guest blogger john hare
Some time ago I learned that I could go to work for any Newspace company I wished, and work on any project I wanted to fund. So any contribution I make to this field before getting rich*, is probably going to be based on my ability to think of things that other people haven’t. To most, that is called thinking outside the box. The problem with thinking outside the box is that most of the time the best answers are the ones in the box. Those boxed answers are there for a reason, they work. They just might not be the best answers all the time.
I made an off hand comment that deep throttling pumps were very difficult to do on the multichamber post. That is another instance of me thinking inside the box. PumpÂ performance Â scales in weird fasions with rpms. A given centrifugal pump volume output rises linear, pressure squares, and power requirements cube with rpm. So if you double the pump speed you get twice as much fluid at four times the pressure requiring eight times as much power. In my off hand remark, I thought “1/7 volume is 1/7 rpms for 1/49 the pressure……won’t work move on”.
When I’m responding in comments there are a few things working against me. My day job requires me to make quick decisions. Not life threatening decisions, but definitely checkbook threatening. When you have four employees, a concrete truck, a pump with crew, and an inspector to deal with when something goes wrong, it is costing $5-10 dollars a minute for me to figure out a solution. So if I dither forÂ ten minutes, that’s a days’ wages for some people. And if the solution is wrong, it gets even more expensive fast. So I tend to fast decisions that are good enough to keep things moving, though never perfect. In comments here, I’m not in my normal field and often I realize a mistake five minutes or a couple of hours after I post it.
Another problem connected to the first is that I try to read the comments of others, and then think about it while I’m doing something else, so sometimes I take a while to respond or even forget to. That part is a balance that I haven’t learned yet.
Another problem is that I am not sure how strong I should defend these off the wall concepts. An idea not worth defending is not worth posting in the first place, except maybe for humor. On the other hand, we all know people that have an idea they are pushing that just won’t listen to anything. After a while most quit trying. I try for the middle road, defend enough to stir the pot, but not enough to put a lid on it.Â Â Â I get it wrong a lot, but it seems to be working in that almost everything I have posted here Â is improved by the input. Yes the circular file is an improvement if the idea sucks.
An hour after replying to Eric, I realized that there just might be an effective way of deep throttling the pump by recirculating some of the output back into the pump feed line as a jet pump. While jet pump efficiency is terrible, in this case it is a matter of getting use out ofÂ equipment that is mostlyÂ there anyway.
As the volume required is reduced for throttling, a diverter valve sends some of the high pressure output propellant back into the feed line as a jet pump to increase the pressure of the fluid entering the pump impeller. The pump impeller pressure rise is additive to the inlet pressure so that the inlet pressure increase allows the impeller to slow down and still deliver the same output pressure. I got tangled up in the numbers trying to figure this out, so I’m throwing it out there to see what happens.
If rpms are 67%, then volume through the impeller is 67%. Impeller pressure rise isÂ ~45% of full speed.Â three quartersÂ of the propellant is tapped off to drive the jet pump to increase pressure on just a thirdÂ as much fluid. With the poor efficiency of the jet pump, the pressure rise before the impeller is a bit over half the difference between the high pressure pump output and the low pressure inlet. The result is enough propellant to drive one chamber at full pressure, though with a serious efficiency loss. With the turbine sized for full flow, there is more than enough power available, even with terrible system efficiency.
This would also allow for the lighter simpler drop tanks Eric suggested. Jsuros suggested pistonless pumps in the drop tanks. That might be another way I haven’t wrapped my mind around yet.
* At current rate of progress, about 1985 I should be a millionaire.