It really is rocket science

[Bob B.]

I wonder what the Prof would make of the images of the Black Arrow launch.... (see attached)
And that was on earth as well.

The Black Arrow was fueled by kerosene and hydrogen peroxide, wasn't it?

[JayUtah]

The Black Arrow was fueled by kerosene and hydrogen peroxide, wasn't it?

Yes, H₂0₂/RP-1

[ka9q]

I wonder why the plume is transparent despite the use of kerosene fuel? Maybe the mixture ratio didn't have to be rich as in most rockets.

[cjameshuff]

I wonder why the plume is transparent despite the use of kerosene fuel? Maybe the mixture ratio didn't have to be rich as in most rockets.

Hydrogen peroxide decomposition produces two molecules of water for each molecule of O2, so the sooty highly incandescent particles would be a bit more diluted in the exhaust. Might also use a more oxidizer-rich mixture due to the fact that just decomposing peroxide contributes some to the thrust, while unburned fuel contributes nothing. Lower temperatures might also be a factor in allowing oxidizer-rich mixtures. Hard information would be nice...

[ka9q]

I later read that the invisible exhaust is attributed to the large amount of water from the decomposition of H₂O₂. I can understand that but it was also my understanding that most rockets are run fuel-rich to avoid an oxidizing environment that could erode the combustion chamber and nozzle, and with RP-1 fuel that would leave some unreacted carbon to glow when it hits atmospheric O₂.

I would think that the atomic oxygen released by the decomposition of H₂O₂ would be at least as corrosive as any oxidizer, but maybe not.

I've noticed in the video from Deltas, Falcons and other RP-1 burning rockets that as they rise in the atmosphere, their plumes broaden out and become less bright, but you can still see what look like random black streaks leaving the nozzle. I assume they're made of unburned carbon that no longer has a chance to burn and glow in atmospheric O₂, but I'm not sure. If you look very closely at some of the Apollo 16mm landing films you will also see those little black streaks on occasion, which I would also attribute to unreacted carbon from the carbon-containing Aerozine-50 fuel.

[cjameshuff]

I would think that the atomic oxygen released by the decomposition of H₂O₂ would be at least as corrosive as any oxidizer, but maybe not.

HTP's been used as a monopropellant in numerous rockets (not often...performance is poor and it's expensive, hazardous, prone to decomposition in the presence of any contamination, etc). Any atomic oxygen has probably recombined to molecular hydrogen by the time it mixes with the kerosene and burns, so you're basically using a mixture of steam and oxygen as your oxidizer, which is probably a good bit cooler burning and less of a corrosion issue than injecting pure liquid oxygen.

[Bob B.]

I seem to recall reading somewhere that the Black Arrow used 85% hydrogen peroxide, though I don't know the mixture ratio of oxidizer to fuel.  That's going to produce a great deal of water in the exhaust.  The temperature is also much lower than when LOX is used.  A LOX/RP-1 engine might have a combustion chamber temperature of around 3300^o C, while a HTP/RP-1 (HTP=high test peroxide) engine might operate at about 2400^o C.  I'm by no means an expert of the incandescence of carbon, but I have to believe the lower temperature would decrease the effect.

[Bob B.]

... it was also my understanding that most rockets are run fuel-rich to avoid an oxidizing environment that could erode the combustion chamber and nozzle

Reducing corrosion might be part of the reason, but I've always understood the reason for operating fuel-rich is to lower the molecular weight of the exhaust gas.  Oxidizing the fuel creates heavy molecules -- CO₂ is heavier than CO, and H₂O is heavier than H₂ or H -- which results in a lower exhaust gas velocity.  The downside of running fuel-rich is that the temperature is less, which also reduces exhaust velocity.  There's a happy medium where we supply adequate oxygen to burn enough of the fuel to get a high temperature, but not too much oxygen that we drive the molecular weight too high.  The optimum mixture that achieves the highest exhaust velocity is on the fuel-rich side of a stoichiometric ratio.

By the way, the Russians operate many of their engines oxidizer-rich in the preburner and turbines.  I haven't studied their designs enough to know how they've overcome the corrosion issue, but they've apparently figured it out.  I think their N₂O₄/UDMH staged-combustion engines operate oxidizer-rich, though the main combustion chamber is still burning a fuel-rich mixture.  They route the oxidizer along with a small amount of fuel to the preburner, and then the oxygen rich gas is routed to the turbines.  The turbine exhaust then goes to the combustion chamber where it is combined with the remaining fuel and burned.

[Glom]

What's the problem with a lower exhaust velocity?

What's the problem with high mass lowering exhaust velocity?  That would mean you could get the same thrust for a reduced power.

[ka9q]

so you're basically using a mixture of steam and oxygen as your oxidizer, which is probably a good bit cooler burning and less of a corrosion issue

Steam can be notoriously corrosive, as any power plant operator can tell you.

[Glom]

so you're basically using a mixture of steam and oxygen as your oxidizer, which is probably a good bit cooler burning and less of a corrosion issue

Steam can be notoriously corrosive, as any power plant operator can tell you.

Water in general as any well operator can tell you.

[ka9q]

What's the problem with a lower exhaust velocity?

What's the problem with high mass lowering exhaust velocity?  That would mean you could get the same thrust for a reduced power.

Yes, but only at the cost of greater propellant mass, so it's not usually worth it.

In a chemical rocket, the energy is stored in the same material that becomes the reaction mass, so you might as well store as much energy as possible in that mass so you can expel it as quickly as possible. That's why you always want chemical propellants with the highest specific impulse available, subject only to practical factors like toxicity, density (which affects tank weight), cost, reliability, safety, etc.

[ka9q]

What's the problem with high mass lowering exhaust velocity?  That would mean you could get the same thrust for a reduced power.

I just realized you probably actually meant to ask what's the problem with a high molecular weight exhaust lowering exhaust velocity.

Every rocket is a heat engine. However the energy is stored, whether it's in the chemical energy of the propellants themselves or added externally from a nuclear reactor or a electrically-powered arc, it first becomes heat. Then that heat is turned into the mechanical (kinetic) energy of the propellant, and that's what makes it a heat engine.

No heat engine can be 100% efficient in turning heat into mechanical energy for a variety of reasons. In a rocket, one of the factors affecting that conversion efficiency is the molecular weight of the exhaust, the lower the better. This can be such an important factor that sometimes it pays to convert less of the stored chemical energy into heat in the first place, if that permits a more efficient conversion of the heat you do generate into mechanical energy.

[cjameshuff]

Steam can be notoriously corrosive, as any power plant operator can tell you.

Not as corrosive as high concentrations of oxygen at high temperatures (http://en.wikipedia.org/wiki/Thermal_lance). The Soviets had to work out some metallurgical tricks to handle such conditions, IIRC, while dealing with steam was well understood by that time.

[JayUtah]

Preburners are run lean to keep the exhaust gas temperatures low until they reach the thrust chamber, to prevent excess thermal stress on the preburner and turbine components.  Those are notoriously difficult to cool.