The first stage engines that exploded were AJ-26 engines. These are remanufactured NK-33 or NK-43 engines built in the Soviet Union in the late-60s or early-70s for the N-1 moon rocket, but never used.
In May, an AJ-26 exploded on the test stand at Stennis space center in Mississippi, severely damaging the test stand.
Even in the computer field, picking up a 30-year-old unmaintained project is very hard, original designers gone, everything around it retired, use case changed and maybe some tricks may never be known.
As for these engines, they're currently retrofitted with new kind of fuel, new electronics and some modifications, after 30 years, by a totally different team.
I really don't think this kind of thing would work.
Pure speculation but I wonder if they rushed to test the engines that just flew. AeroJet's parent company claims they have lost over $31MM on the AJ-26 YTD[1]. There may be pressure from higher-ups to push through the issue seen at Stennis in May.
I wonder if this is going to be a pressing problem, given the Russian's lack of enthusiasm for supplying more rocket gear while under sanctions and the dwindling stock in the US.
AeroJet owns pretty much all of the existing NK-33 inventory and a license to manufacture them. The work for integrating the NK-33 into the Antares rocket was actually done by a Ukrainian firm: http://en.wikipedia.org/wiki/Antares_%28rocket%29#Design
It seems so bizarre that we're using 40 year old, Russian-made engines. I'm sure there are plenty of smart folks who will say not too worry, but I wonder.
It is a fascinating case of uneven technological development.
The Soviet Union went so far with staged combustion engines in the sixties that that level of technology has not yet been reached elsewhere.
USA, Europe and Japan went with hydrogen staged combustion but that's quite different.
The ULA/Blue Origin decision about new indigenous US first stage engine development gets a huge boost from this problem.
It's just what can be reached when you put resources into it. Coming from the IT world it might seem strange that the investment has not lost its value, but it has not. It might be quite a lot easier to duplicate nowadays, but not child's play at all.
Sometimes it's chilling to think what kind of still secret projects were funded by the huge US defense budgets during the cold war. There are some public sides and declassified parts on things like very good glide ratio hypersonic vehicles, but officially they never went far and the space shuttle was the thing.
( http://www.456fis.org/THE%20B-52/FS-011-DFRC_popup6.jpg )
I'm sure the core rocket technology is sound but I've gotta think the sensors and computational capabilities are literally decades out of date. Which is really where SpaceX has a commanding advantage, for example, being able to abort a launch at T-0 and reschedule for later that day having identified and corrected an anomaly.
In May, an AJ-26 exploded on the test stand at Stennis space center in Mississippi, severely damaging the test stand.