I recall this being mentioned in Ignition!: An Informal History of Liquid Rocket Propellants [0] :

Boranes are unpleasant beasts. Diborane and pentaborane ignite spontaneously in the atmosphere, and the fires are remarkably difficult to extinguish. They react with water to form, eventually, hydrogen and boric acid, and the reaction is sometimes violent. Also, they not only are possessed of a peculiarly repulsive odor; they are extremely poisonous by just about any route. This collection of properties does not simplify the problem of handling them. They are also very expensive since their synthesis is neither easy nor simple. But they possess one property which attracted rocket people to them as hippies to a happening. They have an extremely high heat of combustion — gram for gram about 50 percent more than jet fuel.

[0] https://www.amazon.ca/Ignition-Informal-History-Liquid-Prope...

> That being said, there are plenty of propellants used as part of hypergolic combinations that aren't toxic. For example, oxygen.

No, there aren't. From this it is evident you only have a cursory understanding of rocketry and haven't done your research. I suggest reading Ignition! [0].

The reason you are wrong is that there are actually only a few specific different fuels and oxidizers that are used in practice as hypergolic propellants.

For the fuel these are: hydrazine, unsymmetrical dimethylhydrazine (UDMH), monomethyl hydrazine (MMH), or mixtures of any of those three in different proportions (e.g. Aerozine 50 which is 50% hydrazine and 50% UDMH). These aren't the only possible fuel options, many others were explored in the 60s (read Ignition!), but other possible choices such as Furfuryl alcohol got phased out very quickly.

For the oxidizer these are: Inhibited Red Fuming Nitric Acid (IRFNA) or Dinitrogen Tetroxide. That's it. No liquid oxygen. No doubt there are substances that are hypergolic with liquid oxygen, but they've never made it onto a rocket.

Many of these have gone out of fashion as well, so the choices nowadays are even smaller. I don't think anything has used Hydrazine since the Titan IV retired, and IRFNA has universally been phased out in favor of dinitrogen tetroxide. So nowadays, in practice hypergolic propellants consist only of UDMH and/or MMH, and dinitrogen tetroxide. That's it. This may change in the future as there continues to be research into less toxic alternatives, but that's where things stand today.

[0]: https://www.amazon.com.au/Ignition-John-Clark/dp/0813595835

Edit: I see you changed your answer. You have a point about the TEA-TEB used for ignition in the Merlin engine. Certainly, those ignite on contact with liquid oxygen. But that isn't hypergolic -- it's pyrophoric. By definition, for something to be hypergolic it must be a propellant (used to create impulse).

If anyone wants to get into some of the obscure history niches, see the book Ignition!: An informal history of liquid rocket propellants (ISBN13: 9780813507255):

* https://www.goodreads.com/book/show/677285.Ignition_

It's not about rockets in general, but about rocket fuel specifically. It's the "#1 Best Seller in Petroleum (Engineering)" category:

* https://www.amazon.com/Ignition-Informal-History-Liquid-Prop...

I've always loved Ignition![0] for the perspective it gives on working with the chemistry of this stuff.

[0] https://www.amazon.com/Ignition-Informal-Propellants-Univers...

The best place to start is Apollo by Murray/Cox: https://www.goodreads.com/en/book/show/282086.Apollo Rather than focusing on the astronauts, this is very much a story of the engineering and management that made the project possible.

From someone who helped develop the lunar lander, there's Thomas J. Kelly's "Moon Lander: How We Developed the Apollo Lunar Module": https://www.amazon.com/dp/1588342735/

Another good book (so I've heard - I own a copy but haven't made time to read it yet) is Sunburst and Luminary by Don Eyles, who worked on the guidance system at MIT: https://www.sunburstandluminary.com/SLhome.html

For deep technical details (but light on first-person stories), there's How Apollo Flew to the Moon: https://www.goodreads.com/book/show/2323178.How_Apollo_Flew_...

Finally, not strictly related to the space race, but for some truly wonderful first-hand tales from the development of rocket propellants, you can't beat John D. Clark's Ignition!: https://www.amazon.com/Ignition-Informal-Propellants-Univers...

That is pretty much all there is: If earth was more massive, the chemical rockets we've been producing wouldn't have enough thrust-to-weight ratio to reach orbital velocity (about 25k km/h). We would have to load the rockets with so much fuel to break free of gravity that they would be too heavy to lift themselves.

This is all assuming yields and fuels we have now. If we lived on a more massive earth and we were trying to escape its gravity, I'm sure we'd be using more exotic and dangerous fuels (like all those fun fluorine and boron fuels Dr. Clark mentions in Ignition![0]) to do the job. We just happened to have the capability in the middle of the century to use a fuel we were already making (refined petroleum) for jet engines.

[0] https://www.amazon.com/Ignition-Informal-Propellants-Univers...

You can get it on Kindle:

https://www.amazon.com/dp/B076838QS2

Well worth the money. I don't think I'd pay the $90+ people are asking for a hardcover version...

This is probably the best book on how the professionals had fun building rockets back in the day. It's a gread read, and full of useful practical chemical engineering advice and cautionary tales. You might find PDFs, but you can also buy it here: https://www.amazon.com/Ignition-Informal-Propellants-Univers...

Finally back in print after 40 years!

https://www.amazon.com/Ignition-Informal-Propellants-Univers...