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Hybrids For 2015
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I build my own drag bikes. I have used NOS in the past and have a dry system I plan to use soon.
We have had digital controllers as small as 2 x 4 inches that do variable pulse control of the solenoids.
They can throttle from 20% - 100% in multiple ramp speeds and duration.
I was planning on using one of the mini controllers and a 14v Lithium RC battery to drive it and the solenoids.
I am not a fan of carbon fiber bottles but you could make a tank like in the Contrail system pretty cheap.
You could pre-fill the tank, which would have a ball valve with an allen screw. Connect the tank to the solenoid with a 4AN and 6AN Stainless hose.
Then you could open the tank when it is full assembled on the pad with an allen wrench.
The trick will be:
1) to configure some micro relays to trigger the ramp routines
2) have multiple solenoids or one large solenoid large enough to supply the flow rate needed for the motor.
You would have to use high quality racing solenoids to pulse correctly and not fail.
I was thinking of launching at 100% then after say 1000' , throttle back to 75% for the duration of the burn.
What were your thoughts?
AlphaHybrids
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Throttling a hybrid you definitely need some sensors on the motor to help. With your idea I don't think I'd pre-fill a homemade tube tank and transport it. A DOT rated tank would be much safer.
The design I am currently developing is using the servo plug valve from Rocketmoonlighting. Plug Valve I have a pressure transducer connected to the chamber and one also connected to the nitrous tank. The inital program for the valve monitors a pressure differential between the chamber pressure and tank pressure. You don't want them to reverse and splatter your rocket over the field. The 2nd priority of the controller is to maintain a set chamber pressure during the burn. Chamber pressure relates to thrust and this is one way to throttle. The motor works at 100% open in case the valve gets stuck open. I didn't want full open to result in too high of an Ai/At ratio. Approximately 75% flow is the design parameter for the motor. It starts off at nearly full open for 2.5 seconds, then throttles back to the design thrust. Then, because the pressure in the tank is dropping the valve slowly starts opening and keeping the chamber pressure at design thrust. When the pressure in the nitrous tank reaches 100 PSI (blowdown phase) the valve closes and keeps approximately 50 PSI in the nitrous tank preventing any backflow of combustion products into the nitrous tank to keep it clean.
In the cold flow tests with CO2 and using a ball valve to simulate a nozzle we were able to keep the chamber pressure within 25 psi of design even when we drastically opened or closed off the outflow from the combustion chamber. The first hot fire we did we just opened the ball valve 100% and tested at full throttle. We haven't had time to test the throttle but I need to make a small injector change and then we will move on to throttling the hybrid.
Edward
The design I am currently developing is using the servo plug valve from Rocketmoonlighting. Plug Valve I have a pressure transducer connected to the chamber and one also connected to the nitrous tank. The inital program for the valve monitors a pressure differential between the chamber pressure and tank pressure. You don't want them to reverse and splatter your rocket over the field. The 2nd priority of the controller is to maintain a set chamber pressure during the burn. Chamber pressure relates to thrust and this is one way to throttle. The motor works at 100% open in case the valve gets stuck open. I didn't want full open to result in too high of an Ai/At ratio. Approximately 75% flow is the design parameter for the motor. It starts off at nearly full open for 2.5 seconds, then throttles back to the design thrust. Then, because the pressure in the tank is dropping the valve slowly starts opening and keeping the chamber pressure at design thrust. When the pressure in the nitrous tank reaches 100 PSI (blowdown phase) the valve closes and keeps approximately 50 PSI in the nitrous tank preventing any backflow of combustion products into the nitrous tank to keep it clean.
In the cold flow tests with CO2 and using a ball valve to simulate a nozzle we were able to keep the chamber pressure within 25 psi of design even when we drastically opened or closed off the outflow from the combustion chamber. The first hot fire we did we just opened the ball valve 100% and tested at full throttle. We haven't had time to test the throttle but I need to make a small injector change and then we will move on to throttling the hybrid.
Edward
Please keep us posted on your progress.
AlphaHybrids
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Having combustion particles in the tank is something that typically happens with closed nitrous tank systems. The open vent style of monotube hybrids I have not seen this happen.
For the cost of two pressure transducers ($70 total) and an Arduino ($50) you have almost everything you need for a closed loop feedback system. The very first generation I did was on an Arduino but then a friend joined the project and we transitioned a PIC controller. I think I had maybe 150 lines of code for the first generation. Most was just making sure it knew where to go after it adjusted the plug valve. The PIC controller used a PID loop to adjust the chamber pressure.
A DOT 3AL tank that has a rating of 1800 psi will have a minimum test pressure of 3000 psi and minimum burst pressure of 4500 psi. Both of these are beyond the 1800 psi rupture disc pressure. I don't think the Contrail is rated for either of those and would be an unwise choice to pre-fill and transport, unless you like hugging untested pressure vessels.
Edward
For the cost of two pressure transducers ($70 total) and an Arduino ($50) you have almost everything you need for a closed loop feedback system. The very first generation I did was on an Arduino but then a friend joined the project and we transitioned a PIC controller. I think I had maybe 150 lines of code for the first generation. Most was just making sure it knew where to go after it adjusted the plug valve. The PIC controller used a PID loop to adjust the chamber pressure.
A DOT 3AL tank that has a rating of 1800 psi will have a minimum test pressure of 3000 psi and minimum burst pressure of 4500 psi. Both of these are beyond the 1800 psi rupture disc pressure. I don't think the Contrail is rated for either of those and would be an unwise choice to pre-fill and transport, unless you like hugging untested pressure vessels.
Edward
I received my new (to me) Alpha Hybrids hardware and reloads last night. Also picked up a simple GSE set-up and a 10# tank that has a little nitrous in it. Everything I need to fly, except for the airframe...
Hardware:
Nozzle image 1...
Nozzle image 2...
GSE (minus a scale)...
[/URL]
Reloads w/ Pyrodex pellets (reloadsy look color coded)...
Looking forward to flying some of these loads this season.
Hardware:
Nozzle image 1...
Nozzle image 2...
GSE (minus a scale)...
Reloads w/ Pyrodex pellets (reloadsy look color coded)...
Looking forward to flying some of these loads this season.
AlphaHybrids
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I've seen all those before. I'll send you over some other things soon.
Something like this reload
Edward
Something like this reload
Edward
@ Edward, pls keep posting about that beast, it is extremly sexy on the paper
@ dragon64: yeah I opened my 2015 hybrid rocketry year yesterday... 3 motors prepared, 2 beautifull flights and one non-starter.
We tried first to fly my beloved eagle-claw 4 with a K sparky at mid speed. Could not start a proper launch procedure as the fill and dump valves were immediately leaking. After several test we dismantled both fill and dump valves on the field. It was just dirt inside. Easy repair on the field...but the ignitors have been twice frozen and defrozen and were simply refusing to ignite anymore, too humid...Maybe I was swearing too much about the delivery delay of my black sales order 2014..so the eagle claw refused to pleased me ;-)
So we continued with another scratch bird in 4", a light weight glassed phenolic with a lovely contrail J...in WHITE SMOKE and slow nozzle. It went very long into the bavarian skies. WHITE SMOKE is the new fuel formula from contrail. As the name says...it has NO smoke *lol* but an incredible tense white flame....Went just perfect in the sky and cool recovery.
So we finished with the mighty KRAMER X4, now her flight #4, on her favorite fast nozzled K555 BLACK GOLD, a 1200Ns peak thrust beast to lift-off the 38 pound nazi gorilla :kill:
It went just perfect all the way thru.
Now I had a new friend onboard: the full kevlar tubular harness from ONEBADHAWK that did the work just bravely!
Film of said flight coming soon..
Now what I cannot really tell.... is that we share the field with an official student group of the german spatial state agency .. and a 30 men crew and 3 trucks... against me and my friend. They had tons of HW, flat screens under tents etc...and a comparable rocket to our phenolic in one in diameter and length. They went also hybrid, but homemade.
We went good. They had a professional setup.....but...no real high power experience. Result: fin flaterring and broken in flight, motor explosion just above and last but not least a destroyed harness and hard landing pieces without chute.
National Space Agency vs amateur rocketeers 0:2 :kill:
But I must say they were very nice guys to discuss and they even shared nitrous with us, as we went empty with this non starting wildman bird.
It was a great saturday!
I never scrimp on fin thickness if I suspect fin flutter.
Having combustion particles in the tank is something that typically happens with closed nitrous tank systems. The open vent style of monotube hybrids I have not seen this happen.
For the cost of two pressure transducers ($70 total) and an Arduino ($50) you have almost everything you need for a closed loop feedback system. The very first generation I did was on an Arduino but then a friend joined the project and we transitioned a PIC controller. I think I had maybe 150 lines of code for the first generation. Most was just making sure it knew where to go after it adjusted the plug valve. The PIC controller used a PID loop to adjust the chamber pressure.
A DOT 3AL tank that has a rating of 1800 psi will have a minimum test pressure of 3000 psi and minimum burst pressure of 4500 psi. Both of these are beyond the 1800 psi rupture disc pressure. I don't think the Contrail is rated for either of those and would be an unwise choice to pre-fill and transport, unless you like hugging untested pressure vessels.
Edward
You are right once I, too, thought about the engineering of a DOT Al tank.
I would opt for a co2 or other light weight, rated bottle.
I am not a fan of carbon fiber bottles. A dragracer died after a mishap in a trailer after filling a carbon bottle.
Shards of fiber strands and the valve going in all directions! OUCH.
Also, their QA/QC might be great on paper but you depending on (who knows) a person to have clean fabric, mandrel, mold, perfect layup and vacuum bagging, yeah, not a fan!
I also drag race motorcycles and see carbon bottles under the seat of bikes.
If you had a huge fuel leak and fire engulf the bottle, you might get a tank valve fired at your crotch.
Besides, if the rocket took a hard hit on the ground I would be compelled to destroy the bottle, to.
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Love that motor burn Edward, almost looks like a White Lighting clone.
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I finally ordered a valve which should work for my EX M hybrid project. The valve's purpose is to vent N2O for evaporative cooling, effectively setting the flight tank's pressure, temperature, and density to known valuse right before ignition. If one doesn't control for it, N2O density can be all over the place since it is a strong function of temperature. Hopefully fixing the flight tank's N2O condition will yield much more consistent burn results regardless of ambient temperature. Density is higher than normal so tank length is reduced a few inches for the same total impulse. Perhaps I'll actually do the machining soon. I still need a few items but that valve was one of the biggies.
Gerald
Gerald
AlphaHybrids
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https://www.spiegl.org/rocket02/solusn/
Gerald,
That page may help you a lot in doing what you are doing - Mark has went down the path and solved many of the problems that colder than normal N2O can have.
Edward
Gerald,
That page may help you a lot in doing what you are doing - Mark has went down the path and solved many of the problems that colder than normal N2O can have.
Edward
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(A) venting through an inexpensive relief valve to establish the tank pressure. Pressure then establishes the temperature and density through boiloff. (B) Not monitored; factory set valve of acceptible precision over operating temperature range and not really all that worth monitoring for first attempt, sort of a proof of concept. Trying the simple and cheap approach first. (C) Not done yet.
Gerald
Gerald
So lets bump this topic, who has been flying hybrids for the month of June? For financial obligations, I am still without GSE, my only excuse at the moment.
Bump for the month of July...anyone? I am building a new rocket, the Adventurer3, and rebuilding two current fleet rockets to fly this fall, and (like broken record) I am still trying to finalize some ground support equipment... the launch is easy, compared the trials of getting a manifold built for my tank.
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I improved my hybrid design to simplify the machining a bit, and lengthened the combustion chamber. The short chamber was going to be too restrictive of fuel combinations and grain geometry. The original injector assembly wasn't readily modifyable if testing indicated flow rate comes out wrong. Now the injector assembly is swappable. In a few weeks I'll get to machining; need the right metal and a bit of time... I plan to static test this fall.
The plan was on hold with the lowered Potter NY waiver. I lowered the total impulse a little in the redesign to drop a few thousand off the max possible altitude so it should still be flyable - assuming it works of course.
I've had very little time for my own rocket projects these last few months.
Gerald
The plan was on hold with the lowered Potter NY waiver. I lowered the total impulse a little in the redesign to drop a few thousand off the max possible altitude so it should still be flyable - assuming it works of course.
I've had very little time for my own rocket projects these last few months.
Gerald
Alright, looks like I have my GSE woes well in hand... I have ordered a manifold for my 20lb tank from Black Dog Rocketry, and KarlBaum here on the forums, is pulling together the components for a my ground support, including a vent sensor. So I can finally move forward again... looks like I will be launching again this fall.
I have been assisting forum member Stardust on flying his hybrids at our monthly club launches. Maybe he will get on the forum and chime in. He just sent a WAC Corporal up on an H90. All the folks at the launch are real interested, some club members think maybe we a bit crazy to fly such motors. So what if they are not as easy as AP, the sound of the H 90 and a longer burn are worth it. And up to the 3.3 pounds weight limit is they only thing the hybrid flyer needs to worry about, no heavy and inefficient black powder limited to 125 grams.
The issue we had was a short hiccup in power right after ignition; a little scary as it went up the rail. Maybe that chunk of blue thunder burned too fast before the plastic melted and came to full power? Maybe some slower burning white lightnin' will do better?
Just look at those guys way out at the far corner of the range. How silly are they! Look - one of them just wrapped a towel around the tank and poured water on it! They have their own launch system and I think one of them said something about taking a dump! How crude! Are they ever going to get it to work? Is it real rocket science? Are they going Hybrid Crazy? If the regulators have their way they may be the only one's flying.
The issue we had was a short hiccup in power right after ignition; a little scary as it went up the rail. Maybe that chunk of blue thunder burned too fast before the plastic melted and came to full power? Maybe some slower burning white lightnin' will do better?
Just look at those guys way out at the far corner of the range. How silly are they! Look - one of them just wrapped a towel around the tank and poured water on it! They have their own launch system and I think one of them said something about taking a dump! How crude! Are they ever going to get it to work? Is it real rocket science? Are they going Hybrid Crazy? If the regulators have their way they may be the only one's flying.
grouch
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Thanks for keeping this thread going guys. I am interested in trying hybrids one day but I want to learn a lot more first. I have not even seen a hybrid fly yet but I like the fiddling aspect of it.
Good on you for helping Stardust, we need more mentors for fliers who want to know more. There may have been enough time to pass, to issue in another generation of fliers that don't know they would like hybrids... and we should educate them all we can. Love the sound and the long burn myself... But! HyperTEK has a fast burn "I" motor for the 440 system, that will thump your chest when it launches... Also very very cool.
There are a few fliers out there, but not many of them post... almost like many of the "gassers" have gone underground so to speak. As for the fiddle factor; hybrids have it's share of fiddle factoring... that is one of the qualities I really like about the propulsion system. Be warned though, hybrids have very little flavoring in that there are few special effect possibilities... the saving grace for some, is that Contrail Hybrids does make an awesome sparky reload for their motor systems... Nevertheless, hybrids ROCK! Hope you get to see some fly in person.
I just watched Austin Powers again and I am reminded of the scene where Dr. Evil and his son Scott are mulling over what to do with the captured Austin Powers. Dr. Evil has Austin and Vanessa dressed in a pseudo modern outfits and placed in a slow moving, easily escapable trap with an incompetent guard. Scott just wants to get his gun and shoot them now. Dr. Evil tells Scott he just doesn't get it and to ShussHH! Dr. Evil must be a dedicated hybrid flyer. Why mess around with hybrids when you can easily put an igniter in a superior AP motor and rock and roll? ShusHHH! Why try to light a bunch of inefficient clustered BP motors when one super cool AP motor will rock? ShussHH! Why fly a silly, canted and tractored motor oddroc when you know 4FNC with a composite motor in back that is by far better, safer and more reliable? ShussHH! and Zip it!
I did see a big hybrid rocket launch at the first high power launch I attended in '07, but none since assisting Stardust with his 29mm stuff the past few weeks. When starting out it is a good idea to do it as a team since there is a lot to learn and keep track of. You need a "mini me" to move and keep the tank at the right temp and pressure, and to re read the directions and hold things during the motor build. Quickly it becomes second nature and not so hard. Now after years of having stuff in storage Stardust has gone hybrid crazy and is going to use his big stuff on V2's and early Soviet designs.
I did see a big hybrid rocket launch at the first high power launch I attended in '07, but none since assisting Stardust with his 29mm stuff the past few weeks. When starting out it is a good idea to do it as a team since there is a lot to learn and keep track of. You need a "mini me" to move and keep the tank at the right temp and pressure, and to re read the directions and hold things during the motor build. Quickly it becomes second nature and not so hard. Now after years of having stuff in storage Stardust has gone hybrid crazy and is going to use his big stuff on V2's and early Soviet designs.
I would love to have someone local to share my rocket adventures with. I know of one flier, who currently resides in Michigan, who says that when he returns to Alabama we can start flying hybrids together... that won't be until sometime in the middle of 2016. He is busy flying remote control jets I believe... so he is well aware of the fiddle factor involved.
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I received the longer 76mm motor to be the new combustion chamber for my EX M project, and have ordered the aluminum I need to machine for the rest of the parts. I'm figuring on a first static test in a few months if I can get some help with the GSE. I don't have my own yet and won't by then.
If this motor works out well enough, I hope to fly it as an intentionally overweight min diameter in NY presuming the club grants me permission, and as the motor in a large but reasonably light rocket perhaps at Red Glare or some other launch in the spring.
Gerald
If this motor works out well enough, I hope to fly it as an intentionally overweight min diameter in NY presuming the club grants me permission, and as the motor in a large but reasonably light rocket perhaps at Red Glare or some other launch in the spring.
Gerald
AlphaHybrids
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What is your 76mm EX M dimension wise? I have an N that is 84" long and around 17,000 Ns. I'm waiting on a waiver now (October is the next chance) to fly it. I've been tinkering with the idea of using a servo valve to actuate nitrous flow to multiple motors. I typically use PVC for my liner because it is easily available and easy to machine. I cast my grain inside of that. The idea in my head is to use a 4.5 pound nitrous tank and connect that to 3 64mm motors. Small enough diameter that bulkheads and nozzles are easy to obtain and reloads are easy to make. I also know I don't think I could flow 4 pounds of nitrous through a 2.5 motor easily. We'll see.
Edward
Edward
I really like the K240 from RattWorks. I have only ever witnessed one fly, and it was a screamer! Literally, that motor has a unique sound all it's own. Hope you get to launch it sooner rather than later... no worries though, I'' start a Hybrids 2016 thread on Dec 31 all the same.
Are you planning to design/build your own GSE as well? I am having my own GSE built by a forum member. He has it boxed and will be shipping it soon. Black Dog Rocketry i supposed to ship my manifold by the end of this week. I'll get all of it posted on here hopefully by next week.
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Going by memory here as I don't have the stuff here at work, but...
The 76mm motor uses two segments of tubing, one for the tank and one for the combustion chamber.
The combustion chamber uses snap ring retention of a conventional nozzle that I remachined to my specs. The combustion chamber started life as a 3 grain Ed Romani case. For the min diameter flight, I have a metal fin can https://binderdesign.com/store/page13.html.
The forward bulkhead for the tank segment bolts on, and contains a valve being used as a pressure regulator.
The two segments are bolted together via an internal thick wall sleeve. The replacable injector assembly (so I can easily experiment) attaches to the sleeve. Assembled, it resembles a long monotube hybrid, but with the injector assembly bolted in. (or will look like that when the machining is done).
The tank tube is 42" or thereabouts. So the motor is a little over 5' long.
It operates at lower pressures than normal for an N2O hybrid. It gives up ISP doing this, but, the increased N2O density from chilling more than makes up for it in total impulse.
That's it in a nutshell, at least in theory. I'll provide some pictures when farther along. Right now it's some tubes, a fin can, a valve, and a nozzle.
For the GSE, I haven't decided what I want to do. Part of the problem is I have a much more ambitious second project in mind. That is a 5" diameter hybrid with swappable tank segments. It is an N in the minimal configuration, for a minimum diameter project. It could go up to full O potentially with a long tank. It could be flown in the east on the short tank for a relatively stubby rocket. It would be a long slender rocket with the long tank. The motor is designed to be CHEAP to make and easy to use. Part of the cheap is very minimal machining being required, and no big lathe work.
I have the makings - or much of them - for an 8" heavy rocket which may end up being the test bed for the 5" hybrid.
Most any GSE could handle a single fill at least for the 76mm M. But the big motor could gobble some nitrous, and would need higher flow rates during fill. That's $$$ and logistics.
So I'm hoping to get help from someone for at least the first static test of the 76mm, so I can postpone the GSE decisions.
Gerald
The 76mm motor uses two segments of tubing, one for the tank and one for the combustion chamber.
The combustion chamber uses snap ring retention of a conventional nozzle that I remachined to my specs. The combustion chamber started life as a 3 grain Ed Romani case. For the min diameter flight, I have a metal fin can https://binderdesign.com/store/page13.html.
The forward bulkhead for the tank segment bolts on, and contains a valve being used as a pressure regulator.
The two segments are bolted together via an internal thick wall sleeve. The replacable injector assembly (so I can easily experiment) attaches to the sleeve. Assembled, it resembles a long monotube hybrid, but with the injector assembly bolted in. (or will look like that when the machining is done).
The tank tube is 42" or thereabouts. So the motor is a little over 5' long.
It operates at lower pressures than normal for an N2O hybrid. It gives up ISP doing this, but, the increased N2O density from chilling more than makes up for it in total impulse.
That's it in a nutshell, at least in theory. I'll provide some pictures when farther along. Right now it's some tubes, a fin can, a valve, and a nozzle.
For the GSE, I haven't decided what I want to do. Part of the problem is I have a much more ambitious second project in mind. That is a 5" diameter hybrid with swappable tank segments. It is an N in the minimal configuration, for a minimum diameter project. It could go up to full O potentially with a long tank. It could be flown in the east on the short tank for a relatively stubby rocket. It would be a long slender rocket with the long tank. The motor is designed to be CHEAP to make and easy to use. Part of the cheap is very minimal machining being required, and no big lathe work.
I have the makings - or much of them - for an 8" heavy rocket which may end up being the test bed for the 5" hybrid.
Most any GSE could handle a single fill at least for the 76mm M. But the big motor could gobble some nitrous, and would need higher flow rates during fill. That's $$$ and logistics.
So I'm hoping to get help from someone for at least the first static test of the 76mm, so I can postpone the GSE decisions.
Gerald
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I received the metal I need to make the coupler assembly. In a test fit, it slides into the combustion chamber with very little in the way of free play. Actually, rotate it a bit once in and it locks up with no motion. I'll leave that mating surface alone as I couldn't ask for a better fit.
The coupler doesn't slide into the tank. It is slightly oversized. That's good. I'll make a surface pass and then it should fit snugly.
I now need to finalize the design of the coupler. I'd like it to be as short as I can get away with. That would conserve nitrous volume and propellant volume, while dropping motor recovery mass. Since the motor is primarily for a min diameter project where the motor is the body, the coupling can't have any free play. Longer is better for this of course. Otherwise, there need only be room for doubled O-rings at each end of the coupler, a doubled bolt circle at the tank side, and at least one bolt circle at the chamber side.
I plan to have the precombustion chamber liner based on 54mm liner and casting tubes glued up. I'll probably put a preheater in there as well since it would be so easy to do.
The combustion chamber liner is just going to be a good 76mm liner with high phenolic content. Initial propellant is going to be a cylinder made from HTPB with a few minor additives. It's pretty conventional at least for the first static test.
The final rocket is looking to be about 7' long.
When it gets farther along I'll start a thread, here or in the research section, or both. I'm looking forward to seeing how this all works out as it is essentially what I originally wanted to do for my L3, plus a few enhancements such as an EX motor!
I'd like to put video in the rocket as well. I need sources of extra mass anyway, to help keep the max possible altitude down.
Gerald
The coupler doesn't slide into the tank. It is slightly oversized. That's good. I'll make a surface pass and then it should fit snugly.
I now need to finalize the design of the coupler. I'd like it to be as short as I can get away with. That would conserve nitrous volume and propellant volume, while dropping motor recovery mass. Since the motor is primarily for a min diameter project where the motor is the body, the coupling can't have any free play. Longer is better for this of course. Otherwise, there need only be room for doubled O-rings at each end of the coupler, a doubled bolt circle at the tank side, and at least one bolt circle at the chamber side.
I plan to have the precombustion chamber liner based on 54mm liner and casting tubes glued up. I'll probably put a preheater in there as well since it would be so easy to do.
The combustion chamber liner is just going to be a good 76mm liner with high phenolic content. Initial propellant is going to be a cylinder made from HTPB with a few minor additives. It's pretty conventional at least for the first static test.
The final rocket is looking to be about 7' long.
When it gets farther along I'll start a thread, here or in the research section, or both. I'm looking forward to seeing how this all works out as it is essentially what I originally wanted to do for my L3, plus a few enhancements such as an EX motor!
I'd like to put video in the rocket as well. I need sources of extra mass anyway, to help keep the max possible altitude down.
Gerald
