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##### Advanced Concepts / Re: Medusa concept using chemical explosives
« Last post by Beratnyi on Today at 08:11 am »

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? The greater the specific impulse, the lower the mass of exhaust gases for a given amount of energy spent on heating.
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.
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##### Advanced Concepts / Re: Medusa concept using chemical explosives
« Last post by ppnl on Today at 07:45 am »

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.

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##### SpaceX Super Heavy/Starship (BFR/BFS) - Earth to Deep Space / Re: Will a Starship head to Mars in the 2024 launch window?
« Last post by steveleach on Today at 07:24 am »
For anything to happen Starship must make orbit and return intact

The most basic item after that is being able to do propellant transfer. Yes Soyuz has done it and ISS uses it but not a)With cryogens b)On this scale.

With those 2 pieces in place SX will send something Starship sized to Mars in 2024.

With the loose working of the original question "Will a Starship head to Mars in the 2024 launch window" then a Falcon Heavy Demo type launch would qualify: it left an object hearing to Mars' orbit, just not to flyby or land.
A similar Starship test does not seem at all outside the realms of possibility, maybe with less time pressure on launch date (FH demo was needed to qualify the vehicle for paying customers, so could not wait for the correct launch date to fly by Mars itself) to allow for a Mars intercept. Even without propellant transfer - which should hopefully be being worked on as part of HLS by then, so may have a Starship undergoing a prop transfer test sitting in orbit that would need to get rid of a full load of propellant somehow anyway - a stripped-down Starship (AKA 'Starkicker') should just about have the delta V to flyby on launch propellant alone.
These two views seem to be fundamentally opposed to each other.

Could a stripped-down Starship on a throw-away SuperHeavy do a "Tesla Starman" mission without needing either EDL or refuelling?

Would SpaceX launch such a mission if it was the only way to use the 2024 window? Obviously if they have demonstrated EDL and/or refuelling we'd expect them to make use of those.

And would that qualify as Starship "heading to" Mars in 2024?

Or is EDL and orbital refuelling an absolute requirement for a Starship Mars mission?
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##### Space Science Coverage / Re: KARI KPLO/Danuri
« Last post by Phil Stooke on Today at 07:21 am »
Two weeks to orbit:

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##### ISS Section / Re: Expedition 68 Thread
« Last post by FutureSpaceTourist on Today at 07:10 am »

Quote
＼Successfully released 4 microsatellites from Kibo! /
With the support of many people, three BIRDS-5s and one operator's satellite departed from Kibo on December 2nd!
https://humans-in-space.jaxa.jp/kibouser/pickout/73434.html
#ウガンダ #ジンバブエ #九工大 #近畿大

By the way, it seems that it was released over Africa this time 🌍 Is that what the third photo looks like?
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##### ESA Launchers - Ariane, Soyuz at CSG, Vega / Re: Arianespace launch schedule
« Last post by Salo on Today at 07:07 am »
https://nextspaceflight.com/launches/details/1930
Quote
Galileo FOC FM25-FM26
Launch Time
NET December, 2023
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##### ULA - Delta, Atlas, Vulcan / Re: ULA Vulcan Launch Vehicle (as announced/built) - General Discussion Thread 3
« Last post by GWH on Today at 06:51 am »
No, nothing.

Even ULA's official paper on distributed launch (years old now) only list capabilities of a dual launch Vulcan with only 4 boosters which is less than the performance needed. A 6 booster Vulcan could do it but that capability isn't shown anywhere.

Er, aren't the OneWeb launches going to be on 6 booster Vulcan? That would retire some risk for that path...

I didn't mean 6 booster Vulcan performance has never been published,  it has, but not in the context of distributed lift.  The distributed lift paper shows two 4 booster Vulcan's being able to deliver a 24 tonne payload to escape velocity. Orion is roughly 27 tonnes.

Why they don't state what two 6 booster Vulcan's could do (27 tonnes to LEO each) seems like an odd omission .
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##### Advanced Concepts / Re: Medusa concept using chemical explosives
« Last post by InterestedEngineer on Today at 06:48 am »
What are the kinetic and thermal energy components of high grade (e.g. c4 or better) explosives or shaped charges?

Without a nozzle mind you.  A de Laval nozzle converts thermal into kinetic energy, but in doing so gets very hot and and has pressure limits as noted above.

I note Raptor 2 has about a 56/44 ratio of kinetic/thermal energy after expansion on the Vacuum nozzle.  Quite the waste really, but amazing for a thermal engine.

Allegedly, a rotating detonation system doesn't need such a nozzle but you only get about 20-30% more Isp.

Can an explosive system bypass those limits by creating more initial kinetic energy and less thermal energy?

If I did read this chart and do the math correctly, the ratio of kinetic energy to thermal energy for most high grade explosives is 5:1:

https://www.hindawi.com/journals/ijce/2019/4017068/tab3/

IOTW, 83% kinetic 17% thermal.  Much more efficient.
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##### SpaceX Super Heavy/Starship (BFR/BFS) - Earth to Deep Space / Re: Will a Starship head to Mars in the 2024 launch window?
« Last post by sdsds on Today at 06:38 am »
For anything to happen Starship must make orbit and return intact

The most basic item after that is being able to do propellant transfer. [...]

With those 2 pieces in place SX will send something Starship sized to Mars in 2024.
[...] if they demonstrate return to earth by the end of Q223 they can work out propellent transfer and build enough SS's to get to mars.

This timeline makes sense. They'll get a basic Starship to orbit and back. Then hopefully they'll get a prototype depot on orbit, and practice propellant transfer in both directions with a reusable tanker. Once they put a full-sized, full-feature depot on orbit, the (then obsolete) prototype depot could be sent towards Mars....
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##### SpaceX Super Heavy/Starship (BFR/BFS) - Earth to Deep Space / Re: Will a Starship head to Mars in the 2024 launch window?
« Last post by daedalus1 on Today at 06:36 am »
I think you are going of track. Starship and booster are progressing rapidly to first flight and we are 2 years away from the Mars launch window.
For anything to happen Starship must make orbit and return intact

The most basic item after that is being able to do propellant transfer. Yes Soyuz has done it and ISS uses it but not a)With cryogens b)On this scale.

With those 2 pieces in place SX will send something Starship sized to Mars in 2024.

My instinct is if they demonstrate return to earth by the end of Q223 they can work out propellent transfer and build enough SS's to get to mars. If not they probably don't have enough time to do mars and honour other commitments.

There has always been a range of options for baseline SS from pure batteries and expendable coolants for thermal control right up to the full retractable solar-panel-and-radiator package.  With those comes a range of options for what can be accomplished. From a single SS sending "Hi Earth. I'm down and haven't fallen over" to a pair, with the second acting as an orbital relay with high bandwidth and the lander deploying substantial hardware.

But y'know we still have 26 days left......

They have to demonstrate transfer of fuel in orbit by 2024 in order to accomplish the Artemis 3 lander mission the next year 2025.
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