Pros and cons of pressure-fed rockets

Archibald

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Hello to all.
I'm curious about this rocket concept after seeing tons of unbuilt projects (Truax, Percheron, OTRAG, Scorpius, Beal...) using this method.

From what I've understand with my limited knowledge

- pressure-fed are cheaper because they have no turbopump
- but this is more or less offset by heavier tankage

More details ? Debate is open...
 
Basically correct. Pressure feds also suffer from lower performance... turbopump-fed engines regularly get over 1000 psi chamber pressure, but the Dea Dragon, frex, only had 300psi in the first stage and 75 in the second.

The extra tank weight hit is a *big* weight hit. However, an advantage of this is that being thick, they are less prone to damage and corrosion, can be assembled using lower tolerances (Sea Dragon was to be built in a shipyard), and are more readily reusable. But there is a fetish for high performance in the aerospace community... a booster that can get 450 seconds Isp is automatically seen as better than one that can only get 300 seconds, because that first booster will weight far less for the same paylaod. But gross weight really doesn;t mean much... a heavy structure built out of mundane materials by standard labor could easily cost less than a lightweight advanced structure built in cleanroom conditions... and propellant really doesn;t cost much.
 
Ladies & gentlemen (and the rest of you ;) )

Apart from the sheer "performance anxiety" of the launcher design community, I'm given to understand that the problem is, that most of today's space launchers are based on ICBM designs.

As a military system, an ICBM is typically designed for maximum performance, not minimum cost-per-payload/pound.

Launcher designs like the Sea Dragon (see e.g. www.astronautix.com) were designed from the ground up (no pun intended) for minimum cost, and consequently performance was sacrificed. As a result, the launch vehicle got a lot bigger and heavier.

Regards,

Thomas L. Nielsen
Denmark
 
The extra tank weight hit is a *big* weight hit.

Maths are not my cup of tea. Let's try something easy (for me)

Let's take one H-1B Saturn pump-fed engine.
100 tons of thrust, LOX/ Kerosene propellants, one per stage.

Now I'm Bob Truax and my aim is to build a similar engine, but pressure-fed, and mount it on a stage.

How much weight added ?
 
This makes for some very interesting (and lengthy) reading

http://www.dunnspace.com/leo_on_the_cheap.htm

Page 111 starts the chapter on pressure and pump fed.
 
Archibald said:
The extra tank weight hit is a *big* weight hit.

Maths are not my cup of tea. Let's try something easy (for me)

Let's take one H-1B Saturn pump-fed engine.
100 tons of thrust, LOX/ Kerosene propellants, one per stage.

Now I'm Bob Truax and my aim is to build a similar engine, but pressure-fed, and mount it on a stage.

How much weight added ?

ratio empty-full stage for Turbo fed rocket engine is 0,94
ratio empty-full stage for Turbo fed rocket engine is 0,89

so 5% more in empty stage mass (include gas generator)

recovery hardware not include, that increase around 30% in empty stage mass
(here a system like SSRB with parachute.)
 
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