Spaceship Landing

[mike wightman]

All it requires is a bit of Newtonian maths and a few passes through the upper atmosphere.

The preposterous thing about gas giant skimming isn't the orbital mechanics that allow it, but rather how you deal with the hypersonic compression of all that hydrogen into liquid form.

Truly magical heat sinks

 

[AnotherDilbert]

The preposterous thing about gas giant skimming isn't the orbital mechanics that allow it, but rather how you deal with the hypersonic compression of all that hydrogen into liquid form.

Let's see if we can approximate that. Raising the pressure 1000 times as an adiabatic process would raise the temperature by very roughly 10 times, let's say from ~100 K to about 1000 K or a total of 1000 K for simplicity.

A small ship might skim 50 tons or thereabouts. Raising the temperature of 50 tons of hydrogen by 1000 K would take about 50 000 kg × 1000 K × 14 kJ/kgK = 700 000 000 kJ = 700 GJ energy. Spread that out over a few hours that is 700 GJ / ( 2 × 3600 s ) = 0.01 GW = 10 MW power to dissipate.

Compared to the at least 500 MW from the power plant we are already dissipating it does seem to be not all that much extra?



OK, some more energy will be released when it is liquified but that is less than cooling the gas by 1000 K.

 

[BackworldTraveller]

Anyone know which form liquid hydrogen the tanks hold? Ortho or Para?

 

[AnotherDilbert]

I have never seen it discussed, so I assume a natural distribution?

 

[mike wightman]

There is the added complexity of the fact that the gas molecules are effectively travelling at 7-10km/s and need to be slowed down - hence the hypersonic cooling.

50,000kg at 10,000m/s ~ 2.5TJ of energy to get rid of.

 

[mike wightman]

Anyone know which form liquid hydrogen the tanks hold? Ortho or Para?

Since it is cryogenically liquid - although you could take a stab at arguing that it is compressed to its liquid state using extreme artificial gravity fields (grav focussed lasers are a thing after all and they require rather powerful artificial gravitational fields) - then it is going to be 99% para, which in itself is going to release more energy as it changes from the ortho isomer.

 

[AnotherDilbert]

50,000kg at 10,000m/s ~ 2.5TJ of energy to get rid of.

Still considerably less than the power plant of even the smallest ships produce and hence dissipate.

 

[AnotherDilbert]

50,000kg at 10,000m/s ~ 2.5TJ of energy to get rid of.

Come to think about it; that would just slow the ship a bit. Why would we need to radiate any of that as heat?

 

[Straybow]

No, in being compressed and liquified it MUST be removed from the H2 as waste heat. Now all that waste heat is in your ship.

 

[boomslang]

This is exactly why I find the 20 dTon Lifeboat/Launch rather unlikely. A 30 ton Ship's Boat can do 6G, and the 20 ton craft is limited to 1G? Why?? At least give it 2G, and a much improved survivability.

I cannot find the reference (which means it may be in print rather than digitized on my system), but I have a distinct recollection of the late LKW mentioning that it was a mistake of GDW to make the Lifeboat 1G -- in retrospect, it should have been 2G.

So, I figure that puts the idea somewhere between errata and apocrypha and accordingly just retcon Lifeboats to 2G IMTU.

 

[BackworldTraveller]

No, in being compressed and liquified it MUST be removed from the H2 as waste heat. Now all that waste heat is in your ship.

Convert to light and radiate from gun turret as a coherent beam?

 

[AnotherDilbert]

No, in being compressed and liquified it MUST be removed from the H2 as waste heat. Now all that waste heat is in your ship.

Why would the lack of kinetic energy in the collected gas release heat? If anything it has to be accelerated to the same velocity as the ship?

The energy released by the compression is a separate issue, I believe.

 

[AnotherDilbert]

Convert to light and radiate from gun turret as a coherent beam?

That increases entropy, which takes even more energy, which creates even more heat to dissipate, see the second law of thermodynamics.

Or colloquially: TINSTAAFL.

 

[mike wightman]

Convert to light and radiate from gun turret as a coherent beam?

Which generates waste heat - lasers are not very efficient

edit - beaten to it by Another Dilbert

 

[mike wightman]

Why would the lack of kinetic energy in the collected gas release heat? If anything it has to be accelerated to the same velocity as the ship?

The energy released by the compression is a separate issue, I believe.

We have to consider the reference frame.

The ship isn't moving and gas travelling at 10km/s is entering the tanks...

 

[Carlobrand]

I cannot find the reference (which means it may be in print rather than digitized on my system), but I have a distinct recollection of the late LKW mentioning that it was a mistake of GDW to make the Lifeboat 1G -- in retrospect, it should have been 2G.

So, I figure that puts the idea somewhere between errata and apocrypha and accordingly just retcon Lifeboats to 2G IMTU.

I tossed Book-2 boats when Book 5 arrived on the scene 'cause Book-2 boats were obscenely expensive: a 20-ton 1G launch with two seats and no computer was half the cost of a 100-ton jump-2 capable ship with staterooms and a 4-million-credit computer, a 50-ton cutter cost almost as much as the jumper. However, the Book-2 launch lined up with the HG build rules, aside from price: 13 tons cargo, 7 tons other, which in HG meant 4 tons bridge, a ton each fuel, drive, and power plant. With 5% of the mass available for the drive and another 5% for the plant, that lines it up for 2G if it's built at TL13, a wee bit short but within spitting distance at TL9.

Thing is, Traveller only ever understood round numbers up until Striker. You did 1G or you did 2G; you never did 1.2G in the spacecraft universe. There were obvious reasons - it simplified the math - but it left a quandary down at the 1G level: how are these things getting off the ground? Answer became apparent once Striker's fractional-G speed chart was available. The little 1-ton plant delivers enough power at TL9 to pull more than 1G in a launch - 1.67G, actually - it's just rounded off for the space combat scale. So, I just began assuming there were hidden fractional Gs at work getting these 1G-space-combat-scale craft off the ground. Wasn't really important what the numbers were unless I felt an overwhelming compulsion to chase down a launch or a free trader with a grav tank, which so far hasn't come up.

 

[Carlobrand]

That increases entropy, which takes even more energy, which creates even more heat to dissipate, see the second law of thermodynamics. ...

Well, yes, but then so does my refrigerator. :devil: Lasers may not be the answer, but sometimes its useful to create more heat to get rid of heat in one place, so long as all the heat you end up with is in another place more convenient to your needs.

I have on occasion wondered whether we needed to worry about our heat dissipators going up into the soft x-ray range. It'd make life interesting for the dock workers.

 

[aramis]

Convert to light and radiate from gun turret as a coherent beam?

OVerall entropy must increase. Thank-you thermodynamics. So waste heat to laser? Can't happen.

 

[AnotherDilbert]

Well, yes, but then so does my refrigerator.

Your refrigerator or AC works because the power plant is outside your house and hence spills waste heat somewhere else.

With the power plant inside your fridge/house/ship/system all it does it convert usable energy into more waste heat...


But, yes, we can redistribute heat within the system at the cost of increased energy consumption and hence increased total waste heat.

 

[BackworldTraveller]

OVerall entropy must increase. Thank-you thermodynamics. So waste heat to laser? Can't happen.

Can happen because the system is no longer ship but ship + universe. Horrendous conversion overhead though