µMLAS - The First of the Parts

My µMLAS build officially started with an e-mail to Gordon Agnello requesting another Forward Fairing and CM - oh, and while you have your lathe all set up, Gordy, can you do an airframe of the correct diameter too? Gordon was happy to produce more parts, though I'm not sure he stayed that way as we progressed. MLAS was done as a "concurrent design-build" project, which is another way of saying we were ordering hardware and putting it together while the designers were still grinding out drawings. Parts usually showed up on the shop floor just in time for installation. That moved things along much more quickly than they otherwise would have, but we occasionally found ourselves doing some rework when the design changed after some piece of first-generation hardware had been machined. Gordon got a taste of that while working on the µMLAS fairing: the MLAS aero team kept tweaking the original Sears-Haack (SH) profile, and every change resulted in another e-mail to Gordy. MLAS ultimately ended up with a "modified" SH shape ... flattened at the top for minimal drag below mach 1 (max velocity expected of MLAS in flight was about mach 0.6) and flared slightly at the bottom for a smooth transition to the cylindrical Coast and Boost Skirts. The mid section had a slightly more pronounced "flat cone" shape than a true SH - that and the aft flare helped move the aerodynamic Center of Pressure aft slightly. Gordon did an amazing job capturing all those details ... you'll find a link to his working drawing posted here under "Resources: µMLAS Drawings."

The results of Gordon's work are shown at right. The Forward Fairing and CM are both of fiberglass-over-foam construction in 1:20 scale to the actual MLAS. Gordon made the cylindrical section shown from a piece of 12" PML phenolic tubing by cutting out a section lengthwise, pulling the free edges together, and applying a fiberglass patch inside the tube. The CM was identical to the one he produced for µMRA with an open 2-1/2" core up the center, filled in this picture with a piece of scrap Estes BT-80. As with the previous parts, Gordon provided the fairing and CM primed and ready for finishing.

While Gordon was turning parts, I started working on drawings of my own. Those evolved as work on µMLAS progressed, and are posted here in their native MicroSoft Visio format along with a pdf conversion. Feel free to download with the usual caveats: these are for personal use only, I won't guarantee they're free of errors (though I really did try - if you do find something wrong let me know), and if you poke an eye out using them, hey, you were warned.

The Plan

The MLAS flight plan, as it eventually settled out, is shown to the left. I thought I could capture a good part of that with µMLAS ... certainly launch, separation of the skirts, ejection of the CM, and recovery. If I stuck to 1:20 scale and added the skirts, the resulting model would be significantly heavier than µMRA. So I took a leap (for me) and opted for 29mm motors. The G80 was the limit without the expense and hassle of a BATFE Low Explosive User's Permit (LEUP), but at this point that seemed more than adequate. I wasn't ambitious enough to attempt mounting four 29mm motors in the boost skirt, canted through the centerline MLAS-style, but thought I could cluster three easily enough in a 3" motor mount the conventional way and still leave room in the Coast Skirt (CS) for electronics and a parachute compartment. The Boost Skirt would drag separate at burnout and be recovered on its own chute. A timer in the CS would deploy an aft-mounted chute, MLAS style, to separate that section. I'd forgo the turnaround maneuver and use a second timer in the CM to separate it from the Forward Fairing (FF) and recover both those sections on their own chutes.

I crossed the line into high-power rocketry when I opted to cluster those motors, of course. But a smallish 8-fin-nose-cone model seemed a little boring and the complex large-scale option was within reach ... put another way, it seemed like a good idea at the time. My early RockSim runs said I could expect 1000 feet on a cluster of 3 G80's, but my estimate of the weight wasn't much better than an educated guess. In retrospect, I should have controlled weight more carefully during the build, but I wasn't being retrospective at the time and thought I had lots of margin. There was the issue of stability to consider too, but I could address that with ballast ... after all, I had lots of margin.

As I said, it seemed like a good idea at the time.

Evolving Yet Again

To the right is a view of the µMRA motor mount. I stuck with 24mm motors for these models because I was comfortable clustering and flying them and didn't expect µMRZ or µMRA to weigh so much some combination of E or F motors couldn't be made to work. RockSim showed a cluster of 4 E30's would carry µMRA and its 3 pounds of ballast off the pad at about 51 ft/sec to an altitude of some 475 feet -- probably with lots of roll, since the only thing to dampen it was those dummy nozzles. I pictured a pyramid rocket sort of flight. Whether it would have worked or not is anyone's guess ... as MLAS evolved I moved on to the next model and µMRA began gathering dust along with µMRZ. It too is a "when I get back to this" project.

The real MLAS team was struggling with propulsion issues ...

Any rocketeer who has flown a motor cluster knows how important it is that all the thrust vectors be closely-coupled and aligned through the CG of the model. If they're not, and a motor fails to ignite at launch, the resulting thrust offset can pinwheel the model off the pad -- this providing the remaining motors provide enough thrust to even get it off the pad. Solid motor ignition is a mature technology and we weren't too concerned about being able to get the four motors in the MLAS cluster to fire, but did worry about what would happen during the burn or at the end as the motors shut down. No two motors are exactly the same, and the thrust offset caused by one firing with slightly more thrust than the others could cause the ship to fly in an unpredictable direction; one firing slightly longer than the others at shutdown could easily overpower the aerodynamic restoring moment offered by the fins and induce a tumble. If we were designing a custom motor for this application we could build a single thrust chamber with multiple nozzles or design-in TVC, but we were limited to the off-the-shelf MK-70's we had available.

We looked hard at a building a manifold that would tie the head-ends of the four motors together and balance pressures between them, essentially making them act as one. And we stayed on that path until "motor manifold" threatened to consume the project ... it quickly became our biggest project risk and the single-costliest piece of hardware we had to design and build. MLAS was about demonstrating an alternate abort concept, not designing a new propulsion system. So we changed direction. We moved the four motors back to the aft in their own "Boost Skirt" and added another set of fins to provide aerodynamic stability for the whole stack. The motors would be canted to fire through the vehicle centerline with the CG offset slightly to provide a ballistic arc over the water. Our test flight would thus simulate the custom-designed propulsion system of an objective vehicle with its Boost Skirt, and the aerodynamic stability feature of such a vehicle with its Coast Skirt. MLAS would demonstrate drogue-assisted turnaround and capsule separation, and employ a variation of Shuttle SRB parachute deployment for capsule recovery.

MLAS would go through several more design iterations, but the propulsion decision fixed the basic configuration of the vehicle as shown to the left. It also set me on a path to reproduce the ship and as much of the flight as I could in a third-generation MLAS model.

Thus was born µMLAS.

µMLAS Rev A

To the left is µMLAS Rev A ... the evolved MLAS in 1:20 scale. That particular size came about after an e-mail exchange with Gordon Agnello of Roachwerks, my µMRZ nose cose producer. A 10" OD part was about the largest he could comfortably turn on his lathe. The full-scale MLAS as we then envisioned it would be 216" in diameter at the base -- just large enough to encapsulate a full-scale Orion CM with a gap of a few inches around the fairing lip -- so 10" worked out to a 1:21.6 scale. Gordon agreed to push the edge a little and fabricate a 10.8" diameter part which made for a convenient 1:20 scale. As it turned out, I'm glad Gordon didn't have a bigger lathe .... µMLAS and its cousins didn't seem very "micro" when I was filling and sanding.

There's nothing exciting about an empty shell, so Gordon agreed to make me a Crew Module to go with it. The CM fit snugly in the fairing with its forward shoulder resting against an internal bulkhead. Gordon did both parts in fiberglass and provided them sealed, primed, and ready-to-paint. He left the center of the CM open for receiving a piece of 2.5" PML tubing. I wanted a complete model in-hand for our first major design review, so opted to cluster four 24mm motors up the center of the CM instead of mounting them in the fairing. The CM would carry the fairing along with it in flight and be separated at ejection; the two pieces would be recovered on seperate chutes. I used 1" PVC for the dummy nozzles attached to the fairing. The stand shown in the photo was made from music wire and a couple of 10" embroidery hoops. µMRA made it to our design review just-in-time, smelling strongly of fresh paint.

The CM-in-fairing was fine for a show-and-tell model, but for flight there was the little issue of aerodynamic stability to consider. µMRZ required a lot of ballast and µMRA even more. The MLAS aero guys were kicking around options to address the stability issue for our flight test article, and a ballast spike, flared "skirt," and deployable grid fins similar to those flown on Soyuz were all under consideration. A custom TVC-equipped propulsion system could be designed for a crewed flight vehicle that would easily solve the stability problem, but for our concept demonstrator we didn't have the time or budget for such. The aero team ultimately settled on a separable ring with four fixed fins to be attached to the back of the fairing. The fins would be sized to provide stability at whatever margin we chose and would simulate the aero stability mechanism of an objective system whether that was TVC, ballast, grid fins, or some combination of those. Fixed fins would serve our purpose: we could do a concept demonstration with those, including a parachute-assisted turnaround maneuver and capsule/fairing separation, without incurring the cost of custom motors.

So again, the MLAS design evolved. And with its one day of design review glory behind it, µMRA became obsolete.

Evolution - µMLAS Revised

Engineering projects evolve as designers assess and improve upon their work, and MLAS was no exception. The propulsion team's inventory of readily-available solid motors turned up a stockpile of Navy-surplus MK70 Terriers at the White Sands Test Facility (WSTF). This is the same motor used as a booster for several sounding rockets near and dear to modelers, including the Orion and Black Brant. A cluster of four Terriers would give us enough total impulse to meet our mission objectives and, since WSTF already had a stock in-hand, there would be no delay for procurement. So gone was the 8-motor cluster we had been considering -- µMRZ was suddenly obsolete.

It became even more obsolete when we considered carefully how a MLAS-type system might be used on a crewed flight vehicle (the objective of the project, or "objective" system). If the abort motors were placed next to the Orion SM, separation during an emergency was significantly complicated. The CM would have to be separated from the SM and the motors flown away from the stack without damage to themselves, the forward fairing, or the CM. Since baseline plans for Orion had the SM encapsulated in a fairing, the problem was even more complicated: we would have account for a "plunger effect" that would occur as the motors pulled free of that fairing or provide some means to alleviate it. There was room for our four Terriers in the forward fairing, providing we canted them so the motor casings roughly paralled the Outer Mold Line (OML) of the CM. The MK70's provided enough total impulse to fly the mission even allowing for the loss caused by mounting them off-axis, and the forward-mount allowed for clean separation at abort -- an objective system so-designed would not have to deal with the problems inherent in aft-mounted motors.

The third big change was driven as much by economics as engineering. We found we could procure a fiberglass fairing custom-manufactured by Northrop-Grumman's Gulfport, MS shipyard in any shape we wanted more quickly (and cheaply!) than buying one designed for the Atlas. Our concept demo was not weight-constrained as a crewed flight vehicle would be, and we didn't need the aerospace-quality Atlas fairing to meet our objectives. Fiberglass would be just fine. Our aerodynamics team recommended a Sears-Haack shape for minimum drag at our expected flight conditions. Thus, MLAS evolved to a four-motor flying nose cone ... and as it did, µMLAS Rev A was born.

Incidentally, the "Max" part of MLAS surfaced somewhere in the midst of the early discussions and design assessments. Maxime Faget designed the tractor-rocket escape system flown on Mercury and Apollo and, in 1961, patented the "Aerial Capsule Emergency Separation Device" seen to the right. We named our alternate launch abort system in his honor ... thus MLAS.

µMLAS Rev Zero Redux

Here's µMLAS Rev Zero viewed from behind. Motors are canted in pairs through the centerline; one motor mount of each pair passes through the bulkhead into the parachute compartment to provide a flow path for ejection charge gasses. While µMRZ is flyable, it hasn't flown for a couple of reasons: it's not aerodynamically stable without a lot of nose weight - a LOT of nose weight - and evolution of the real MLAS had me hard at work on µMLAS Rev One shortly after µMRZ was completed, leaving it to gather dust on the bookshelf. Eventually, I'll dope out a motor / ballast combination that works and fly the thing though. That canted motor cluster is way too interesting to just ignore.

MLAS - Initial Concept

It so happens the Atlas 5-meter fairing is just big enough to fully encapsulate an Orion crew module. Thus was born the first MLAS concept: we would procure an Atlas fairing and Orion CM mockup and launch them on some combination of aft-mounted solid motors. Several missile programs had readily available designs we could cannibalize; some even came equipped with Thrust Vector Control (TVC), an added bonus. When we started down this path we envisioned four clusters of two motors arranged around Orion's Service Module (SM). One motor of each pair would have a high-thrust, short-burn profile suitable for accelerating the CM and fairing off the pad. The second motor in each pair would fire at burnout and provide a lower-thrust, long-duration impulse to complete the flight trajectory. If we could find a suitable motor with TVC, it could even be used to fly the turnaround maneuver and orient the CM for separation; if not, we would could use parachutes to accomplish the same thing.

At about this point, I started thinking about models .... I had this piece of 6" phenolic tubing left over from another project and a drawing of an Atlas 5 fairing. I had a lathe, too, but didn't want to eat a lot of balsa and time learning to use it again, so for the nose cone I turned to Gordon Agnello, aka "Sandman" of Roachwerks Little Joe II fame. Gordon rapidly produced a beautiful cone, all pre-primed and sealed for me. I added 8 24 mm motors, clustered as we proposed clustering the MLAS motors (but canted through the vehicle centerline) and 4 Nike-style built-up fins. Thus was born the original µMLAS, shown on the left in this photo. It made a great show-and-tell model, quite useful for demonstrating the concept to folks as we set about building a real MLAS. And plopping it down on the conference room table that first day was certainly entertaining.

µMLAS Background

Pictured to the left is my 1:20 scale rendering of the NASA Max Launch Abort System (MLAS) Flight Test Article as displayed at the Project's third "Independent Technical Review" (ITR). I spent about two years working on this thing, on-and-off, while at the same time working on the real MLAS. It picked up the moniker "micro-MLAS" somewhere along the line and the name stuck, shortened for convenience to µMLAS. Design of the model evolved along with that of the rocket -- if I had it do again, I'd do some things differently. And while MLAS flew sucessfully on 8 July 2009, µMLAS has yet to fly. But it will. In the meantime, here's some background to the story ....

Abort systems for crewed launch vehicles were pioneered in the late 50's by NASA Langley engineer Maxime Faget. His tractor-rocket design graced the nose of the Mercury capsule and the Apollo command module. A variant is still flown on Russia's Soyuz. The baseline plan for NASA's newest crewed vehicle, the Orion, calls for a scaled-up version -- and that's where MLAS enters the picture. My organization, the NASA Engineering and Safety Center (NESC), was asked to conceptualize and demonstrate an alternate Launch Abort System (LAS), one that did not require the tower or tractor rocket. The idea was to have a completely different approach in-hand as a backup should development of the baseline LAS prove too expensive or schedule-busting to accept. We were given a set of basic requirements: demo a system that could pull the crew module off the stack in event of an emergency and carry it about a mile up and a mile out to sea where it could be recovered. Intact. We were to minimize costs and complete the project within a year.

Lobbing a heavy object out to sea with a rocket motor is not difficult. Doing it in a way that makes that object safely recoverable is a different matter. Not only does the encapsulated Crew Module (CM) have to fly a stable parabolic arc in a pre-described direction, it has to be reoriented at apogee so the CM can be separated from its fairing and recovered; i.e., it has to be turned around so it's no longer flying nose-first but heatshield-first. The Apollo LAS relied for stability on ballast at the top of the tower to pull the Center of Gravity (CG) ahead of the aerodynamic Center of Pressure (CP) during powered flight. Canards were deployed at apogee to pull the CP forward and destabilize the stack so it would flip over, the process assisted along by a side-firing solid motor. Since ballast is not a reasonable option for the scaled-up Orion LAS, it relies on an exotic control motor for active stability and reorientation. MLAS' minimium cost / one-year constraints pretty much eliminated any kind of active control system and drove us to seek a solution that could be passively stabilized, yet still allow for clean separation of the CM from its fairing in a recoverable orientation.

And that's where we started: building a team, developing a workable concept, and looking at off-the-shelf hardware we could rapidly procure.

Starting

I've done model rockets since the early 70's. My first model was an MPC Titan IIIC - an all-plastic beast one could build to fly or display. That's geeky me at 12 or 13 posing with the Titan just before it's maiden flight. As it turns out, a C-whatever MPC motor will not effectively lift an all-plastic model (a fact the MPC designers seem to have overlooked) and my first attempt at rocket science ended up as a pile of smallish pieces. I stuck with it though, motivated by Apollo moon landings and an Estes catalog.

Many years and an engineering degree later and I still do model rockets. Which brings me to the purpose of this blog. Having learned a lot sifting through the model builds others have posted on the various hobby rocketry forums, I'm going to document some of my own here. I'll try to add some real rocket science, too, where that's appropriate, along with whatever other ramblings come to mind.

By the way, "rebuild the Titan III" is on my list of "someday" projects ... I still have most of those pieces.