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The "Real Aerospace" Thread 
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Post Re: The "Real Aerospace" Thread
Zarya wrote:
Mjolnir wrote:
edit: Correction, the Vulcain is actually a gas-generator engine like the Merlin. It pumps both fuel and oxidizer into the chamber with a pump driven by a turbine running from a small fraction of the propellant flow and dumping its exhaust separately. Simpler to develop but less efficient.


heh - you're more than 99% right about the Vulcain, although it also re-injects the exhaust product of the pre-burner to create a 'film' inside the engine that protects the nozzle & engine bell. The original idea to create a slightly cooler chamber wall by injecting something extra, such as fuel, dates back to the A4 motor that powered the V-2 and it was also applied in the F-1 used for the Saturn V. My point: the Vulcain is not simply dumping the exhaust product of the pre-burner over the side, while not being a full-blown staged combustion either.

The Bristol Siddeley Gamma is brilliant in its own right, it applies the properties of H2O2 to achieve "staged combustion like efficiency".


It's a pretty standard technique, the Merlin 1D Vacuum does the same, but the turbopump exhaust contributes little to thrust, it just lets you use a lighter nozzle extension. It's still a gas generator cycle, with the limitations on chamber pressure and less efficient use of reaction mass that come with it...the Vulcain only runs at 100 bar, the Merlin 1D at 97 bar. The RS-25 runs at 206 bar, RD-180 at 270 bar. The Vulcain gets a vacuum Isp of 431 s, compared to the staged combustion RS-25's 452 s. The Merlin 1D's 311 s is quite good for a kerosene burning gas generator engine, but the staged-combustion RD-180 gets 338 s.

Raptor gets 380 s with methane...near the performance of hydrogen with a far denser propellant that's a close match in physical properties to liquid oxygen.

The Gamma: that depends on how brilliant you consider using HTP to be. As far as I'm aware, the Black Arrow was the only launch system in existence to ever get to orbit using HTP as an oxidizer, which it did exactly once before being canceled. Many others have tried, with the expectation that a room temperature oxidizer would be easier to work with than cryogenic oxygen, but nobody's succeeded. Instead, the main room temperature alternative to LOX has been nitrogen tetroxide. People get misled by thinking of the heavily diluted stuff you can buy in the store, hydrogen peroxide in high concentrations is quite dangerous and difficult to handle. (Its instability is the reason the Soyuz can't stay longer at the station...the Soyuz uses it for attitude control, but it decomposes over time no matter what you do.)


Sat Aug 31, 2019 7:24 am
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Post Re: The "Real Aerospace" Thread
You're right about the Vulcain as well as about the higher chamber pressures - up until the late 1990s considered 'extreme' - for closed-cycle/staged combustion engines. Although the RD-180 is still used to launch NRO satellites and has a spotless track record, it has never been man rated, presumably because of the chamber pressures.
The Raptor and in due course the BE-4 or their successors may be among the first staged combustion engines with high chamber pressures that will ever be used for manned flights. I think they will be literally have to be tested to destruction over a lengthy period before they will be trusted.

I also feel we should be kind about the Bristol Gamma: its story started well over 60 years ago in the age of slide rulers and engineers wearing woollies on stone cold test sites, with the UK looking to develop its own launch vehicles 'on a shoestring' to develop a capability that'd perhaps allow the country to field its home-grown nuclear deterrent. For the delivery vehicle they needed a storable propellant and oxidizer and for that they went with HTP. The Royal Navy already had experience with high-test peroxide for their torpedoes - including a serious mishap with it - so at that time it was probably considered not as exotic and dangerous as nitrogen tetroxide.

Looking at the Black Arrow launches the exhaust plume is super-clean and the rocket itself is relatively small. Those are all advantages. Puck/Prospero, the satellite it launched during its very last launch, is indeed still in orbit. In short HTP and kerosine allowed for a very efficient launcher that was never adequately funded and that came too early to lift commercial payloads into orbit.


Sat Aug 31, 2019 9:54 am
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Post Re: The "Real Aerospace" Thread
Zarya wrote:
You're right about the Vulcain as well as about the higher chamber pressures - up until the late 1990s considered 'extreme' - for closed-cycle/staged combustion engines. Although the RD-180 is still used to launch NRO satellites and has a spotless track record, it has never been man rated, presumably because of the chamber pressures.
The Raptor and in due course the BE-4 or their successors may be among the first staged combustion engines with high chamber pressures that will ever be used for manned flights. I think they will be literally have to be tested to destruction over a lengthy period before they will be trusted.

I also feel we should be kind about the Bristol Gamma: its story started well over 60 years ago in the age of slide rulers and engineers wearing woollies on stone cold test sites, with the UK looking to develop its own launch vehicles 'on a shoestring' to develop a capability that'd perhaps allow the country to field its home-grown nuclear deterrent. For the delivery vehicle they needed a storable propellant and oxidizer and for that they went with HTP. The Royal Navy already had experience with high-test peroxide for their torpedoes - including a serious mishap with it - so at that time it was probably considered not as exotic and dangerous as nitrogen tetroxide.

Looking at the Black Arrow launches the exhaust plume is super-clean and the rocket itself is relatively small. Those are all advantages. Puck/Prospero, the satellite it launched during its very last launch, is indeed still in orbit. In short HTP and kerosine allowed for a very efficient launcher that was never adequately funded and that came too early to lift commercial payloads into orbit.


The BE-4 actually has a weirdly low chamber pressure. Only 134 bar, more in line with a high-end gas generator engine than a staged combustion engine. It's their first large engine, so perhaps they're just being very cautious with its design. (Though they've also had problems bringing it up to full thrust, so maybe there's other issues.)


Sat Aug 31, 2019 11:41 am
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