totm nov 2023 SpaceX Discussion Thread

[PB666]

1 hour ago, YNM said:

Interestingly, there were plans for for Saturn-Centaur that shows Centaur uses LH2 over LOX ; So is the DCSS (D-IV) with two separate tanks.

I guess there are a lot of reason, but none of them really meant to address / aleviate explosion.

Also, if H2 will rise faster than O2, wouldn't that meant the two shall meet if you put O2 over H2 ? ( @PB666 )

Im not sure why the External Tank on the shuttle had the O2 over the H2. It might be because it was easier to lift the H2 and feed it to the orbiter. The ET was relatively safe until an SRB ruptured the top of the tank and allowed O2 to travel.

It seems to me in most rocket designs you want to keep the weight as close to the engines as possible. The tanks for the RL10B-2 have the H2 tanks setting over an 02 taurus.

[Cunjo Carl]

35 minutes ago, PB666 said:

Im not sure why the External Tank on the shuttle had the O2 over the H2. It might be because it was easier to lift the H2 and feed it to the orbiter. The ET was relatively safe until an SRB ruptured the top of the tank and allowed O2 to travel.

It seems to me in most rocket designs you want to keep the weight as close to the engines as possible. The tanks for the RL10B-2 have the H2 tanks setting over an 02 taurus.

Without any specific knowledge of the system, I'd hazard a couple guesses. Perhaps having the denser LOx up higher had aerodynamic stability advantages, or perhaps since that tank fed off the bottom, it was better to have the LH2 down there because the volumetric flow rate of LH2 was much greater than that for LOx. Or maybe so the obnoxiously cold LH2 with its obnoxious metal embrittlement properties wouldn't need to go through as many long (=expensive) pipes.

Goodness, come to think of it making space ships sounds hard!

[Antstar]

2 hours ago, YNM said:

Interestingly, there were plans for for Saturn-Centaur that shows Centaur uses LH2 over LOX ; So is the DCSS (D-IV) with two separate tanks.

I guess there are a lot of reason, but none of them really meant to address / aleviate explosion.

Also, if H2 will rise faster than O2, wouldn't that meant the two shall meet if you put O2 over H2 ? ( @PB666 )

Why does everyone keep thinking that liquid H2 will rise, it will not

 

 

Density (LH2) 70.85 g/L

At sea level and at 15 °C air has a density of approximately 1.225 kg/m³ (1.225 x10⁻³ g/cm³

Latent Heat of Vaporization of Hydrogen is 0.44936 kJ/mol. (1mol~2g => 15kJ/cubic meter of LH2)

Air has VERY APPROXIMATELY 150kJ/m^3 of energy (sea level, room temperature), however to access all of this you end up liquefying the air

finally found a place which didn't use EVERY useless unit of measurement ever invented: https://www.h2tools.org/hyarc/hydrogen-data/hydrogen-density-different-temperatures-and-pressures

We see that cold gaseous H2 has a density of ~1g/L, at approx STP it is 0.08g/L

 

What conclusions can we draw from this:

1) Liquid hydrogen will fall like a brick - well not a brick, but 1/12 the density of water.

2) Hydrogen will want to float once boiled... but;

3) 1L of LH2 will become ~70L of cold gas, ~800L of gas at room temperature.

Has anyone here ever filled a coke bottle with LN2, fitted the lid and thrown it in a river. Kids, try this at home - its fun!!!

 

So, bulk discharge of hydrogen (not a leak) will either fall and explode once it hits the hot ground, or explode from mixing with hot rocket sides and air.

Edited March 2, 2018 by Antstar

[Antstar]

14 hours ago, PB666 said:

There are many forms of turbulence. Two types encountered Mach shock and sub Mach churning. At the launch site Mach shock is directed in the direction of exhaust gas travel. The venturi effect of the engines also concentrated around the engines. There is turbulence from the redirected exhaust plume but this is largely local. and velocity drops with the square of distance. Once the rocket is moving the flows are laminar with respect to the rocket up until about Mach 0.85 at which point bow shock starts to occur. There is considerable turbulence under the shock wave and this persists until the rocket is above 70 km (depending on its speed). Again if the rocket is 15 km when it hits max Q then sonic shock wave extends out at 45' angle downward to about 20 km on the ground, its likely someone on the ground would notice a slight boom. But the d

I was vaguely remembering a documentary about building the A380. Apparently the 747 had caused problems from the vortex (helical?) turbulence at the wing tips. They had to change the minimum distance between takeoffs and/or landings as a result. A380 needed to make sure it produced NO MORE turbulence than a 747.

It was not this documentary, but this has the same footage :

 

EDIT: And yeah, I guess it is not applicable for a rocket launch (I'm just stoopid)

Launch is nowhere near max Q, and by then the rocket is in a place with huge sheer winds that do sometimes cause cancellation of launch, so yeah, stoopid

Edited March 2, 2018 by Antstar

[YNM]

24 minutes ago, Antstar said:

Why does everyone keep thinking that liquid H2 will rise, it will not

Yeah, you can pour LN2 indeed...

But that'd mean upper stages are stupendously dangerous XD

Edited March 2, 2018 by YNM

[Antstar]

3 minutes ago, YNM said:

Yeah, you can pour LN2 indeed...

Yeah I use it most days. however LN2 is nearly the density of water. A side note: https://www.washington.edu/alumni/columns/dec97/car1.html

IIRC LN2 to room temp N2 gives 10% of the energy storage density of petrol, but your radiators will ice up

 

[YNM]

1 minute ago, Antstar said:

however LN2 is nearly the density of water.

I guess this could change with pressure and temp. Perhaps the usual LN2 is, but some pressurant inside a rocket might pack more or such.

[Antstar]

36 minutes ago, YNM said:

I guess this could change with pressure and temp. Perhaps the usual LN2 is, but some pressurant inside a rocket might pack more or such.

??

nitrogen LN2. I don't think it is suitable for rockets unless you mean for pressurising the tanks. BTW liquids don't like to be pressurised, its the basis of hydraulics.

I suppose you COULD make densified LN2 (I was really surprised this was practical for LOX), I don't think there is a point though, IIRC it goes from hot (100K) liquid to STP gas with 100x increase in volume, so since it is not going to be propellant, getting a 10% improvement (this is what falcon gets from densified LOX?) may not be worth the difficulty and the surface area of the vessel does not scale down well with volume, although I guess you can put it inside the densified LOX tank.

Would anyone disagree that for hydrolox, there is no need for densified LOX, since you cant densify LH2 and this is most of your fuel volume...?

Edited March 2, 2018 by Antstar

[YNM]

2 hours ago, Antstar said:

nitrogen LN2. I don't think it is suitable for rockets unless you mean for pressurising the tanks. BTW liquids don't like to be pressurised, its the basis of hydraulics.

I was just saying that you could have LH2 at a density denser than the "usual" densing point (at 1 atm). Indeed adding pressure doesn't sound the smartest but you could want it for the simple reason of keeping it liquid, sort of the cooled RP-1 F9 uses.

[tater]

 

[sevenperforce]

 

[PB666]

8 hours ago, Antstar said:

Why does everyone keep thinking that liquid H2 will rise, it will not

 

 

Density (LH2) 70.85 g/L

At sea level and at 15 °C air has a density of approximately 1.225 kg/m³ (1.225 x10⁻³ g/cm³

Latent Heat of Vaporization of Hydrogen is 0.44936 kJ/mol. (1mol~2g => 15kJ/cubic meter of LH2)

Air has VERY APPROXIMATELY 150kJ/m^3 of energy (sea level, room temperature), however to access all of this you end up liquefying the air

finally found a place which didn't use EVERY useless unit of measurement ever invented: https://www.h2tools.org/hyarc/hydrogen-data/hydrogen-density-different-temperatures-and-pressures

We see that cold gaseous H2 has a density of ~1g/L, at approx STP it is 0.08g/L

 

What conclusions can we draw from this:

1) Liquid hydrogen will fall like a brick - well not a brick, but 1/12 the density of water.

2) Hydrogen will want to float once boiled... but;

3) 1L of LH2 will become ~70L of cold gas, ~800L of gas at room temperature.

Has anyone here ever filled a coke bottle with LN2, fitted the lid and thrown it in a river. Kids, try this at home - its fun!!!

 

So, bulk discharge of hydrogen (not a leak) will either fall and explode once it hits the hot ground, or explode from mixing with hot rocket sides and air.

liqH₂ has a low vaporization temperature at STP (something like 20'K and pretty much if it comes spewing out of a tank it will rise just on ambient heating. Air is 15 grams per 22.4 liters, H2 is 2 grams per 22.4 liters, IOW it is 1/7.5 the density of Air, as the Hindenberg rose, so will H₂. liquid hydrogen is not like water, it has a very low thermal energy and an extreme temperature differential with air. If you could spray it out of the sprinkler in your garden the drops would never hit the ground.

 

[tater]

 

stay up til 1:30 mountain time? Ok, I probably will, lol.

[Antstar]

46 minutes ago, PB666 said:

liqH₂ has a low vaporization temperature at STP (something like 20'K and pretty much if it comes spewing out of a tank it will rise just on ambient heating. Air is 15 grams per 22.4 liters, H2 is 2 grams per 22.4 liters, IOW it is 1/7.5 the density of Air, as the Hindenberg rose, so will H₂. liquid hydrogen is not like water, it has a very low thermal energy and an extreme temperature differential with air. If you could spray it out of the sprinkler in your garden the drops would never hit the ground.

 

I'm sorry, you're wrong. If you look at the figures above you will see that the THEORETICAL amount of air that the liquid hydrogen will have to interact with to boil - if it cools the air to 0K - is 10 x its volume in GASEOUS air. in combination with the Leidenfrost effect (which I believe applies, since I have seen it for LN2), I absolutely guarantee you that a garden hose spewing LH2 at its boiling point would have most of the droplets hitting the ground. If you think I'm wrong, find an error in my maths.

EDIT: and to elaborate on my point in the previous post, if a bulk discharge is aerosoled it will boil and expand >100x, we call this an explosion - an explosion does not need to be on fire....

11 minutes ago, tater said:

 

stay up til 1:30 mountain time? Ok, I probably will, lol.

dammit, what time zone am I even in? OK, GMT + 1, sigh, that may be too early

Edited March 2, 2018 by Antstar

[kerbiloid]

53 minutes ago, PB666 said:

Air is 15 grams per 22.4 liters,

Why 15?  29.

_{(wrong quote corrected)}

Edited March 2, 2018 by kerbiloid

[wumpus]

9 hours ago, Antstar said:

Why does everyone keep thinking that liquid H2 will rise, it will not

The point is that LO2 will fall faster (if it is on/close to the pad).  If the rocket has sufficient momentum the H2 will of course still be rising, but likely slowing down faster due to aerodynamics than the much denser LO2.  A full detonation of the fuel seems unlikely (considering how many rockets have launched), but the effects would be devastating.

[PB666]

12 minutes ago, kerbiloid said:

Why 15?  29.

_{(wrong quote corrected)}

29 it is.

[Antstar]

12 minutes ago, wumpus said:

The point is that LO2 will fall faster (if it is on/close to the pad).  If the rocket has sufficient momentum the H2 will of course still be rising, but likely slowing down faster due to aerodynamics than the much denser LO2.  A full detonation of the fuel seems unlikely (considering how many rockets have launched), but the effects would be devastating.

Okay, I'm not going to say this is completely untrue, but I think that balancing the rockey is the primary reason for the positioning of the tanks - that and feed lines to the engine. Looking at the numbers:

Buoyancy and surface area are key here. LH2 is 70x air, LO2 at the boiling point is 1000x - both will really want to sink. Not so much difference really as they are both way over unity.

So, comparing surface areas. LO2 is 16 x as dense as LH2 (again at boiling points). so, say its got 16 ^ (2/3) = 6 x the surface area

So, yeah it wont fall quite as fast, but it will meet some exploding boiling oxygen (if they both burst at once) once the oxygen hits the ground....

 

EDIT: I never considered a moving rocket... If it can maintain pressure (Ie the leak isn't total) I would have thought that the propellant will be spread out over so many kilometers it doesn't really matter??

Edited March 2, 2018 by Antstar

[PB666]

7 minutes ago, wumpus said:

The point is that LO2 will fall faster (if it is on/close to the pad).  If the rocket has sufficient momentum the H2 will of course still be rising, but likely slowing down faster due to aerodynamics than the much denser LO2.  A full detonation of the fuel seems unlikely (considering how many rockets have launched), but the effects would be devastating.

If the H2 tank is above the O2 tank, the O2 weighs 32 grams per 22.4 liters and will fall the H2 has 2 grams per 22.4 liters and will rise. H2 can always explode because it is desolving in air. [I've actually been in an H2/O2 explosion in a lab, it was a big noise and things (mainly glass shards) flying. I have seen methane O2 explosion, turned a house into toothpicks, one of the residents survived (below the level of the gas) the other, at the level of the gas, did not. (Methane at 16 grm per 22.4 liters also rises).

[sevenperforce]

7 minutes ago, wumpus said:

The point is that LO2 will fall faster (if it is on/close to the pad).  If the rocket has sufficient momentum the H2 will of course still be rising, but likely slowing down faster due to aerodynamics than the much denser LO2.  A full detonation of the fuel seems unlikely (considering how many rockets have launched), but the effects would be devastating.

I would argue that without very specific mixing conditions, you have deflagration at worst, not detonation. AMOS-6 was a rapid but entirely subsonic deflagration, and it had the nearly-perfect mixing conditions of a total pad RUD without FTS activation. CRS-7 didn't even ignite (at least not appreciably) and it was an in-air FTS.

[PB666]

19 minutes ago, Antstar said:

Okay, I'm not going to say this is completely untrue, but I think that balancing the rockey is the primary reason for the positioning of the tanks - that and feed lines to the engine. Looking at the numbers:

Buoyancy and surface area are key here. LH2 is 70x air, LO2 at the boiling point is 1000x - both will really want to sink. Not so much difference really as they are both way over unity.

So, comparing surface areas. LO2 is 16 x as dense as LH2 (again at boiling points). so, say its got 16 ^ (2/3) = 6 x the surface area

So, yeah it wont fall quite as fast, but it will meet some exploding boiling oxygen (if they both burst at once) once the oxygen hits the ground....

Your missing the point, H2 liquid holds very little energy in the energy of liquification, such that once it is surrounded by air, its gas. In fact if you put enough liquid H2 in the air, the air will liquify before the gas begins to persist.

https://web.njit.edu/~muratov/hazards.pdf

Edited March 2, 2018 by PB666

[tater]

I lost track of what the hydrolox discussion has to do with SpaceX, since they use 3 different propellants, none of which are hydrolox.  

[PB666]

6 minutes ago, tater said:

I lost track of what the hydrolox discussion has to do with SpaceX, since they use 3 different propellants, none of which are hydrolox.  

Coming soon to a SpaceX rocket near you.

[Antstar]

4 minutes ago, tater said:

I lost track of what the hydrolox discussion has to do with SpaceX, since they use 3 different propellants, none of which are hydrolox.  

Indeed. Although *SpaceX* actually uses another additional propellant. Its called charisma Not sure you can power a rocket with it though, but helps build it

[tater]

The new possible launch date might well slip due to weather at sea if they want to try recovery. The nor’easter has apparently increased the sea state down south.