In May, Russian deputy prime minister Dmitry Rogozin announced via Twitter that the country would end its export of rocket engines - designated the RD-180 - to the US if they were for military purposes. Clearly irritated at being included on a US sanctions list, he tweeted: "Russia is ready to continue deliveries of RD-180 engines to the US only under the guarantee that they won't be used in the interests of the Pentagon."
If Russia banned the export of the RD-180 rocket, the Pentagon estimates it would take nearly five years and $1bn to design and manufacture a home-grown rocket engine. At the moment, the US stockpile of RD-180s covers only two years, which potentially opens a gap of nearly three years without launches.
A gap of three years could have serious consequences for sensitive areas such as US military satellite communications and missile defence.
Satellite communications and defence warning systems at risk
Only a year ago, it would have been hard to imagine that political events in Ukraine would have serious knock-on effects for US military programmes in space. But that is exactly what has happened as tensions on terra firma threaten to end several US-Russia space initiatives.
Russian engines are used to power classified military satellites into space on a United Launch Alliance (ULA) Atlas V rocket. Atlas V launches the Advanced Extremely High Frequency (AEHF) constellation of satellites into orbit which relay secure communications for the US President and the military.
Atlas V also launches satellites into geosynchronous orbit for the Space-Based Infrared System (SBIRS), designed for space surveillance and missile defence.
"For the Americans not to take RD-180s any more would probably be quite disruptive of their space programme in the medium term," Brian Harvey, an author of several space books, told the Guardian newspaper.
The engines are sold to ULA by RD Amross, a joint venture between Russian company Energomash and United Technology Corp.
Elon Musk has rattled the US military space launch market, challenging the monopoly of United Launch Alliance by suing the US Air Force.
The liquid-fuelled RD-180 is designed as an expendable launch vehicle, meaning it is only good for one launch and cannot be used again. Since its introduction in 2000, experts say the technical performance of the RD-180 has been "outstanding" with no other rocket coming close to matching its capability.
"No domestic rocket engine that uses liquid oxygen and kerosene propellants has anywhere near the same performance," wrote Jeff Foust, editor and publisher of The Space Review.
US industry clambers to find rocket alternative
Buying Russian-made RD-180 engines was seen as a good idea in the aftermath of the Cold War, especially when relations between the US and Russia had thawed and developing domestic RD-180s would have meant a significant investment from the US Government. The US is licensed to produce a domestic version of the RD-180 if it chooses to, but due to lack of investment from government and industry it has never done so.
It's a decision which is coming back to haunt the US space industry as it clambers to find alternative ways to power the Atlas V. An unpublished Pentagon study, written by retired Air Force Major General Mitch Mitchell in May and obtained by Space News, said an eventual Atlas V grounding would delay as many as 31 missions and cost the US nearly $5bn.
"Actions must be taken in [fiscal year 2014] to mitigate current risk and preserve future options," the report's summary said.
In June, lawmakers in the US House of Representatives added a provision in the 2015 defense appropriations bill which would allocate $220m of funding to a RD-180 replacement programme. That was quickly rejected by the White House saying that the provision was "premature" and did not reduce US dependency on Russian rockets.
"An independent study recently concluded that such a program would take eight years to field and could cost $1.5bn with another $3bn needed to develop a suitable launch vehicle," said the White House statement.
It added: "This approach prematurely commits significant resources and would not reduce our reliance on Russian engines for at least a decade. With a goal of promptly reducing our reliance on Russian technology, the Administration is evaluating several cost-effective options including public-private partnerships."
SpaceX - the government's saviour?
The use of public-private partnerships may see a bigger role for companies such as SpaceX, founded in 2002 by tech entrepreneur Elon Musk. SpaceX was the first company to develop a privately funded liquid-fuel rocket which could reach orbit and now manufactures the Falcon rocket family. Musk has previously said that the Pentagon should end its dependency on Russian rockets by using his Falcon 9 rocket instead.
In its recent court case against the US Air Force which began in May, SpaceX said ULA payments to Russia could contravene US sanctions.
Referencing the US's dependency on RD-180 and sanctions against Rogozin, SpaceX's court complaint said: "It is hard to imagine any way in which entrenching reliance on Russian rocket engines while funding the Russian military industrial complex with U.S. tax dollars serves national security interests, especially at a time when the Administration has sanctioned individuals associated with the same military industrial complex over the Ukraine annexation."
The court issued a preliminary injunction blocking payments to Energomash, effectively implementing an import ban on the RD-180.
After opposition from the US Department of Justice and ULA, which said SpaceX had made "unfounded speculations to create negative perceptions of a competitor", the injunction was lifted.
The outgoing head of Air Force Space Command, General William Shelton, recently told reporters at the 30th Space Symposium in Colorado that he would prefer to see the US develop its own rocket engines rather than build Russian rockets under license. "There will still be a reliance on Russian system engineering and subject matter expertise and all that. You haven't necessarily solved the problem," Shelton told reporters.
Studies begin for a next-generation rocket engine
Some see reliance on space assets as a possible weakness, especially if satellites are destroyed by space debris or anti-satellite weapons.
In June, ULA announced it had signed contracts with several US companies to study next-generation liquid-fuelled rocket concepts, the ultimate aim being to replace the RD-180. Each company will conduct technical feasibility analysis and identify cost and technical risks. The first launch of the next-generation rocket is set for 2019.
"While the RD-180 has been a remarkable success, we believe now is the right time for American investment in a domestic engine," said Michael Gass, ULA president and CEO. "At the same time, given that ULA is the only certified launch provider of our nation's most important satellites, it is critical that America preserve current capabilities and options while simultaneously pursuing this new engine."
ULA says it will continue working with RD Amross to evaluate the long-term feasibility of the RD-180.
Recent actions in Ukraine have forced many countries and organisations to re-assess and recalculate their relationship with the Russian Government. What may have seemed like safe investments two or three years ago are now looking a little more fragile. The drama surrounding the RD-180 has shown that the US space industry is not immune to these political calculations.
While studies are now finally underway to address the RD-180 supply chain, questions still remain as to how the US got into this vulnerable position to begin with.