very small when compared to the costs of the engineering program as a whole. As a result, a cheap rocket with a poor mass fraction may be able to deliver more payload to orbit with a given amount of money than a more complicated, more efficient rocket. Launch assists Many vehicles are only narrowly suborbital, so practically anything that gives a relatively small delta-v increase can be helpful, and outside assistance for a vehicle is therefore desirable. Proposed launch assists include: sled launch (rail, maglev including Bantam, MagLifter, and StarTram, etc.) air launch or aircraft tow in-flight fueling Lofstrom launch loop/space fountains And on-orbit resources such as: Space tether tugs Nuclear propulsion Due to weight issues such as shielding, many nuclear propulsion systems are unable to lift their own weight, and hence are unsuitable for launching to orbit. However, some designs such as the Orion project and some nuclear thermal designs do have a thrust to weight ratio in excess of 1, enabling them to lift off. Clearly, one of the main issues with nuclear propulsion would be safety, both during a launch for the passengers, but also in case of a failure during launch. As of December 2021, no current program is attempting nuclear propulsion from Earth's surface. Beam-powered propulsion Because they can be more energetic than the potential energy that chemical fuel allows for, some laser or microwave powered rocket concepts have the potential to launch vehicles into orbit, single stage. In practice, this area is not possible with current technology. Design challenges inherent in SSTO The design space constraints of SSTO vehicles were described by rocket design engineer Robert Truax: The Tsiolkovsky rocket equation expresses the maximum change in velocity any single rocket stage can achieve: where: The mass ratio of a vehicle is defined as a ratio the initial vehicle mass when fully loaded with propellants to the final vehicle mass after the burn: where: The propellant mass fraction () of a vehicle can be expressed solely as a function of the mass ratio: The structural coefficient () is a critical parameter in SSTO vehicle design. Structural efficiency of a vehicle is maximized as the structural coefficient approaches zero. The structural coefficient is defined as: The overall structural mass fraction can be expressed in terms of the structural coefficient: An additional expression for the overall structural mass fraction can be found by noting that the payload