What fire will Firefly fly, and when? (update/clarification on planned launch vehicle)

Question

This answer from 2017-Dec-03 says:

Firefly Aerospace will use an (annular) aerospike engine with their Alpha rocket. They have a planned launch in 2018.

The linked Wikipedia article shows this (from here) and says it is Methane/LOX, with the central spike cooled by the Methane:

But their website shows this, using RP-1/LOX:

Question: I'm trying to reconcile the linked answer and Wikipedia article with the website. Did the design for the Alpha rocket used to call for an aerospike engine similar to the design in Wikipedia, but now it does not? Is the plan now for four methalox engines with more standard combustion chambers and expansion nozzles? Not being a rocket scientist (nor a brain surgeon for that matter) I don't understand what I'm looking at in the image on the web site at all.

Did they scrap or postpone the aerospike plan?

Answer 1

The WikiPedia article lists the aerospike design as the original intended powerplant from Firefly Space Systems. The Firefly Aerospace "Revised Firefly Alpha" reportedly has a bigger payload and higher cost per launch.

The Firefly Aerospace site shows the following:

PROPULSION: STAGE 1

Engine / 4X Reaver 1
Propellant / LOX / RP-1
Propelleant Feed / Turbopump
Combustors / 4
Thrust (vac) / 728.8 kN (163,841 lbf)
lsp (vac) / 295.7 seconds

and

PROPULSION: STAGE 2

Engine / Lightning 1
Propellant / LOX / RP-1
Propelleant Feed / Turbopump
Combustors / 1
Thrust (vac) / 69.9 kN (15,714 lbf)
lsp (vac) / 324.1 sec

and states:

Alpha utilizes well established propulsion technology. Both stages use common designs: copper regen-cooled LOx/RP-1 thrust chambers, a simple tap-off cycle which drives single shaft turbopumps, nozzle-mounted turbine exhaust manifolds, and hydraulic actuators. Innovations in Firefly engines include our simple “Crossfire” injector, tap-off geometry, dual-mounted electrically actuated, trim-able propellant main valves, and ultra-compact horizontal turbopump mounting. The upper stage engine, “Lightning,” includes a turbine exhaust cooled refractory metal high area ratio nozzle extension. The first stage “Reaver” engines feature simple single axis gimballing. Consistent with the overall Alpha vehicle design, cost and performance are traded and optimized in Lightning and Reaver components to provide the best payload performance value.

so it indeed does appear the new design no longer uses the aerospike engines.

Interestingly, if you modify the URL http://www.fireflyspace.com/vehicles/firefly-a to http://www.fireflyspace.com/vehicles/firefly-b you get a page from the old Firefly Space Systems site showing the design of the original proposed Firefly Beta.

Answer 2

Did the design for the Alpha rocket used to call for an aerospike engine similar to the design in Wikipedia, but now it does not?

I wasn't able to find the aerospike rendering in a few minutes of poking around on archive.org, but that seems likely.

Is the plan now for four methalox engines with more standard combustion chambers and expansion nozzles? Not being a rocket scientist (nor a brain surgeon for that matter) I don't understand what I'm looking at in the image on the web site at all.

The current rendering appears to show conventional gas-generator engines with the gas generator exhaust delivered (by the gold-colored ducting) to the edge of the nozzle. The exhaust gas from the turbopump comes into the nozzle through the horizontal slots seen in the interior. A similar configuration is used by e.g. the Saturn V's F-1 engine, but here the injection is much closer to the end of the nozzle, so it's less useful for cooling. I will (baselessly) speculate that the gas generator exhaust is being used as a virtual nozzle extension -- helping keep the primary exhaust plume confined beyond the mechanical exit plane without the weight of a metal bell. I'm not sure if that's even physically possible, but it's the kind of thing one might think of when thinking about aerospikes.

I am confident, however, that the red lines are rendered reflections and that the colors/textures of the final product will not be close to those of the rendering.