An F-1 engine ready faster than AR-1? Surely you are kidding. Neither will happen, though.
It's a low pressure engine that doesn't push the limits of it's construction materials as much and would be mostly based on technology the industry is familiar with so design and testing should go faster.
It's similar to how building a large displacement big block V8 for a hotrod is a lot easier then building a formula one engine.
But yes both are unlikely considering the 2016 date so the RD-193 or RD-181 are probably the most likely candidates as they'd require the least amount of rework.
Second most likely candidate is a SRB stage from ATK though this would likely have to be two stages to offer then same performance of the present first stage.
Yea. Which I think ATK could do. As Ed Kyle did some numbers, two equal sized motors, while not being optimal, could replace Antares's core dV, with Castor 30XL on top, if I remember correctly. That means just one new motor to develop, and I think they did Castor 30XL in about a year? So that's feasible.
The larger issue there I think is where to launch it from? Many have said Wallops is not really an option for several reasons. Jim has said there's no way they'd launch from KSC, but that they would have to launch from the Cape to compete for government contracts. So I guess that would leave OrbATK to purchase/lease an old pad from SpaceFlorida or the USAF itself (as I believe SpaceX is doing for LC-40?)
And then build a new pad there, which could accomodate vertical solid segment stacking on the pad, like is done with Minotaur, but on a larger scale. Could such a new pad be constructed at the Cape within the 2-2.5 years tweeted about? That's what I think might be difficult.
SpaceX razed and rebuilt LC-40 and SLC-4E I think in around 2 years each? (someone correct me on that). But Those would be more simple in the horizontal integration buildings and simple strongback construction. A solid Antares would need some sort of MSS with a heavy duty gantry to lift the big solid segments onto the pad and stack them. And while they are at that, they'd probably need some sort of vertical payload integration room as they probably can't integrate horizontally and erect the stack on the pad as Antares is currently done. So it would be a more expensive/complex pad I would think than the SpaceX pads. Could that be built in that time frame? I'm thinking that'd be pretty tight.
Although...one thing of note. The Tweet talked about a 6 month overlap between a re-engined Antares and the last AJ26's. Do they just mean they'd have the first new core ready 6 months before the last current core flies? Or do they mean they plan to have a new LV on a new pad 6 months before the last AJ26 Antares flies from Wallops?
I would think the former, but it's not quite clear from the Tweet.
PS: One other thing of note. If anything comes out of the new Pegasus II/Stratolaunch/Scaled Dreamchaser information, that would mean that ATK would have those 3.71m wide composite casings anyway. Pegasus II would use shorter motors probably, but a longer Antares motor could use the same casings, and samle nozzles and TVC. If they have figured out a launch solution for such a solid booster, it would offer some interesting benefits to OrbATK to do it. Railoads capable of carrying the motors already run from ATK-Utah to the Cape. And they'd need to launch from the Cape -anyway- if they ever want to try to get some government contracts like they publically stated they do.
I still think Russian engines are more likely on a stretched Antares core launching from the existing Wallops facility...at least as the first step...is the most likely. But it'll be interesting to see when they release more info later.