TOPIC: Rapiers: Questions and answers

Hi Jim,

many thanks for this. We who fly aeroplanes and not hydroplanes usually make our motor tubes from rolled paper with either PVA (Mike Stuart) or epoxy (me). I have never put in 'cooling slots, but then our motors are perhaps subject to a greater flow of cooling air ... until an early arrival with the motor still blazing away. So the mounting tubes can get scorched but (so far) I haven't had one fail.

Incidentally, here is a 'Hydrojet', one of Jim's marvelous models:






There is a video of one of Jim's models on you tube - has anyone got the link?

A further complication to flying with Rapiers is their storage. Last year I bought a large batch of really excellent L1's (100 mN 10 sec).

I now find they are difficult to light and need fairly extensive reaming out to get burning.

OK, I know how to do this, but what about for someone just starting out in rocketry?

Any thoughts on (a) storage of motors; (b) lighting those which are reluctant to start?
I have heard of guys using a drop or two of lighter fuel, or ground match heads - but I really hoped Dr Z had addressed these issues so that we could promulgate Rapier flying with a clear conscience. Oh dear!

Your thoughts on the present Rapier situation are needed!

Hi
Me again, you mention storage, I never use silica jell for one reason, remember silica jell is used to soak moisture up in the air so basically it stores the moisture you are trying to guard your motors from.
I store all my L3s, and any others at that in a thick cardboard box in a dry atmosphere room, in my case the spare bedroom
JimF

Hi Guys,

Dr Edward Jones has a few pertinent observations about Rapiers, their propellant and storage.

Dr Ed writes,

"silica gel if used can be changed (or heated, to freshen it) from time to time, but in a small, hermetic container, a single gel pack can and will keep things safely drier. AND, at the same time, I'm leary of blaming moisture on Rapier problems. I honestly have seen no evidence, nor do I see any reason to think they'll be even trivially affected--even in the sometimes less-than-sunny parts of UK.
My reasoning: the cases are densely wrapped epoxy layered tubes, with a tightly sealed compressed clay with epoxy end cap. That leaves only the nozzle port for moisture entry (short of some days perhaps in direct drizzle: I shall in fact test this later today, sealing ONLY the nozzle hole, and giving them some days of drizzle in the shower.
I'll report!).
Rapier nozzles being less than 1.5-mm diameter offer only 1.77-sq mm area for moisture to enter, as I see them. AND, if some H2O vapor/vapour were permitted to circulate within the nozzle ... Dr Zigmund's instruction sheet reads simply,
"Do protect the jet units from damp".
Most importantly, consider the propellant grain itself. I've dissected (forensically?) a few of various sizes, and in every case the propellant is hard-as-a-rock, very dense, and has a waxy feel, which I attribute to the known 2% added zinc stearate additive. The formulation is based upon a 6% epoxy binder-fuel, as well. The 6% dicyandiamide content is not particularly water-attractive. The ammonium perchlorate, while soluble in water, isn't regarded as particularly hygroscopic either. And it would be really, really hard for many molecules of H2O to find any way into that hard, dense, epoxified grain of propellant, IMNSHO.

Consider: Other than perhaps black powder, AP is the most widely use solid rocket oxidizer on the planet, used in rockets, missiles, boosters of all sizes, biggest to smallest (Rapier). I would guess that there's a lot more AP consumed than KNO3 for rockets worldwide these days. AP hasn't been a guilty party with moisture, even over decades of Polaris, Tartar, and others that I know of.

My conclusion (and current operating practices) is that moisture and Rapiers is a red herring. To wit: here in this little lab, I've left 10-gm batches of Rapier propellant, powdered and mixed minus the epoxy, open to the air on humid days ... for several days ... and it shows no reduction in performance (and that's bad practice, it's not a habit, just saying). Dr Zigmund's Rapier propellant is not nearly as easy to ignite as I had thought, even totally fresh. The high zinc content (37%) acts as a terrific heat sink, to begin with, and the epoxy don't start burning until the zinc begins reacting with the AP and DICY. It's a lot harder to ignite than black powder, or Jetex pellets!
I think Rapier motors are quite stable, if simply stored in a cool, dry place--the same conditions that you might use to keep important old photographs, for instance. And the silica gel packs won't hurt”.

Much to consider here, and I'm glad my storing Rapiers with silica is not thought to be a problem.

But the fact remains: Rapier L-1 motors that lit easily in September now need a fair bit of reaming and fetttling to get them to ignite and burn through to the end.

So what is going on? Are vertical cracks appearing in the propellants so that only the first portion burns and then 'pop'? Such motors can still be lit with a cuterizer tool with a long 30 swg Nichrome filament and three fresh AA batteries. But why the change?

I'll try the old Jetex trick of ground up match heads and see if that is effetive.

Note the L-2 motors seem OK - so what is the difference?

Questions, quiestions, when I'd rather be building and flying!

Hi all
Read with interest Ed comment about silica gel, and water getting into rapier motors. All I can say is that these are my own experience with the substance especially in regards to photographic equipment .
As for the construction of the motor cases the ones that I have come in contact with (and there have been quite a few ) are not water resistant at all, on quite a few occasions when the the boats I have been running have capsized the motor cases have swollen so badly that they have had to be cut from the model so moisture ingress is in my mind is very possible.
JimF

Over $10 a flight WOW!

(this comment refers to a box of L-3's on eBay which went for an eye-watering amount!)

A few weeks ago George Foster wrote about problems he had had with last year's (2013) L-1 Rapiers.

These had hitherto behaved in an exemplary manner, but after over winter storage George found that 2/3 were now difficult to light, and had a tendency to go 'pop' after 2-3 seconds of ignition when they did light.

This was disappointing to say the least, as George now had many L-1 Rapiers which appeared U/S and his JP 5 (jet Provost 5 was running out of propulsion.

I suggested George tried boring them out and add a sprinkle of 'fairy dust' before trying again.

George has now written:

"It was good to meet you at Old Warden a fortnight ago. I just want to endorse your method for resurrecting 'popped' Rapier motors. I tried it a couple of days ago with complete success - drilling out and adding a pinch of ground safety match heads - instant result!

Perhaps it should be more widely known (perhaps it is), but it has removed one large reservation about Rapiers for me.

My JP5A for L-1 is now going well after repositioning the motor further forward to give more effective noseweight on initial climb".


This is good news. A 1mm drill bit is fine,but don't go too deep (not more than a couple of mm).

Onwards and upwards!