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I guess you could add smaller auxiliary engines to correct off-centred thrust.

Very interesting shapes for the exhausts, nice to see some innovative use of unusual materials :)

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Assuming it's using something like a VASIMR drive, there's no issue with assymetrical thrust, you would just tune the EM field of the exhaust to compensate. :nerd:

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I'm not sure, Will. In a vacuum there would be no other forces working on the craft, suggesting that the CoG would be irrelevant. An aircraft or ship would suffer uneven drag, causing the vessel to pivot around the part offering greatest resistance. Without drag it becomes a matter of how much angular velocity is built by the degree of offset.

If math is your thing I found these- http://physics.stackexchange.com/questions/55631/thrust-center-in-space , http://physics.stackexchange.com/questions/43232/force-applied-off-center-on-an-object?lq=1

From what I can gather there would be a rotational force, but sticking some numbers in shows the force as quite small at smaller angles. The equations assume a spherical object, which would spin if given an offset thrust. If the greater portion of your ship's mass is in the after part and evenly distributed, the induced spin might not be significant enough to make the single engine useless. Occasional course corrections with manoeuvring thrusters might be enough to compensate.

Edit- ninja'd! :lol: Like sgt s says, you could just use thrust vectoring motors!

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I have always like Chris Foss's work, especially the illustrations on the covers of Isaac Asimov novels. I am looking forward to seeing this develop.

I am pretty sure the hose connector will be moulded in polypropylene so paint adhesion could well be an issue. You can get surface activators for difficult to bond / paint materials such as this product from Loctite:

P1060603_zpse464a10d.jpg

3M also make a similar product.

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I like the idea of two large main engines and a couple of smaller auxiliary ones, would look good. I also like your choice of bases for the engines, great idea.

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If I may chip in regarding the engines: I personally like the idea of one large central engine flanked by two smaller ones.

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As long as the thrust line is through the centre of mass, all would be well. It might fly like a crab, but it would fly true. So, had you thought about gimballed thrusters (AKA, thrust-vectoring)?

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Lots of helpful responses, thanks!

I had considered thrust vectoring using exhaust vanes/petals, but only after I posted. That's probably the best hand-waving answer since it lets you move the thrust line. I guess you could do the same thing by firing auxilliary engines or thrusters, but then they'd need to be relatively close in magnitude to the main engine. So you'd have to dial the main engine back, in which case you could just as well use the auxilliaries to do the burn.

Maybe I should work out some numbers, but from what I remember writing simulations off-centre thrust gets out of control quite quickly.

Gimbals would look really cool, but maybe would be a bad engineering solution on something this big?

Incidentally I assume this kind of thing isn't a problem for aircraft as the shape in the air produces some force to counteract off-centre thrust, particularly if the control surfaces are trimmed right?

Will

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:lol: - depends on which flight sim you believe! I'm quite lucky to be utterly pants at video games, so the nuances tend to pass me by- I crash no matter what I'm simming when half my engines die! :giles:

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There's an old saying....

When one engine dies,

The other will provide just enough power

to get you to the crash site.

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It's too, I don't know, clean and perfect? There should be flames and black bits and re-entry heating and so on, not just "I turn on my AG and float away".

I did some material tests with my various engine bits. The good news is that the threaded pipe coupler rings are ABS, or at least they stick really well with Tamiya ABS cement. They also stick to ping pong balls with CA. The bad news is that nothing else sticks to anything else. So I think the coupler rings with ping pong ball inserts are going to be the main drive nozzles. I now need to figure out what to fit them to since they're a bit too flat against the ship. I quite like the idea of having half a ping-pong ball on the other side, as though it's some kind of combustion or detonation chamber. Then some struts and gubbins to tie it all together. I might also build up a truncated pyramid from sheet as a base - not sure.

Incidentally the Tamiya ABS cement is very thick. It's like going back in time!

Cheers,

Will

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I'm looking forward to the paint job on this one. Been a fan of Chris Foss' work ever since I saw some of it gracing the covers of the 'Lensman' series in the school library.

Incidentally, two engines wouldn't be a problem if you offset them slightly so that the line of thrust from each passes through the centre of mass. That way, if one fails you don't get a spinning starship.

Edited by TonyG

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That's true, I think there's a Martin Bower 1999-like design that does that.

I was just reading up about ping pong balls and they're made from celluloid. Not sure I want that in my model!

Will

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Forget ping pong balls.

Xmas tree ornaments.

Huge range of sizes & materials.

They'll be in the shops soon!

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Great idea, maybe not soon enough although I'll have a look! How about Kinder Surprise, are they glue and paintable?

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Lensman series... oh my word, those were utterly wonderful book covers! I've managed to track a set down from amazon and ebay over the yearsand revisit my childhood reading them again and marvelling at the completely audacious arms escalation portrayed within. battleships not powerful enough? How about a negasphere? Oh hang on how about a moveable planetoid... oh no how about a moveable planetoid with an faster-than-than light intrinsic velocity... bonkers, utterly bonkers, but as a 12 year old boy reading this stuff, it was magical!

Always wanted to have a model of the Dauntless!

Will you should paint this matt black as a stealth speedster :)

Now where did I leave my Delameters....

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I love the Lensman books as well - there are wonderful descriptions of all the enormous relays and shunts and coils and things, it all feels deliciously analogue.

"Why do all you spacemen swear to Klono?"

Will

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"Oh no they've got a purple ray!" :shoot:

EE Doc Smith just took that pesky science bit right out of his science fiction didn't he? :D

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I got fed up with things that didn't stick to other things, removed the bit of ping-pong ball from the hose ring, and used an internet tool for metalworkers to lay out a frustum touching the ID of the ring at the back and again at the narrow point. I extended this a bit further since I found a nice piece for the "flame holder" or whatever spacey equivalent it is:

14997241832_946ca01991_c.jpg

The frustum (truncated cone, fact fans) insert is from 0.5mm styrene sheet, cut with a compass cutter and formed around a wooden spoon handle with boiling water. I didn't brace the back of the cone join except for the narrow end, since it would have fouled the ring. I think it would have been a lot easier to brace it and trim or file away the bracing where it touches the ring, since I had to do a fair bit of filling and sanding to remove a dip and seam where the edges wanted to pull apart.

Given the sanding, I thought I'd better fit some ribs to disguise the smoothness (or not!) of the inside.

14810913049_522b7c2c81_c.jpg

The mounting is a 1/72 AFV part, I'm sure plenty of people will spot what it is :) I might raise the bell up a little bit with some tubes and such, not sure yet.

Cheers,

Will

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I like the internal ribs, they really add something.

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Thanks Nigel! I liked the idea of ribs running the other way but that struck me as something that would be hard to do well - these were pretty easy. I found some tank grab-irons that I might use to finesse the bare ends of the ribs a bit, I haven't made my mind up yet.

Nothing very exciting to report in progress terms but I did some boring stuff - cut a hole in the rear plate, cut and filed some K&S square tube to make a stand socket and fitted against the internal bulkhead, and then got the rear plate on.

I suppose the socket is a little bit interesting - I've actually used two tubes, one about 1" either side of the spine on the bulkhead that sits behind the bridge. It's a bit far back for balance but not too bad. They have wires soldered to them so the idea is to have two prongs (of the appropriate size of telescoping brass) from the stand to provide power. Then the ship can be lifted off (maybe with internal batteries too?) if necessary.

I tacked them in place with CA, then boxed around them with chunky styrene strips and covered the whole thing with Araldite. Hopefully that'll be strong enough, if not I'll have to have a cradle-type stand with the nose supported as well.

Sorry no pictures since I can't find the thingy that holds the memory card :(

Will

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My current computer only has a slot for a microSD card, so I need an SD card adapter to use it in the camera. I have two but can find neither.

(Downloading images over USB is slow vs. importing them from card).

The camera itself is right there, I can see it!

Will

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Found it! Here's the stand socketey thing:

15042194222_fa247ce8cf_c.jpg

and here's the view from the inside. I've now sanded, filled, primed and sanded the hull so hopefully I can think more about the details and less about the shape from hereon in.

14856005667_96d851af16_c.jpg

I also did a bit more on the engine mount - first of all it got a backing plate so I can get it glued nicely onto that, and then have a nice easy job to fit it to the hull later. Then I added various AFV bits including some track links as mounting points for the engine bell.

15042551455_fe8e9f1f6f_c.jpg

and some grab-handles inside as mentioned earlier:

15042548975_e1f1142956_c.jpg

I boxed in the recess at the bottom where the gun would've been, in future it would be easier to do this before fitting all the details rather than afterward, oh well :)

Total number of different AFVs which have donated parts to this build so far: 4 (or maybe 5?)

Cheers,

Will

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