if for whatever reason the peak temperature reached is higher than planned, temperatures will rise on the inner side of tge heat shield, and again Stainless will perform much better than Aluminum.

38
Edit to add: mis-attributed, likely for this launch

Based on the times, they're all OneWeb.

OneWeb:

Primary Day = Tuesday, December 6 at ~22:37 UTC (~17:37 EST) (convert time).
Backup Day #1 = Wednesday, December 7 at ~22:32 UTC (~17:32 EST) (convert time).
Backup Day #2 = Thursday, December 8 at ~22:27 UTC (~17:27 EST) (convert time).
Backup Day #3 = Friday, December 9 at ~22:23 UTC (~17:23 EST) (convert time).
Backup Day #4 = Saturday, December 10 at ~22:19 UTC (~17:19 EST) (convert time).
Backup Day #5 = Sunday, December 11 at ~22:14 UTC (~17:14 EST) (convert time).
Backup Day #6 = Monday, December 12 at ~22:10 UTC (~17:10 EST) (convert time).

HAKUTO-R:

Primary Day = Wednesday, December 7 at ~08:04 UTC (~03:04 EST) (convert time).
Backup Day #1 = Thursday, December 8 at ~07:56 UTC (~02:56 EST) (convert time).
Backup Day #2 = Friday, December 9 at ~07:49 UTC (~02:49 EST) (convert time).
Backup Day #3 = Saturday, December 10 at ~07:44 UTC (~02:44 EST) (convert time).
Backup Day #4 = Sunday, December 11 at ~07:38 UTC (~02:38 EST) (convert time).
Backup Day #5 = Monday, December 12 at ~07:31 UTC (~02:31 EST) (convert time).
Backup Day #6 = Tuesday, December 13 at ~07:25 UTC (~02:25 EST) (convert time).
39
SpaceX General Section / Re: Starlink : Speed Discussion
« Last post by Mandella on Today at 05:13 pm »
My own experience with Starlink is that average speed ratings are pretty useless. Speedchecks can vary wildly minute to minute, and of course from peak to off peak. So far, it has certainly not dropped below what I was provided as routine by Viasat even at the worst of peak. And of course nighttime downloads can only take an hour or so to bring down files larger than my total bandwidth cap under Viasat.

Connectivity and lag are more useful metrics for me for daily use, and in those areas reliability has been improving for me in the US Southeast, and it wasn't terrible to begin with. Momentary obstructions or loss of signal are measured in seconds, not even noticeable when surfing or streaming, and tend to only occur hours apart. Even online gaming just registers these as lag spikes, rarely a disconnect. I've even hosted games on my home system and got no complaints from other players that did not know they were accessing a host on the other side of a satellite link.
40
One obvious missing capability to turn the Option A LSS into Option B is the ability to refuel in cislunar, which is how the LSS becomes reusable. 
Don't they also need some way to inspect/test the vehicle before reuse? That seems to me to be the biggest missing piece--although it provides a great reason to have a crew at Gateway. :-)
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