Alien: Nostromo's Engine room towers become thrusters

leading from

Alien: Creating The Nostromo

initial Nostromo engine room miniature (source www.martinbowersmodelworld.co.uk)

a) Ridley makes the engine room towers the rear thrusters
Ridley came in one day and saw them finishing off the angular engine room towers, and asked them to see what they would look like if we they were to stick them onto the Nostromo lander. 

He grabbed hold of the two front towers and carried them from the model shop to the stage where the large Nostromo sat on its rig. 

He offered the towers up the back of the Nostromo, asked a couple of the people there to hold them there and stood back. 

He liked the way they looked, and so modelling crew were sawing, chiselling, filing, added extra dressing such as flaps to make them look like secondary engines between the three main exhausts and this brought the model up to nearly 9 feet in length. 

This was a fairly common event; it was very much a case of "Let's try that"

The final version of the  9ft long Nostromo ready for filming.  It went through many changes 
on its road to the screen. The two hexagonal rear jets are sitting upright on the floor, right in 
the foreground, so that they could get to the inside of the model to attach light.  They were 
 originally part of the Engine Room model set, but Ridley wanted more engines so the model 
had these towers altered to become additional engines

b) Nozzles, studs and exchanger flange rings
Every single one of the 57 nozzles within the octagonal engine bulkheads, 19 to each main engine, was made from 12 studs from the Horny inclined piers sets, sandwiched between three EMA "exchanger flange rings" (code VX 200, VX 225 and VX 32 in the catalogue).

That required 684 studs and 171 rings and that was just for the three main engines.

c) Parts from Tamiya Panzerwagon Kit
Once Ridley had added the one time engine room towers, what had been the base of the tower now shows as another gaping space needing to be filled with detail in order to look like an engine.

So the 17 chassis bases from the Tamiya Panzerwagon kit was stuck inside each engine to detail the sides and then EMA cones and tubing clad with "Beam Splices: (code KBJ-10) and yet more studs and rings to form the extra jet nozzles.

EMA Cones (VC-500) - Sheet of 5 Assorted Shapes

Quote source

1. Bill Pearson: Ridley came in one day and saw us finishing off the angular engine room towers, and asked us to see what they would look like if we were to stick them onto the Nostromo lander.  He liked the way they looked , and so we added extra dressing such as flaps to make them look like secondary engines. This was  a fairly common event; it was very much a case of "Let's try that"(Sci-Fi & Fantasy FX  #48, p27)
2.  Martin Bower: We has also built a miniature set of an engine room. No, not the one with the shuttle parts, that came later. Ridley too one look at the first engine room and said he didn't like that... He grabbed hold of the two front towers and carried them from the model shop to the stage where the large Nostromo sat on its rig. He offered the towers up the back of the Nostromo, asked a couple of us to hold them there and stood back. Next thing we knew we were sawing and chiseling, and filling and filing and we had these two former towers now added as extra engines between the three main exhausts and bringing the model up to nearly 9 feet in length. (Sci-fi and fantasy models, No.?,  Alien: The Models, the definitive story part 1, p30)
3.  Martin Bower: I've often been asked what the multiple nozzles within the octagonal engine bulkheads were made from. "Were they aircraft rotary engine kits?" one person asked. "No" is the short answer. Every single one of the 57 nozzels, 19 to each main engine, was made individually from 12 studs from the afore mentioned "Hornby" inclined piers, sandwiched between three EMA "exchanger flange rings" (code VX 200, VX 225 and VX 32 in the catalogue). So for those alone it doesn't take much maths to tell you that required 684 studs and 171 rings! And those were only for the three main engines! However , once Ridley had added the one time engine room towers, what had been the base of the tower now showed as another gaping space needing to be filled with detail in order to look like an engine.  So the 17 chassis bases from the unpronouncable Tamiya Panzerwagon kit were stuck inside each engine to detail the sides and then EMA cones and tubing were clad with EMA "Beam Splices" (code KBJ-10) and yet more studs and rings to form the extra jet nozzles. It also meant we needed to build a new engine room, hence the use of all those Airfix Shuttle parts! But the odd bits we didn't use did not get wasted either. We kept seeing this strangely familiar face peering round the workshop door. Terry Gilliam was his name and he needed a few bits for a space sequence he was doing next door for a film called Life of Brian (Sci-fi and fantasy models, No.?,  Alien: The Models, the definitive story part 1, p30 and 51)
4. Bloody Massive Engines. he large main engines (built separately, and clad in sheets of plastic card) ended up the smoothest part of the ship, in the pre-CGI world of the late 70s, the crew found the best way to create an engine glow was to place 30-40 projector bulbs inside each one. (SFX#6, p35)
5. Bill Pearson: Ridley wanted more engines on the back and we added them, so it got really messy. The shape was so odd by the end of it, without looking at the pictures I'd be very pressed to sit down and actually draw the Nostromo. I think anyone would. (SFX#6, p35)
6. Martin Bower: This is the final version of the NOSTROMO ready for filming. Note the rear hexagonal engines (which were originally built as parts of an engine room set that was never used) sitting upright on the floor so that we could get to the inside of the model to attach light etc. ( Facebook 2 April 2013 at 11:09)
7. Martin Bower: The almost finished large 9ft long NOSTROMO went through many changes on its road to the screen. Here you can see the 2 rear hexagonal rear jets sitting on the floor, right in the foreground. These were originally part of the Engine Room model set, but Ridley Scott wanted more engines so the model had these towers altered to become additional engines! ( Facebook 3 May 2013 at 10:53)