Dinitrogen Tetroxide vs Nitric Acid

[Silavite]

After some study of the history of rockets (and playing some RP0), I noticed that the most common storable oxidizer in the late 1950s through the early to mid 1960s seemed to be nitric acid. However, by the late 1960s and beyond, nitric acid had been supplanted in most applications by N₂O₄. Why did this switch occur? Was it motivated by ISP, corrosiveness, handling characteristics, density, boiling/freezing point, or something else entirely?

Edited March 31, 2019 by Silavite

[Xd the great]

Mostly handling.

N2O4 did not give out poisonous NO2 gas when poured.

[IncongruousGoat]

28 minutes ago, Xd the great said:

Mostly handling.

N2O4 did not give out poisonous NO2 gas when poured.

Actually, it's got very little to do with handling, and a lot to do with chemistry, the nature of warfare in Siberia, and technological inertia.

RFNA is a pain to handle in the field, and it has worse performance than N2O4, but it has a much lower freezing point than N2O4, which freezes at -9.8 C. This high freezing point eliminated N2O4 from use in missile systems, which needed to work without warming up in cold conditions. Like, say, Russia. Or Alaska. Since early space launch tech had substantial missile heritage (where it wasn't just a re-purposed ICBM), it's no surprise that many early space launch vehicles found themselves using RFNA as an oxidizer somewhere. The switch to N2O4 in space vehicles (and Titan II) was motivated by specific impulse, since temperature wasn't a concern for such systems in the same way it was in a missile.

The reason for early use of WFNA in the AJ-10 can be attributed to historical weirdness. There's no reason to use WFNA over IRFNA-III, but IRFNA-III had only been somewhat decided as the superior acid in 1957, and the engineer in charge of that choice was probably either A: a bit weird, or B: not as well informed as they should have been. Or both.

Oh, and N2O4 is also nasty to handle in the field. Those clouds of NO2 fumes that RFNA produces come from the N2O4 that it contains.  N2O4 forms an equilibrium with NO2 at room temperature, so you get a lot of NO2 vapor. It wins over early RFNA on handling not because it's easier to handle, but because it doesn't corrode the tanks it's put in, and therefore can be loaded at the factory. Or at least, it won this until 1951, when IRFNA was invented and the whole issue became moot.

[Cunjo Carl]

1 hour ago, IncongruousGoat said:

Actually, it's got very little to do with handling, and a lot to do with chemistry, the nature of warfare in Siberia, and technological inertia.

Technological inertia is surprisingly strong in rocket science.  I wonder if it's because rocket science so expensive and difficult, people try to cling to whatever works!

 

1 hour ago, IncongruousGoat said:

The reason for early use of WFNA in the AJ-10 can be attributed to historical weirdness. There's no reason to use WFNA over IRFNA-III, but IRFNA-III had only been somewhat decided as the superior acid in 1957, and the engineer in charge of that choice was probably either A: a bit weird, or B: not as well informed as they should have been. Or both.

Echoing that, the opinion of John D. Clark in Ignition about the initial choice of RFNA over N₂O₄: " So the first order of business was choosing an oxidizer. ...  and Malina, Parsons, and Forman who, with the assistance of Dr. H. R. Moody, did a survey of the subject, considered that N2O4 was impractical. It is difficult to say why, but the extremely poisonous nature of the beast may have had something to do with its rejection. "

Edited April 1, 2019 by Cunjo Carl

[Silavite]

 

1 hour ago, IncongruousGoat said:

Actually, it's got very little to do with handling, and a lot to do with chemistry, the nature of warfare in Siberia, and technological inertia.

RFNA is a pain to handle in the field, and it has worse performance than N2O4, but it has a much lower freezing point than N2O4, which freezes at -9.8 C. This high freezing point eliminated N2O4 from use in missile systems, which needed to work without warming up in cold conditions. Like, say, Russia. Or Alaska. Since early space launch tech had substantial missile heritage (where it wasn't just a re-purposed ICBM), it's no surprise that many early space launch vehicles found themselves using RFNA as an oxidizer somewhere. The switch to N2O4 in space vehicles (and Titan II) was motivated by specific impulse, since temperature wasn't a concern for such systems in the same way it was in a missile.

The reason for early use of WFNA in the AJ-10 can be attributed to historical weirdness. There's no reason to use WFNA over IRFNA-III, but IRFNA-III had only been somewhat decided as the superior acid in 1957, and the engineer in charge of that choice was probably either A: a bit weird, or B: not as well informed as they should have been. Or both.

Oh, and N2O4 is also nasty to handle in the field. Those clouds of NO2 fumes that RFNA produces come from the N2O4 that it contains.  N2O4 forms an equilibrium with NO2 at room temperature, so you get a lot of NO2 vapor. It wins over early RFNA on handling not because it's easier to handle, but because it doesn't corrode the tanks it's put in, and therefore can be loaded at the factory. Or at least, it won this until 1951, when IRFNA was invented and the whole issue became moot.

Thanks for the answers! I've read Ignition before, but it's been a few years. After reading your post, I now remember the bit about the corrosion problems with RFNA and the discovery of IRFNA via the addition of small amounts of HF, but I had totally forgotten about the freezing point concerns in tactical missiles. Perhaps this shows that I should give it another read in the near future...

[IncongruousGoat]

1 hour ago, Cunjo Carl said:

Technological inertia is surprisingly strong in rocket science.  I wonder if it's because rocket science so expensive and difficult, people try to cling to whatever works!

That, and infrastructure. Rockets require lots of expensive infrastructure that's often very sensitive to small changes in the design of said rockets. Tooling, test stands, transportation infrastructure, integration facilities, tracking systems, launch facilities, range support...

[MaverickSawyer]

Yeah, gotta respect the ability of N2O4 to make a BFRC... 

[DDE]

7 hours ago, Silavite said:

 

Thanks for the answers! I've read Ignition before, but it's been a few years. After reading your post, I now remember the bit about the corrosion problems with RFNA and the discovery of IRFNA via the addition of small amounts of HF, but I had totally forgotten about the freezing point concerns in tactical missiles. Perhaps this shows that I should give it another read in the near future...

Indeed. At one point USN wanted an oxidizer rated for -100° F.

Also, oh dear, I’ve left the forum for a week and it has a dislike button now.

[Elthy]

40 minutes ago, DDE said:

Also, oh dear, I’ve left the forum for a week and it has a dislike button now.

Hm, may have something to do with the date...

[kerbiloid]

2 hours ago, DDE said:

Also, oh dear, I’ve left the forum for a week and it has a dislike button now.

At probably, it will disappear tomorrow.

[Ultimate Steve]

2 hours ago, kerbiloid said:

At probably, it will disappear tomorrow.

Hmm, April Fools?

[kerbiloid]

1 minute ago, Ultimate Steve said:

Hmm, April Fools?

According to "Dislike Reactions" thread, yes.

P.S.
Or maybe tomorrow it will create dislikes, lol.

[hideurgin]

On 3/31/2019 at 7:01 PM, Xd the great said:

Mostly handling.

N2O4 did not give out poisonous NO2 gas when poured.

you are actually incorrect in that statement. N2O4 is just NO2 in liquid form. considering its NO2's melting point is above stp conditions, when removed from a cryogenic thermos or not actively being cooled, it will boil off into NO2 gas.

[sevenperforce]

On 11/11/2023 at 1:38 PM, hideurgin said:

On 3/31/2019 at 8:01 PM, Xd the great said:

Mostly handling.

N2O4 did not give out poisonous NO2 gas when poured.

you are actually incorrect in that statement. N2O4 is just NO2 in liquid form. considering its NO2's melting point is above stp conditions, when removed from a cryogenic thermos or not actively being cooled, it will boil off into NO2 gas.

Holy necro batman!

But anyway this is sort of true. N₂O₄ is a different molecule than NO₂; it's a dimer of NO₂, two -NO₂ groups bonded together. But the N-N covalent bond in N₂O₄ is a very weak bond because the paired oxygen atoms on either side of the dumbbell are holding onto the electron shells tightly, so you only need a small temperature transition to break or form the bond.

Red fuming nitric acid, a mix of nitric acid, N₂O₄, and water, has a lower freezing point than pure N₂O₄ (as noted above four years ago by @IncongruousGoat) which is great. But it will continue to "fume" toxic NO₂ continuously, whereas pure N₂O₄ will produce very little NO₂ gas as long as it is kept below ~12°F. So you can load the propellant tanks at low temperature, tamping down fume production, and then you're good as long as you aren't trying to launch at significantly lower temperatures where N₂O₄ will freeze.