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

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?

• Are you seriously asking whether Wikipedia has better information than Firefly's own web site? – Russell Borogove Feb 22 '18 at 5:31
• @RussellBorogove yes and no; yes I am seriously asking, and no about what information is better. I don't know what's inside those four holes, and while those four red lines are likely to be rendered reflections from a concave surface, they might be something else. Is there any chance that there are there aerospike engines inside there? Explicit, long-form Question added at the end of the post. – uhoh Feb 22 '18 at 6:44

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.

• Interesting that they are using a tap-off cycle. Nice answer, +1. – Organic Marble Feb 23 '18 at 0:09
• Thank you for this excellent research and write-up! Finding the "b" page makes the picture of what's happened complete. I noticed at the bottom of the b page they've also added a link to their Feb 6, 2018 tweet of their "we're not worthy!, we're not worthy!" hat tip to SpaceX after the FH launch. – uhoh Feb 23 '18 at 0:21
• Tap-off cycle being: start the turbopump electrically (or with an expendable hypergolic charge or something), then use a small amount of primary chamber exhaust to keep it going? – Russell Borogove Feb 23 '18 at 1:01
• en.wikipedia.org/wiki/Combustion_tap-off_cycle – Organic Marble Feb 23 '18 at 1:17

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.

• Thank you for taking my question seriously! I appreciate you explaining what can potentially be inferred from the rendering; it seems there's much more going on there than I'd realized. – uhoh Feb 23 '18 at 0:24