Propellant Tank Vent Valves

As the propellant feed system was being developed for MOCH4, I identified a key area in need of improvement: propellant tank venting. The implementation on PTR worked, but was suboptimal in a few ways. Recognizing the opportunity for optimization, I felt confident in taking ownership of redesigning the propellant tank venting system.

Project Overview and Conceptual Design

The PTR vents are depicted above and below. As I've written before - manual ball valve and electronic solenoid valve in parallel. I already mentioned why this configuration is imperfect, and the image above further highlights the pure bulk of all the fittings and tube needed to route these valves within the rocket (green highlighted paths). And this is required for both propellant tanks! One main goal with my redesign was to consolidate these into a single valve and reduce mass and packaging size along the way.

The image to the right shows my buds Thomas and Gabe in the tractor "mancage" at FAR closing the manual ball valve vents on our rocket during launch operations. They are suspended ~10ft in the air in a rickety basket, operating valves on a rocket that has both LOX and LNG tanks fully loaded, with ~3000psi in the COPV. This sucks. This should not be part of our venting solution. Another goal of my vent redesign is to eliminate the need for manual actuation. My valve will be remote actuated, and will fail open to ensure nobody has to go up to the rocket and perform procedures that risk their safety.

In summary, the goal of my vent valve redesign is to consolidate into a single remote vent valve (1 per prop) with adequate CdA that fails open, bearing strong potential to decrease envelope and mass.

Based on the new engineering design process I helped develop, my first step was to work towards a Conceptual Design Review, largely focusing on preliminary requirements, component working principle and trade studies, and a conceptual design.

Copy of VentValveCoDR

After researching commercial and custom solutions, I landed on a custom piston style valve as the optimal component for the needed function. My biggest concerns with a custom solution were feasibility and price. This would be the first actually custom valve the UCIRP has developed, and based on my prior internship experience with valve design, I was aware of how expensive these things can get. The feasibility concern was easily alleviated, as I was very confident in my abilities to complete this, and I did have some relevant experience to lean on. Any optimized solution to this vent redesign was going to be expensive, and while I couldn't place a precise guess on how much this valve would end up costing, I was confident I could do it relatively inexpensively by leveraging the fact that my labor is free. Therefore, I would wear all the hats, especially machinist, and all I would pay for is materials. I did a very early instant quote on a mock housing part. This is shown below. If I were to outsource the machining for this project, it would easily reach into the thousands of dollars.

Moving on from the CoDR, I began working on a detailed design for my vent valve. A large majority of the work during this phase was CAD, sizing calculations, component selection, dev test planning, and manufacturing planning. A side note - one of the hardest parts of this project was that nobody on the team knew much about valves, so getting help or asking for peoples thoughts on my design was quite challenging. I will shoutout my friend Michael and some OG rocket project alums - Owen, Austin, Sebastian, Wes, and George - that all provided extremely valuable insight during my detailed design phase.

I went through 4 major versions before I arrived where I am at now (03/12). I will briefly recap the major changes (I'm skipping a few minor ones). I will firstly give a brief explanation of the various parts and functions.

Brief Valve Explanation (on older version)

Although this explanation pertains to an earlier version, it should still offer sufficient information to comprehend the current version discussed later.

The spring stays in the middle cavity, it does not phase through the valve. The orange parts are Rulon J bushings, the white parts are PTFE spring energized seals / poppet tip. The poppet parts, housing, and fittings are all brass. I am machining all of these parts, so I didn’t really want to deal with stainless, and while I think Al would've been fine, I wanted to take less gox compatibility risk.

The inlet is the top vertical port which is routed to the tank. Spring + tank pressure opens the valve, pushing left on the poppet part that retains the spring.

A 3/2 solenoid is used to de/pressurize the left side w GN2 to change valve state. I might implement an orifice to control actuation time. The ½ NPT plug on the left is drilled out 90deg, and acts as a hard stop for the poppet. I might just switch it to a 90 fitting if I have extra space in the vehicle. The fitting on the right is a ⅜ NPT plug drilled thru with conical sealing geometry.

Open

Closed

Revision History

V1 - Piston Thing

Basically just a more realistic representation of the sketch I made in my CoDR. I wont get into the details, but I essentially scrapped this one as I had very large concerns about the rightmost spring energized seal (SES) catching on the inlet port (vertical holes in housing perpendicular to poppet) when actuating.

V2 - Traditional Poppet

This was probably the biggest change throughout all the revisions. I shifted my focus to a more traditional poppet style valve with a seat, eliminating the seal shearing concerns brought up in V1.

V3 - Seal Shift and Part Split

I had underestimated how much deeper my drill would have to go than my tap, so the leftmost SES in V2 would actually go into the larger drill diameter shown in V3 upon valve actuation. I moved it to the right of the bushing, so now it should only see the smooth housing interior wall. I also split the poppet up into 5 parts instead of 3. I did this to reduce complexity of each part, as another huge challenge of this project is working around the machine shop hours.

V4 - Almost There

Returned to the 3 poppet parts as they actually weren't that much harder to machine. This meant I had to figure out a way to cut slits into my bushings (see more below). Moved a SES to the right for better force balance. Added bleed hole in housing between seals to vent any minor interstitial leaks and prevent seal blowout.

V5 - I Think I'm Done?

Final spring energized seal gland geometry and 20% housing mass reduction through excess material removal.

Somewhere in between V2 and V3 I held my PDR. I'm just gonna drop that here and move on because there is too much information on this page for me to organize it perfectly. Most of the PDR information will be brought up and explained below.

Copy of VentValvePDR

Valve Design (Current Version)

Essentially every component in this system required some sort of sizing calculation or analysis in order for me to make informed decisions. These analyses are presented below.

Fitting/Tap/Drill Depth

I am using NPT fittings to seal and interface with my valve, and act as hard stops for the poppet motion. When the valve closes, the poppet seat slams into the conical sealing surface cut into the 3/8" NPT fitting shown to the right. When the valve opens the poppet gets pushed to the left until it hits the 1/2" NPT fitting shown to the right. The NPT fittings need to be installed deep enough for the male and female thread tapers to mesh in order to form a seal. The tap depth is also a variable here.

In order to determine how deep I needed to tap, and how deep my fitting would need to go in order to seal with a reasonable amount of teflon tape, I threaded the taps I planned on using into a COTS NPT manifold. When hand tight, I measured the length from the manifold face to the end of the tap shank, then subtracted from the total tap length to determine tap depth. I then threaded the fittings I planned on using in the same manifold. As shown in the fitting measurements spreadsheet below, I measured the distance between the manifold face and the fitting hex as a function of turns. I then fit a curve to this data to obtain an equation for fitting depth as a function of number of turns. I used this equation to predict number of turns for a fully tightened NPT fitting.

In order to determine how deep I needed to drill, I made the hole test block on the right. I drilled and tapped in increments until I could achieve the desired tap depth. Due to the amount of lead-in chamfered threads on my tap (I don't think this is standardized), I had to drill deeper than I hoped to achieve my desired tap depth. I didn't want to have to drill that deep though, as it reduces the amount of available contact area I have for my bushings. To reduce the depth of the hole I had to drill, I purchased another tap and angle grinded about 0.15" off the tip. This allowed me to reduce the required drill depth as I could achieve larger diameter threads at shallower tap depths! I would tap with the unaltered tap first, then finish with the shortened one.

Open Closed

FittingMeasurements

The trendline got nuked when I imported this into google sheets

Force Balance

An internal force balance is one of the most important analyses to verify the valve will function as expected. The concept is pretty simple - just add up the internal forces and make sure the net force is in the right direction and of reasonable magnitude. The nuance is ensuring the internal force balance checks out in every state the valve can be in. When I want the valve to be closed, I want the net force to be to the right. When I want the valve to be open, I want the net force to be to the left. Until I did testing, I could not predict the internal friction, so that is omitted (Read more in testing below). Another thing I need to consider is the stress the poppet tip endures at a given closing force. This force balance was used to determine the seal sizes I would use, the spring I should use, what my actuation pressure should be, what 3 way solenoid and regulator I should use based on the actuation pressure, and probably more I'm forgetting.

ForceBalance

Tolerancing

My valve needs to operate at some pretty extreme conditions. Cryogenic temperatures will cause parts to shrink - some more than others. Ensuring the valve operates nominally at ambient and cryo conditions requires careful part dimensioning and tolerancing. I performed a tolerance stackup on internal parts at LMC, exact, and MMC tolerances, at ambient and cryogenic temperatures. As shown below, I have no cases with negative tolerance or unreasonably high clearance, meaning all of my parts are toleranced adequately. Based on some literature review, I chose an RC5 running fit for my bushings in the housing, and an LC4 for the brass part locational connections.

Tolerances

Sealing

I am using the only off-the-shelf spring energized seals I could find for my radial sealing. The specs are super vague and are different across distributors... One of my biggest concerns was determining what gland geometry to use. I considered using Orings as I could use the parker guide and calculator as a crutch, but found that they had muchhh more friction to the point where I was forced to use SES (more in testing below). The spreadsheet below shows my actual measurements of the seal as well as recommended gland geometry (at least that's what I think it is) from the Grainger product page. I was still very suspicious of the recommended values. I even reached out to the manufacturer to ask if they could provide recommended values and they were no help. Therefore, in one of my first sealing tests I machined a few parts with different gland geometry to close in on "good" gland geom. I ended up going with a gland rod diameter of 0.4" for the 9/16 seal and 0.525" for the 11/16 seal, and both have been sealing wonderfully.

SealGlands

Structural

I performed some basic pressure vessel calculations and backed them up with a internal pressure load FEA with NX Nastran to ensure my valve wont explode when pressurized. The simulations match pretty closely to the analytical solution.

PresVesselCalcs

Von-Mises Stress (psi): Linear statics - Internal pressure load 500psi - Inlet and pneumatic ports fixed - Ambient Temp - Tetra10 elements

Von-Mises Stress (psi): Linear statics - Internal pressure load 500psi - Inlet and pneumatic ports fixed - Cryo Temp - Tetra10 elements

^cryo case with custom material. Kept everything the same as stock brass except:

Youngs - 1.6e7 psi
Temp: -180C
CTE: 0.0000118 1/K

I attempted to do a FEA to better calculate seal stress when squishing the poppet seal against the brass conical sealing surface. The result seems at least on the correct order of magnitude but visually the simulation looks really weird, so I may have done something wrong.

Von-Mises Stress (psi): Linear statics - 20lbf push on poppet - screw countersink and fitting end fixed - Amb Temp - Tetra10 elements

Flow

I ran a CFD simulation using NX Flow to estimate the valve CdA. ADD MORE

Fastener Torque

FastenerTorqueCalcs

Valve Manufacturing

I manufactured almost every part on this valve for cost savings and personal interest. The only parts I did not make are the screws, fittings, spring energized seals, helicoils, and spring. I am accustomed with GD&T, but since I was machining these parts, spending the time to make my drawings more "to spec" was not super valuable. My inspection abilities are also pretty crude. All of these parts were made on a manual mill and lathe.

3D Printed Bushing Slitting Jig!

Valve Testing

Vent Valve Test Article 1

Vent Valve Test Article 2

Vent Valve 1

lol

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