Ozone oxidiser?

[wumpus]

4 hours ago, kerbiloid said:

Let's return from the world of fantasies to the real world.

Fuel: https://en.wikipedia.org/wiki/Pentaborane
Oxidizer: fluorine-stabilized ozone

Gotta love that 4-4-4 triangle (Magnemoe beat me to it).  Flash point: 30C?  Not for use near Cocoa, Florida (KFC), Santa Maria, California (Vandenburg), and even winter use is iffy in Chincoteague, Virgina (Wallops, Island).

Can't tell if the 50mg/kg LD50 really means milligrams, the concentration LC50 values  imply you wouldn't need close to that to kill you.  Explosive limits of .42%?  Does that mean any significant concentration explodes if you look at it funny?

[sevenperforce]

3 hours ago, kerbiloid said:

We should mix it with liquid ozone.
Because it's cool, too. Very cool, -200°.

Maybe add some lithium, too.

Rocket propellant chemistry is (unsurprisingly) a rather complex issue. Maximizing specific impulse is a porkchop plot of two variables: specific energy and product molecular mass. Both have to be maximized in order to get highest average exhaust velocity.

Specific energy is the energy released when one kg of reactants is burned and converted into (slightly less than) one kg of reaction products. To get a high specific energy, you want lightweight atoms which can each form multiple bonds and have high bond energies in the reaction product. At the same time, you want its pre-combustion molecular configuration to have low-energy bonds, because energy you use to break those bonds has to come out of your reaction. Atoms like nitrogen have really high bond energies, but their pre-combustion state has as much energy as their post-combustion state and so they don't really contribute. 

Product molecular mass is the average molecular mass of your reaction product. You want this to be low, because exhaust velocity is inversely proportional to the weight of each exhaust molecule. The lowest possible product molecular mass (barring molecular disassociation in nuclear thermal rockets) would be simple compressed heated hydrogen gas. Of course, compressed hydrogen gas isn't very high-energy, so it's not exactly ideal as a monopropellant. But if you have an effectively-infinite heat source (like a nuclear reactor or a solar thermal rocket) then hydrogen (probably in liquid form, for storage considerations) is def the way to go.

The highest specific energy is achieved by operating an engine in stoichiometric ratio, but adding low-molecular-mass fuel can increase specific impulse (as in the SSME) because you can sometimes get better exhaust velocity if you lower your average product molecular mass, even when that decreases temperature (because some of the hydrogen passes through unburnt).

Beryllium has higher specific energy than lithium because even though it's heavier and has lower bond energies, it can form multiple bonds with a single atom. Laying aside silly foibles like extreme toxicity, it's a better rocket propellant, as long as sufficient quantities of diatomic hydrogen are available to carry that energy away at high exhaust velocity.

Liquid ozone beats out liquid oxygen because its precombustion bonds are weaker and therefore require less energy to break, but deliver the same amount of energy in the reaction products.

Fluorine beats out liquid oxygen, even though it only forms one bond per atom to oxygen's 2, because its bond energies are so much stronger.

FOOF beats out fluorine and ozone because it is even denser than liquid ozone, but has single-bonds in its precombustion state, which require less energy to break than LOX or ozone or liquid fluorine.

So FOOF + LH₂ + Be, in perfect ratio, is pretty much the highest-specific-impulse triprop combination chemically possible.

59 minutes ago, wumpus said:

Explosive limits of .42%?  Does that mean any significant concentration explodes if you look at it funny?

Eh, that's probably a reference to fuel LEL/UEL. It means that when mixed with air at STP, it will detonate when ignited below 42% but conflagrate when ignited above 42%.

LEL/UEL stand for Lower Explosive Limit and Upper Explosive Limit. As an example, natural gas has its explosive limit between 5% and 15%. When natural gas is mixed with air at standard temperature and pressure, ignition will result in simple conflagration below 5% (because it is too oxidizer-rich to detonate) or above 15% (because it is too fuel-rich to detonate) but will explode in a destructive, supersonic wavefront if the ratio of methane to air is between 5% and 15%.

[kerbiloid]

Spoiler

5 hours ago, sevenperforce said:

Beryllium has higher specific energy than lithium because even though it's heavier and has lower bond energies, it can form multiple bonds with a single atom. Laying aside silly foibles like extreme toxicity, it's a better rocket propellant, as long as sufficient quantities of diatomic hydrogen are available to carry that energy away at high exhaust velocity.

Btw, you've given a nice _{(in both senses)} idea, why the alien invaders should occupy the Earth and send the captured humans into mines.

https://en.wikipedia.org/wiki/Emerald

Quote

a variety of the mineral beryl (Be₃Al₂(SiO₃)₆) colored green by trace amounts of chromium and sometimes vanadium.^[

They will force humans to mine emeralds and pound them, to extract beryllium as rocket fuel.

 

[wumpus]

16 hours ago, sevenperforce said:

Product molecular mass is the average molecular mass of your reaction product. You want this to be low, because exhaust velocity is inversely proportional to the weight of each exhaust molecule. The lowest possible product molecular mass (barring molecular disassociation in nuclear thermal rockets) would be simple compressed heated hydrogen gas. Of course, compressed hydrogen gas isn't very high-energy, so it's not exactly ideal as a monopropellant. But if you have an effectively-infinite heat source (like a nuclear reactor or a solar thermal rocket) then hydrogen (probably in liquid form, for storage considerations) is def the way to go.

I was under the impression that monatomic hydrogen was up there (along with metalic hydrogen) as wildly superior to ordinary rocket fuels.  The issue is that monatomic hydrogen isn't stable enough to even have the horror stories in this thread, you simply can't make it without it decomposing into H₂.  Presumably you meant simply compressing hydrogen gas, as that is pretty pointless.  It takes up too much volume, needs to be refilled right before launch, and is unlikely to deliver more impulse than compressed nitrogen (which is trivial to work with, although dangerous in large volumes.  I'd guess you'd include some oxygen to avoid asphyxiating technicians).

Don't forget that beryllium as a metal has some amazing strength to weight ratios.  You might want to use that more to decrease dry mass than to increase Isp.  Just be careful what you expose it to and how you mix it in an alloy.

[sevenperforce]

22 hours ago, wumpus said:

I was under the impression that monatomic hydrogen was up there (along with metalic hydrogen) as wildly superior to ordinary rocket fuels.  The issue is that monatomic hydrogen isn't stable enough to even have the horror stories in this thread, you simply can't make it without it decomposing into H₂.  Presumably you meant simply compressing hydrogen gas, as that is pretty pointless.  It takes up too much volume, needs to be refilled right before launch, and is unlikely to deliver more impulse than compressed nitrogen (which is trivial to work with, although dangerous in large volumes. 

Oh, using compressed hydrogen gas as a monopropellant by itself is a very bad idea; it has no specific energy. But hydrogen gas is the lightest thing (short of doing something like monatomic hydrogen) you can use as a propellant, so mixing it with a high-specific-energy heat source (like fluorine and lithium, or a nuclear thermal reactor) will give you the best specific impulse.

[wumpus]

1 hour ago, sevenperforce said:

Oh, using compressed hydrogen gas as a monopropellant by itself is a very bad idea; it has no specific energy. But hydrogen gas is the lightest thing (short of doing something like monatomic hydrogen) you can use as a propellant, so mixing it with a high-specific-energy heat source (like fluorine and lithium, or a nuclear thermal reactor) will give you the best specific impulse.

My understanding is that hydrogen is already used this way in current hydralox engines.  They run fuel rich to include hydrogen in the output (as well as water), this helps both keep temperatures from melting all known materials and increase Isp.  As far as I know, I think the materials are at thermal limits, so using an increasingly rich input won't help much (unless you want to build a cheaper engine with slightly less Isp and not quite so thermally constrained).  Building a turbopump for a source of hydrogen is likely too complicated for any currently used engine, while hydralox engines are simply designed for a different oxidizer/fuel ratio.

As far as the nuclear thermal reactor (or possibly solar thermal reactor), hydrogen is the key and the achilles heel.  They might list a 4 digit Isp, but don't expect that from anything but hydrogen.  Water will give you less Isp than chemical hydralox, but might be easier to find in space.  Keeping hydrogen in your tanks beyond Earth orbit is easier said than done.  I don't think anyone as worked with fluorine or lithium in rockets since Ignition! was published.

[Zeiss Ikon]

24 minutes ago, wumpus said:

 I don't think anyone as worked with fluorine or lithium in rockets since Ignition! was published.

Well, not as propellants.  The last generation External Tanks for the Space Shuttle were skinned with an aluminum-lithium alloy -- which, to me, seemed prone to very rapid corrosion in air, but might have used the natural aluminum oxide surface layer to protect the lithium.  It was used because it saved several percent on the dry weight of the tank assembly (there's a LOT of skin on one of those tanks).

[NSEP]

4 hours ago, Zeiss Ikon said:

Well, not as propellants.  The last generation External Tanks for the Space Shuttle were skinned with an aluminum-lithium alloy -- which, to me, seemed prone to very rapid corrosion in air, but might have used the natural aluminum oxide surface layer to protect the lithium.  It was used because it saved several percent on the dry weight of the tank assembly (there's a LOT of skin on one of those tanks).

Falcon 9 also uses Aluminium Lithium for their first stage tanks.

[Zeiss Ikon]

1 hour ago, NSEP said:

Falcon 9 also uses Aluminium Lithium for their first stage tanks.

Really?  I thought all their actual tanks were composite (the composite helium tank inside the oxygen tank was blamed for the on-pad explosion a while back). 

[NSEP]

55 minutes ago, Zeiss Ikon said:

Really?  I thought all their actual tanks were composite (the composite helium tank inside the oxygen tank was blamed for the on-pad explosion a while back). 

The main fuel tanks aren't made of composites. The interstage and fairing are.

[StrandedonEarth]

Although BFR is supposed to use all-composite tankage...

[sevenperforce]

23 hours ago, wumpus said:

My understanding is that hydrogen is already used this way in current hydralox engines.  They run fuel rich to include hydrogen in the output (as well as water), this helps both keep temperatures from melting all known materials and increase Isp.

Right. Running LH₂+LOX at stoichiometric ratio will melt the engine, so adding unburnt hydrogen reduces chamber temperature while simultaneously decreasing molecular mass in order to kick Isp up a little bit.

The key is that Isp is the combination of average propellant specific energy and average propellant molecular mass, and there is a porkchop plot which defines the maximum Isp.

True tripropellant engines (which mix three propellants at essentially the same mixture ratio the entire time) exist to pair a high-specific-energy fuel combination with a low-molecular-mass propellant.

16 hours ago, NSEP said:

17 hours ago, Zeiss Ikon said:

Really?  I thought all their actual tanks were composite (the composite helium tank inside the oxygen tank was blamed for the on-pad explosion a while back). 

The main fuel tanks aren't made of composites. The interstage and fairing are.

As are the COPVs.

13 hours ago, StrandedonEarth said:

Although BFR is supposed to use all-composite tankage...

Unlined composite, which is scary.

[Zeiss Ikon]

8 hours ago, sevenperforce said:

Unlined composite, which is scary.

As I understand the process of making a COPV, you get a sort of "automatic liner" in the form of a layer of resin laid down on the form before you start the overwrap.  With conventional COPV, like SCBA tanks for emergency responders, the form is the liner, a steel or aluminum inner tank too thin to hold the full design pressure (COPVs are used in this situation because they're lighter for the same pressure and volume than either steel or aluminum).  "Unlined" COPVs are made on a collapsible form that can be removed after curing -- essentially a big balloon that will hold the desired tank shape, but can be removed through one of the formed-in ports.  The COPVs used for helium in the Falcon 9 tanks (helium to pressurize the tank, which both adds extra column strength to the stage and assists the turbopumps by reducing cavitation) are unlined, but the inner continuous layer of resin keeps the helium in.  The outer layers are where the oxygen found a way to freeze in the interstices, causing a combination of tank rupture and ignition energy as the helium pressure expanded the inner tank.

I don't think resin-surface tanks, if made with adequate pressure margins and expansion allowance, will be any more hazardous, even for oxygen service, than putting the (genuinely, on the outside) unlined helium tank inside the liquid oxygen.  Which, yes, blew up a rocket -- and now won't, because the fueling procedure was changed to prevent formation of oxygen crystals within the wrap.

[sevenperforce]

16 minutes ago, Zeiss Ikon said:

As I understand the process of making a COPV, you get a sort of "automatic liner" in the form of a layer of resin laid down on the form before you start the overwrap. 

Is COPV resin inert to oxidization?

16 minutes ago, Zeiss Ikon said:

With conventional COPV, like SCBA tanks for emergency responders, the form is the liner, a steel or aluminum inner tank too thin to hold the full design pressure (COPVs are used in this situation because they're lighter for the same pressure and volume than either steel or aluminum).  "Unlined" COPVs are made on a collapsible form that can be removed after curing -- essentially a big balloon that will hold the desired tank shape, but can be removed through one of the formed-in ports.  The COPVs used for helium in the Falcon 9 tanks (helium to pressurize the tank, which both adds extra column strength to the stage and assists the turbopumps by reducing cavitation) are unlined, but the inner continuous layer of resin keeps the helium in.  The outer layers are where the oxygen found a way to freeze in the interstices, causing a combination of tank rupture and ignition energy as the helium pressure expanded the inner tank.

The COPVs used for helium in Falcon 9 are Al-lined. 

 

[Zeiss Ikon]

10 hours ago, sevenperforce said:

Is COPV resin inert to oxidization?

This likely depends on what resin is used.  Completely inert?  Probably not.  An internal coating of PTFE or similar fluorinated polymer could be applied after curing for oxygen service.

Quote

The COPVs used for helium in Falcon 9 are Al-lined. 

Okay, I was incorrect -- still, it was the outside of the COPV that caused the problem, not the inside.