WASHINGTON — Virgin Galactic is proposing to raise $300 million in additional capital to accelerate production of suborbital spaceplanes and a mothership aircraft the company says can fuel its long-term growth. In a Nov. 6 earnings call to discuss the suborbital spaceflight company’s third quarter financial results, Virgin Galactic executives said that while work on the first two Delta-class spaceplanes remains on budget and schedule, they see an opportunity to raise money to add vehicles to the fleet sooner that previously planned. “We have an exciting opportunity to captures economies…