low-cost pressurization concepts

[rocketpoke]

Hey guys, I'm another newbie introduced by the SpaceX discussion. If I remember correctly, didn't SpaceX originally propose being pressure fed? Interesting if the failure is in the turbopump. I'm pretty sure there was a redesign of some sort. Pressure fed seems to be the method de jour for the "private" launchers. Does anyone have any insight as to why they went pump-fed? And for that matter why folks like AirLaunch and Microcosm use "novel" pressurization systems dating back to the 50's and 60's and if those methods are so great why they haven't been used? AirLaunch uses vapor pressure in their tanks to feed propellant and I guess that's an old Aerojet idea. Microcosm uses helium with fractions of oxygen and hydrogen fed through a catlyst to provide heat to the tanks. I guess that goes back a ways too. If there's such a weight and cost savings why isn't anyone else doing this? I'm posting after work from home, so forgive me if I don't reply right way. Thanks in advance.

[braddock]

Welcome to the site, rocketpoke.

SpaceX's Kestrel second stage engine is pressure fed.  I found an '03 Elon Musk quote about using a first stage pressure fed engine at http://www.spacex.com/index.html?section=media&content=http%3A//www.spacex.com/media8.php

"SpaceX believes the turbopump is worth its expense for the tank weight it saves", Musk said.

Remember, SpaceX intends to recover and reuse that first stage eventually which might have influenced their decision.    

I'm sure others here can add more technical depth to that statement than I can.

At the end of the day, SpaceX probably went with whatever their VP Propulsion Tom Mueller was comfortable with.  That included the low-cost, pintle-injected, LOX/RP-1, turbopump-fed TRW engine development experience that he came from TRW with.  The similarities are so striking that SpaceX had to settle a lawsuit with Northrup-Grumman to use it.

http://spaceflightnow.com/news/n0009/26trwpintle/
http://www.grc.nasa.gov/WWW/RT1995/5000/5310k.htm

[dmc6960]

Here is an interesting concept I ran into last week.  To achieve a large pressure-fed rocket, you need heavy tanks to hold the higher pressure.  What these people have done, is elliminated the large pressure tanks, and replaced them with normal lightweight fuel tanks, then send the fuel into two alternating small high-pressure tanks pressurized by a single small super-high-pressure tank.  However with 8 valves per fuel tank constantly changing state, it certainly isn't as simple as an ordinary pressure fed design.  Have only been used on some sounding rockets and a test stand with their 1000lbs engine, but they say it can be scaled up to millions of pounds of thrust.

http://www.flometrics.com/rockets/rocket_pump/rocketpump.htm

[Jim]

Beal Aerospace was going to straight He pressurization of the propellant tanks.  The result of this was 1/3 of the vehicle was Helium tanks

[braddock]

Jim - 4/4/2006  11:15 AM
Beal Aerospace was going to straight He pressurization of the propellant tanks.  The result of this was 1/3 of the vehicle was Helium tanks

Is the Helium stored in a gaseous state?  Has there been any work done on using a fluid with a viable liquid-to-gas state change?  

Seems to me the heat of the engine could be used to boil just about anything, and use the resulting vapor as the pressurizing gas.

[dmc6960]

braddock - 4/4/2006  10:39 AM

Is the Helium stored in a gaseous state?  Has there been any work done on using a fluid with a viable liquid-to-gas state change?  

Seems to me the heat of the engine could be used to boil just about anything, and use the resulting vapor as the pressurizing gas.

Thats exactly what the Flowmetrics folks say.

[braddock]

Nifty animation buried on the Flowmetrics site.

Apparently, this isn't an entirely new idea, according to the Flowmetric's own site:

This pump is similar to one mentioned in 1960 in Exploring the Solar System by Felix Godwin p21- p22 and to one patented by Sobey,(3,213,804) Jim Blackmon and Eric Lanning (6,314,978)
 

[Tap-Sa]

braddock - 4/4/2006  8:29 PM
 
Apparently, this isn't an entirely new idea, according to the Flowmetric's own site:
 

IMO one can succesfully argue that the prior art goes way beyond 1960. Hero of Alexandria described in ~130BC (but apparently did not build) steampowered temple door mechanism which involved pumping water from closed container using steam. Italian Giovanni Battista della Porta repeated the feat in 1601. Englishman Thomas Savery patented and built in 1698 the first practical steam operated pump to drain mines.

Flometric's application of the idea is very promising, offering many advantages over turbopumps without the usual mass penalty of pressure fed design. Hopefully it finds it way to real test flights soon. I predict that before space elevators one of first applications of superstrong CNT fabric is building pressure fed rocket tanks without pretty much any mass penalty.

[braddock]

Is dissolution of the gas into the fuel an issue with any of these concepts?

And how exactly does this work without gravity or acceleration to force the fuel to the bottom of the chamber?  For example, in a pressure-fed re-startable second stage (such as the Kestrel), once in zero-gee, what keeps the remaining fuel from breaking up into liquid bubbles before the re-start?  (or for that matter, how does this work with a turbopump feed?)

[dmc6960]

braddock - 4/4/2006  1:39 PM

Is dissolution of the gas into the fuel an issue with any of these concepts?

And how exactly does this work without gravity or acceleration to force the fuel to the bottom of the chamber?  For example, in a pressure-fed re-startable second stage (such as the Kestrel), once in zero-gee, what keeps the remaining fuel from breaking up into liquid bubbles before the re-start?  (or for that matter, how does this work with a turbopump feed?)

Settling motors.  Small solid fuel thrusters fire to give just enough acceleration on the rocket to drop all the propellant to the bottom of the tanks.  In small scales this can also be done with cold gas thrusters or RCS.

[aero313]

dmc6960 - 4/4/2006  2:57 PM

Settling motors.  Small solid fuel thrusters fire to give just enough acceleration on the rocket to drop all the propellant to the bottom of the tanks.  In small scales this can also be done with cold gas thrusters or RCS.

Actually, there are a lot of ways to do it.  The solid settling rockets are no-brainers but add the complexity and safety hazards of additional systems.  All launch vehicles will have RCS thrusters for roll control and pitch/yaw control during coast.  By slightly angling these thrusters rearward you can get a small axial acceleration to settle the propellant - it doesn't take much.  Also, some tanks are built with surface tension screens or PMDs in the outlet end of the tank.  These can provide just enough propellant to get the main engine started and thus provide the settling of the rest of the propellant.

[Tap-Sa]

Other commonly used propellant management options are positive expulsion and surface tension. Former means either piston (rare) or flexible bladder (common with storable propellants) which pushes and always separates the remaining propellant from ullage space. Surface tension involves fine mesh compartments near propellant outlets. Surface tension keeps the propellant attached to the mesh and it does not want go through it in microgravity so outlet remains always wet.

[rocketpoke]

Cool pics. Funny how even this part of rocket science can date back a couple thousand years. And I forgot about Flometrics. I hear their system makes a pretty good margarita too.  

Does anyone know anything about the other concepts (AirLaunch and Microcosm)? Neither of them makes much practical sense, but on paper they look pretty good.

Thanks for all the replies!

[aero313]

rocketpoke - 4/4/2006  9:50 PM

Does anyone know anything about the other concepts (AirLaunch and Microcosm)? Neither of them makes much practical sense, but on paper they look pretty good.

Why do you say that?  From what's been published, the AirLaunch system looks pretty good.  Unlike SpaceX (just look at the pictures of their Stage 1 plumbing in AvWeek), the AirLaunch rocket is dirt simple.  Hardly any parts means low recurring cost and fewer chances to screw up during a launch.  The self pressurized propellants do have a low feed pressure - something like 250 psi I think, but starting at 30,000 ft instead of sea level helps make up the performance loss.

[publiusr]

Jim - 4/4/2006  10:15 AM

Beal Aerospace was going to straight He pressurization of the propellant tanks.  The result of this was 1/3 of the vehicle was Helium tanks

It made for a big LV. It could launch some satelites side by side. It was to have more thrust than an SRB.

[rocketpoke]

aero313 - 5/4/2006  9:04 AM

Why do you say that?  From what's been published, the AirLaunch system looks pretty good.  Unlike SpaceX (just look at the pictures of their Stage 1 plumbing in AvWeek), the AirLaunch rocket is dirt simple.  Hardly any parts means low recurring cost and fewer chances to screw up during a launch.  The self pressurized propellants do have a low feed pressure - something like 250 psi I think, but starting at 30,000 ft instead of sea level helps make up the performance loss.

I guess I have a hard time seeing such a low pressure system being effective in getting a meaningful payload to orbit. Think about the size/weight impact of the low pressure nozzle alone. A 250-ish psi chamber would require a large throat and since it's being launched at altitude, a high expansion ratio too. So that tells me this is going to be huge. Does the weight/complexity benefit of the pressurization system outweigh the weight/size penalty of the nozzle?

It's a tough trade. Do you chose the performance benefit of a TPA and accept the complexity/reliability/cost issues, or go with a pressurization system for simplicity and cost and accept the weight/volume/performance penalty? Or do you go a step further like AirLaunch or Microcosm and choose a novel pressurization system to try and bypass the usual disqualifiers for a pressurized system? And I still have a hard time wondering if these methods are so novel and effective, why haven't they been used in the last 40+ years since they were first conceived?

I guess I'm concerned about this following the SpaceX failure. Especially if it turns out to be related to their feed system. I need to get back to the post-failure thread and catch up.

[aero313]

rocketpoke - 7/4/2006  9:54 AM

I guess I have a hard time seeing such a low pressure system being effective in getting a meaningful payload to orbit. Think about the size/weight impact of the low pressure nozzle alone. A 250-ish psi chamber would require a large throat and since it's being launched at altitude, a high expansion ratio too. So that tells me this is going to be huge. Does the weight/complexity benefit of the pressurization system outweigh the weight/size penalty of the nozzle?

So what?  The launch vehicle is just a truck.  If the vehicle is not mass efficient, who cares so long as the cost is low?  Frankly, in the launch vehicle world, liquid propellant is pretty much free as compared to the rest of the system.  If you can build low-cost, reasonable weight structures using modern composites, that's not necessarily a bad trade.  Besides, the AirLaunch rocket does fit in a C-17, so it can't be that big.  Payload is supposed to be 1,000 lbs to 100 nmi due east - pretty much the same as SpaceX and Pegasus.

It's a tough trade. Do you chose the performance benefit of a TPA and accept the complexity/reliability/cost issues, or go with a pressurization system for simplicity and cost and accept the weight/volume/performance penalty? Or do you go a step further like AirLaunch or Microcosm and choose a novel pressurization system to try and bypass the usual disqualifiers for a pressurized system?

Well, in my experience, the parts are only half the cost.  The other half is integration and test.  Added complexity brings not only more I&T cost but also more rework and repair during the prelaunch integration and checkout phase.  That means a larger logistics footprint for critical spares, more tech and QA labor, and longer checkout times (which translates to higher standing army costs). These, by the way, are costs that are always overlooked by launch vehicle novices.  That's another reason why the cost of Pegasus went up from $6M a flight and why I maintain that SpaceX will also raise prices eventually.  Solid rockets are historically used for responsive systems (ie, ICBMs) because the simplicity of the system reduces these costs - albeit at a much higher initial production cost.  I think AirLaunch is attempting to get the operational benefits of a solid with the lower fabrication costs of a liquid.  That's not a bad thing.

And I still have a hard time wondering if these methods are so novel and effective, why haven't they been used in the last 40+ years since they were first conceived?

Well, performance has always been king.  In the early days of the Atlas and Titan programs, paying extra for higher performance was deemed necessary.  As those vehicles made the transition to space launch, the incremental cost of a "small" mod was always cheaper than starting from scratch with a new design.  There's also the issue of the space launch versions (initially at least) benefitted from the ICBM infrastructure.  Also, we now have low cost, high performance materials and fabrication methods that we didn't have in the 1960s.

[publiusr]

I like big simple pressure-feds myself. But if they are small and you want to drop them from a 747?

 Forget it.