Propellant Tanks – Case Study

For this post, I’ll go over the basic design of the fuel tank on the Earendel Sounding Rocket.

Step one in any design is the requirements, so here we go:

– 9.6 kg of Isopropyl Alcohol

– 220 psi tank pressure

– 2.5 x Factor of safety

– 3% Ullage (% initial gas) , 3% residual propellant (unburnt at the end)

– For a Sounding Rocket so cylindrical form factor

With these requirements, let’s get started with sizing the propellant tank. The first step is the volume of the tank. At a density of 785 kg/m^3, that gives us a propellant volume of 746 in^3 and a tank volume of 791 in^3. If we want a 4-1 cylinder, that gives us a 6.25″ diameter. But to match up with a standard composite tube of 6.02″ ID, we will choose 6.02″ as the ID. Now, for the endcaps, we choose a 60% ellipse as needing roughly the same wall thickness as the sidewall of the cylinder, as you can see in the chart below.

Now this gives us a tank with a 6.02″ ID wall, 25.4″ tall, with 1.8″ tall endcaps. You can see the rough size below and it looks pretty reasonable.

Now a stress analysis for the wall thickness. We will use aluminum 6061-T6 as it is common in 6″ pipes and tubes as well as rods for the endcaps. It has a 42 ksi ultimate strength, and a 35 ksi yield so we can just use the ultimate. The sidewall calculation is fairly easy with:

42 ksi / 2.5 = 220 psi * (6.02″ / 2) / t      ==>  thickness = 0.0394 that we will just round to 0.040″

For the endcap, using the modifier from SP-125 Fig 8-7 of 0.92 and equation 8-19, we get 0.0344″ for the ellipse thickness. Let’s just round that up to 0.040″ as well to give us margin on any discontinuities.

Now for attachments. There should be a bulkhead on both of the endcaps; let’s make it 2.5″ diameter as, from my past experience, that is about as small as is useful for multiple ports. We could also choose to machine ports directly into the tank. This is more expensive and harder to machine and clean, but with the benefit of less seals and a robust mount. So using our flat plate stress analysis, and assuming a bolt diameter of 3.0″, that gives us a thickness of  0.075″. The pressure load is good for 3900 lbf with the 2.5x FoS so that is either 20 #4-40 screws or 14 #6-40 screws or 9 #8-32. So we will just use 10 #8-32 screws.

And the last attachment is the endcaps, which we will use bolts for again. Since the stress area will be around 5.5″ after the bolts go in, that is a load of 13,100 lbf. We want around 20 bolts so that means we have to use 1/4-28 bolts, once again assuming we have Al-6061 bolts.

One word about aluminum bolts: The most common bolt is 7075-T73 or 2024-T4 which are OK for cryogenic service and are stronger than 6061. So we are being conservative with the bolt strength. Please double check your vendor though because some bolts are 3000 or 1000 series alloys which are very weak by comparison.

Now for the seals. There are two of them: the bulkhead and the endcap. We will use viton o-rings as they are cheap, available, and work very well. We will also use 1 O-ring per seal; there is no redundant seal, which is fine for our application, but sometimes frowned upon in high risk fault intolerant systems. I highly recommend the Parker O-ring handbook. In this case, I used it and will be using a face seal gland with a #38 O-ring on the bulkhead and a #160 O-ring on the endcap.

This is the end of the initial design. At this point, you will want to model the tank, check its weight, run a stress analysis, and see how it lines up and mounts with other structures in your system.  Good luck, and be prepared for the inevitable redesign!

Propellant Tank – Joints

Successful creation of propellant tanks requires proper sizing, proper analysis, and proper joint design. This post we will cover various joints in metallic tanks and then we will complete the tank post series in the next post with a case study.

There are 2 main ways to make joint on propellant tanks: Bolted or Welded. Other methods exist such as brazing, epoxy, rivets, etc., but 99% of the time either welding or bolting are used.

Welding is generally used to join the major structural pieces of propellant tanks with the main welds longitudinal welds to form a cylinder from a rolled segment, circumferential welds to join cylindrical segments and endcaps, and, in the case of very large tanks, gore welds to form endcaps. Small tanks can avoid some of these welds, but sheet stock is usually limited to 144″ x 72″ so anything larger will require many more welds.

Most welds on propellant tanks are either butt welds or  lap welds as shown on the picture below.

It is almost always desired to have thicker material at a weld to minimize any stress associated with weld discontinuity and lower strength due to heat affected zones. As a rule, try to minimize the number of welds and keep welds away from high stress areas; for instance, by making the dome of the tank extend a half tank diameter into the body before welding on the cylindrical section. This may cost more to make, but it should also have better performance. In addition, you can make tank cylindrical segments from spin forging, eliminating the high stress longitudinal weld.

One final note on welding is that most materials lose strength when welded so either account for that in thicker parts, or plan on post weld heat treatment. Heat treatments need to have minimal thermal shock as the large, thin structures of a tank are prone to buckling or deforming under high thermal loads such as quenching. For most heat treatable aluminums, this means only aging after welding, not a full solution heat treat.

The other option for connections are bolts with seals. Most propellant tanks, even composite or large welded structures, have bolted port holes or man hole covers. A simple method, or at least comparatively simple, of constructing a propellant tank is a seamless tube with flat caps at both ends.

The first part of the system is the bolts. These are ideally the same material as the base structure, or at least the same CTE (Coefficient of Thermal Expansion), to ensure changes in temperature do not over-stress or unload the joints. The two main gotchas on the system are you need to pre-stress the bolts to a higher level than they are stressed in the pressure load case to minimize cyclical loading and bolt pull out strength is a function of the base material or the thread material, whichever is weaker.  So if you are using 7075-T6 bolts on a 5052 H32 tank, you need to calculate the stress assuming the thread will rip out of the 5052.

Part number two is the seal. For a fluid between -65F and 200F, just use an o-ring. Most of the time either Viton or silicone will work, and Parker has a great o-ring handbook for design guides. Outside of this temperature range, the easiest option is a Teflon gasket or sealing ring, and the best option is a metal o-ring. Size the Teflon sealing ring just like an o-ring but make the Teflon a square cross section instead of a circle for easy fabrication. It will smoosh out in use and is usually reusable until you take the seal out of the groove. They are cheap and easy. A metal o-ring or c-ring, otherwise known as a metal energized seal, will work very well on a flat and good surface finish surface. They can work down to liquid hydrogen temperatures and up to ~1200F, but they are expensive and long lead items and, if you scratch the sealing surface, that part is scrap. So only use metal seals if it is colder than LN2 or hotter than 400F.

I hope this was informative, and we’ll do a case study for the Project Earendel tank next week.