ramjet strap-on pod concept

These images have to do with my ramjet engine sizing for a ramjet-assist strap-on pod for space launch. This post is also for the benefit of my NewMars.com buddies.

This first image is the concept behind an integral rocket-ramjet engine pod that is supposed to be a fly-back, reusable unit. I'm still learning how to get images embedded where I want them in this text. Tough for an old slide rule guy.

For the first few seconds off the pad, a solid propellant booster housed within the ramjet chamber provides considerable thrust, until vehicle flight speed reaches somewhere about Mach 1.5 or so. Depending upon the acceleration levels, that could be as low as 5000 feet, or substantially higher for significantly lower accelerations.

At that takeover point, with suitable geometry changes, the pod transitions to ramjet thrust in a fraction of a second. The pod continues operating as a ramjet up to approximately Mach 6 at 100,000 feet, at least as maxima. The geometry changes are an ejectable nested booster nozzle, and an inlet port cover. These are similar to those used on SA-6 "Gainful", ASALM-PTV, ALVRJ, and more recently, the SS-N-22 "Sunburn".



I had an old DOS Basic language code I wrote some 15 years ago that I made run again on an old Windows 98 / 486 machine from my shop. I have had no success trying to run this ancient DOS / Basic code in Windows ME or Windows XP machines.

This old code was a ramjet engine sizing code. I used it to size the ramjet pod's engine for a takeover point of Mach 1.5 on a standard day at 5000 feet. I used equivalence ratio 1.1 (110% of ideal mixture strength) at inlet pressure margin 2% (to cover manufacturing tolerances). That result is depicted as follows:



As you can see, the pod features an air plenum beghind the inlet structure, feeding a center coaxial duct, in turn feeding a sudden-dump entry into the combustor. There are some very necessary equipment items packaged within the air plenum space. The sudden dump is the flameholder, very similar to the old ASALM-PTV. I assumed RJ-5 (a.k.a. Shelldyne-H, a synthetic substitute for kerosene that is denser than water), although the basic sizing would be just about right for JP-5 jet fuel, or RP-1 rocket fuel, both petroleum-derived kerosenes.

This pod features a pivoting wing, wrap-around fins, and retractable landing gear, for recovery and re-use. I'm thinking inert mass fraction about 40%, like the X-15, to ensure the structural robustness for 1000's, if not tens-of-1000's, of flights. The X-15 was the most inexpensively reusable rocket vehicle in all of history, after all. Depending upon whose data you believe, its inert mass fraction was about 40%, within about 2-3%.

Not shown are dive brakes for deceleration after staging. I may switch to fixed fins, and use the dive brake panels differentially, for steering control. Not sure yet.

In any event, the idea is to fly this thing back by remote control from the ground, like a big model airplane. It has a nosewheel and skids, like the X-15. That ought to provide sufficient capability for a runway recovery adjacent to the launch site. For the translating-spike inlet design, I would prefer a land landing, over ditching and towback at sea. Less risky.

I do not yet have a performance-mapping code here at home (or anywhere else, for that matter). I am attempting to re-write one from that old DOS-Basic sizing code, but I am nowhere near done. Meanwhile, I have been programming the basic engine balance into an extensive sequence of Excel-spreadsheet worksheets.

Using a constant 5.5 gee vertical acceleration as a rough guide to a fast ascent trajectory that reaches Mach 6 at 100,000 feet, I picked off several Mach-altitude points and ran point performance in the spreadsheet at a "sweep" of fuel/air equivalence ratios (relative richness, 1 = perfect mixture, under 1 is lean, over 1 is rich). All the plots looked exactly like this one at takeover, only the numerics were different. This one went from ER = 0.4 to ER = 1.5 by 0.1's. Rich is at lower right, lean is at upper left.


I did a manual graphical "aft tangent" analysis on each one of these plots. That effort determined the "best" compromise between impulse and thrust for a vertical accelerator engine to be equivalence ratio 1 (stoichiometric, or perfect mixture). That's quite different from the "best" ramjet cruising missile design, which is max Isp consistent with a thrust coefficient matching the drag coefficient for the design cruising speed, typically around ER = 0.7.

I am not yet at all sure that the nose inlet / center duct design approach is the best for this application. Because of severe aeroheating, even for a short transient, at Mach 6 conditions, this geometry offers "coking" potential for fuel residuals on the tank surfaces, both outer, and especially inner along the center duct. This is hard to avoid, and very much harder to make easily refurbishable and reusable.

Perhaps a side-mounted multiple-inlet geometry similar to SA-6 or ALVRJ would be better. At least the hot ducts would be outboard of any exterior tank insulation. However, inlet pressure and massflow recovery characteristics are a little different from what I used here. Although in the same basic ballpark as what I used, they are a tad lower, and far more sensitive to off-angle attitudes.

I just dunno. So, I will push this center duct "design" a few steps further before I give up. Watch this space for further details.