Saturday, October 18, 2014

Advanced Cryogenic Expendable SSTO: The Future?

Research into Single Stage to Orbit (SSTO) rocket designs has been ongoing for decades. During that time, numerous designs, such as the X-33, DC-X, and Roton, promised to deliver exceptional performance, but failed. However, they all failed, due to technology issues, lack of funding, an inherently flawed design, some combination of the above, or other variant problems. Like fusion power, I have heard it said that SSTO rockets are always a few years away.

I recently acquired a paper (which is unfortunately behind a paywall) entitled "ACE: Practical SSTO" (written by Paul W. Gloyer, Tim S. Lewis and Zachary R. Taylor), which claims to present a workable design for an SSTO launch vehicle. Given that engineers at Lockheed Martin, McDonnell Douglas, and other companies probably claimed the same thing about their designs, I'm a bit skeptical. Still, after reading the paper, I'm a bit more optimistic, though not entirely certain that their design will revolutionize spaceflight.

Many SSTO designs wanted to achieve both SSTO performance and a reusable vehicle. There is some logic behind this; with a reusable SSTO design, the entire vehicle could be reused (unlike designs such as the X-37 or STS, where only a portion of the stack is recovered). The only costs would be fuel, and whatever (theoretically) minor repairs would have to be done to the vehicle between flights. However, reusability adds more complexity and mass to the vehicle, in the form of a heat shield, landing gear, and the like. Additionally, as demonstrated by STS, the amount of time and money needed for refurbishment between flights is nontrivial.
In the paper linked, ACE stands for Advanced Cryogenic Expendible. Since the design is not intended to be reusable, heat shields and similar parts are not required. Some might ask, then, what is the advantage of an SSTO over a conventional staged rocket if it cannot be reused? In the paper, primary advantages are identified as reduced complexity, and reduced operating costs due to lack of spent stages falling to Earth. While I am somewhat skeptical on how important the second point will be, the reduced complexity of a single-stage design is definitely a valid point. A single-stage, expendable vehicle has the potential to be a very simple design indeed, and anything which reduces the number of failure points is usually good.

A diagram of the vehicle described in the paper.

Another aspect of the SSTO described which differentiates it from most earlier SSTO designs is its relatively small size. While both the DC-X and X-33 were eventually planned to be developed into manned versions, the ACE is designed to only launch small satellites. The configuration in the paper is listed with a total weight of under 3500 kg (7500 lbm), and a payload of about 70 kg. Not developing a manned vehicle means that life support systems are not needed, and eliminates the need to satisfy NASA (or any other space agency's) requirements for man-rating a launch vehicle. The small size would also reduce support and transportation costs of such a launch vehicle. However, the small size does introduce new technical challenges; volume increases with the cube of size, but surface area increases quadratically. This means that larger launch vehicles have to devote a lesser portion of their mass to fuel tanks, typically the largest portion of a rocket's dry mass.

ACE dimensions, in Imperial units for whatever reason.

The team developing the ACE proposes to eliminate this problem by using advanced composite fuel tanks. Specifically, they plan to use a type of composites known as BHL. Those familiar with the X-33 program may recall how issues with composite fuel tanks were one of the issues that led to the scrapping of that program. However, given that the ACE is a geometrically simpler design (roughly cylindrical, compared to the X-33s shape), and the advancements in composites over the past decade, I think that using composite fuel tanks is a viable strategy. The ACE team plans to achieve quite low mass/volume ratios with composite tanks; on the order of .5 lb/ft^3 o (8 kg/m^3). Naturally, larger diameter tanks will be more efficient. Since larger tanks are required for LOX/LH2 (the only commonly used fuel which could provide reasonable SSTO performance), this could be a major breakthrough.

Comparison of BHL with aluminum-lithium metal tanks, which are some of the lightest metal tanks currently in use.

One interesting thing mentioned in the paper was the relatively large size of the vehicle (for its payload) in orbit. While the actual satellite payload might only be a few dozen kilograms in mass, there would still be several meters of vehicle remaining. As mentioned in the report, if covered in solar panels, there would be a very large amount of power available for the payload. It is also mentioned that scaling up the vehicle, and/or fitting an electric propulsion system for in-orbit use, could give the payload sufficient delta-V for transfer to GEO or beyond.

While the concept is, overall, very interesting, I do have a few minor nitpicks. For one, they assume use of a LOX/LH2 engine with a thrust/weight ratio of roughly 75. This is in excess of both the J-2 and RS-25 (I have heard that the latter was considered to be a very 'hot' engine). While a T/W ratio of 75 might be possible with new developments in rocket engine technology, I would prefer to see an actual test of such an engine, especially one of such a small size as the ACE would use, before making any conclusions.

Another one is the economic issues. While there may be a market for putting relatively small satellites into orbit, I am skeptical of whether the ACE would provide sufficient savings over simply launching them as a secondary payload on a larger rocket. Additionally, the STTO concept suffers from an inherent disadvantage compared to staged rockets; the entire rocket must be carried for the whole flight, while a staged rocket can simply dump useless mass (empty fuel tanks and associated structure) partway through the flight. How the ACE SSTO compares with a small two-stage design incorporating similar technologies would be an interesting thought exercise.

Despite these misgivings, I feel that the ACE SSTO concept has merit, and could potentially be quite good. At the very least, I hope that a couple test launches of the concept are made; that would put it far ahead of any of its peers to date.

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