Comments on: Target Fixation

By: Sage

Sage — Wed, 07 Jan 2009 20:07:48 +0000

Alan:
Tank pressure in a pumped system is to provide positive pump feed, and in many tank designs, also increase the structural rigidity. You need a positive inlet pressure, especially with centrifugals; note that the output pressure is related to the positive head, and increasing this reduces some of the inlet requirements and affects items such as cavitation onset as well; the physical characteristics of the pump design are thus affected by the inlet pressure. As you pump from the tank you need to maintain positive feed which is generally achieved through an adjunct pressurization system. As John noted earlier, about 3 – 5 bar is a reasonable target for pump fed systems but this depends on your design; 10 bar is not unreasonable.

By: Alan Kruppa

Alan Kruppa — Wed, 07 Jan 2009 18:28:03 +0000

Thanks David. That makes sense. I’m a bit naive about turbo pumps but I think I’ve got this correct. Turbo pump mass vs. tank (i.e. inlet) pressure will approximately follow an inverse (negative slope) logarithmic curve, right? I understand how tank weight scales linearly with pressure, so I’m asking the question to better understand the trade off.

By: David Summers

David Summers — Wed, 07 Jan 2009 15:39:55 +0000

Alan, I think it is a little of both. Most importantly, the turbo pump mass goes way up if the tank pressure is too low – so there is a relatively easy trade off there in favor of higher tank pressures. The structural integrity benefit seems to be mostly incidental – it is still somewhat rare for designers to rely on “balloon tanks”, as they are known (only early Atlas, as I recall). It is true that all liquid rockets I know of rely on it somewhat, though. (I think the Space Shuttle was supposed to be the exception – it was spec’d to not require tank pressure at any point, but they missed the spec by a bit during the worst case loading.)

By: john hare

john hare — Wed, 07 Jan 2009 01:18:50 +0000

My understanding is that pumped systems usually shoot for 2-4 atm in the tank, while pressure fed hit about 20 on average. For small vehicles though, minimum guage on the tanks tends to diminish the mass savings possible.

By: Alan Kruppa

Alan Kruppa — Wed, 07 Jan 2009 00:44:21 +0000

David, I saw your point about tanks holding the propellant at 10 atm or so. Isn’t that more for structural integrity against buckling of the tank skin? Or is it just a nice benefit born of the need to provide some inlet pressure to the turbine? Forgive me if that’s a stupid question.

By: Roderick Reilly

Roderick Reilly — Mon, 05 Jan 2009 23:35:32 +0000

One issue relevant to the notion of “target fixation” are the huge expenses and long development times that seem to have become an accepted part of the aerospace culture, at least with government-funded programs. This forces program managers and policy makers to lock into a concept which will not survive challenges from more agile competitors. Of course, it has been the culture for so long because agile competitors have been nonexistent.

By: Monroe

Monroe — Fri, 02 Jan 2009 21:59:11 +0000

We shall see I hope! keep plugin.

By: Sage

Sage — Fri, 02 Jan 2009 19:52:07 +0000

1)
Monroe,
You’re incorrect in this case. A conventional launcher is an excellent and intelligent choice for an N-Prize attempt; your conjectures of hardware mass limitations and other items are simply not compatible with the physics.

2)
Tim,
In the case of a “low performance” pump as you mention (i.e., low chamber pressure) your overall mass savings with a pump are going to be small (if any). As I mentioned, practically all pump fed systems still require a pressurized tank (to some extent). But, using both systems for increasing chamber pressure (much beyond the tank pressurization requirements currently used for pumps) suggests to me an increase in complexity (and probably weight), but there may be some uses (for instance, maybe as a possible multi-string/backup implementation and/or potentially reducing pump inlet requirements). Still though (and regardless of the merits/drawbacks of such a system), I do agree with your point that we should be careful not to exclude synergistic approaches (and that’s true in general).

~Sage
http://littlemonsterrocket.webnode.com/

By: David Summers

David Summers — Fri, 02 Jan 2009 16:26:59 +0000

John, another way to look at it is that most of the currently flying engines already are pressure fed / pumped hybrids – it’s not like the tank hold the propellant at ambient pressure! I’m pretty sure that most tanks are at 10 atmospheres or so (I’m not sure of the numbers – but non-trivial pressure).

Turbopumps require a certain amount of pressure to suppress cavitation – and they also are a pressure multiplier, so higher inlet pressure = higher outlet pressure.

By: john hare

john hare — Fri, 02 Jan 2009 09:21:09 +0000

Tim.

One of the points of pressurization and pump is the ability to bolt a new type of pumped engine to a thoroughly tested pressure fed vehicle. By making the engine compatible with other companies’ vehicles, your market expands. I think it is safe to speculate that Lynx, Pixel, and MA-02 will each have much more investment in getting the total vehicle right, than in the core engine. The ability to bolt on an engine that gets another 25 seconds Isp could be attractive to several companies when they are starting to reach the performance limit of a particular vehicle and want more.

With good pressure into the pump, the suction characteristics are relaxed to the point that you can almost ignore cavitation, which makes the pump both easier to do, and less critical to the vehicle. The two methods can possibly be designed in as safety back ups for each other.