单级入轨
单级入轨的设计难题

The design space constraints of SSTO vehicles were described by rocket design engineer Robert Truax:
Using similar technologies (i.e., the same propellants and structural fraction), a two-stage-to-orbit vehicle will always have a better payload-to-weight ratio than a single stage designed for the same mission, in most cases, a very much better [payload-to-weight ratio]. Only when the structural factor approaches zero [very little vehicle structure weight] does the payload/weight ratio of a single-stage rocket approach that of a two-stage. A slight miscalculation and the single-stage rocket winds up with no payload. To get any at all, technology needs to be stretched to the limit. Squeezing out the last drop of specific impulse, and shaving off the last pound, costs money and/or reduces reliability.
The Tsiolkovsky rocket equation expresses the maximum change in velocity any single rocket stage can achieve:
where:
(delta-v) is the maximum change of velocity of the vehicle,
is the propellant specific impulse,
is the standard gravity,
is the vehicle mass ratio,
refers to the natural logarithm function.
The mass ratio of a vehicle is defined as a ratio the initial vehicle mass when fully loaded with propellants  \left(m_{i}\right) to the final vehicle mass  \left(m_{f}\right) after the burn:
where:
is the initial vehicle mass or the gross liftoff weight ,
is the final vehicle mass after the burn,
is the structural mass of vehicle,
is the propellant mass,
is the payload mass.
The propellant mass fraction ( \zeta ) of a vehicle can be expressed solely as a function of the mass ratio:
The structural coefficient ( \lambda ) 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  \left({\frac {m_{s}}{m_{i}}}\right) 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 mass fraction  \left({\frac {m_{\text{pl}}}{m_{i}}}\right), propellant mass fraction and structural mass fraction sum to one:
Equating the expressions for structural mass fraction and solving for the initial vehicle mass yields:
This expression shows how the size of a SSTO vehicle is dependent on its structural efficiency. Given a mission profile  \left(\Delta v,m_{\text{pl}}\right) and propellant type  \left(I_{\text{sp}}\right), the size of a vehicle increases with an increasing structural coefficient. This growth factor sensitivity is shown parametrically for both SSTO and two-stage-to-orbit (TSTO) vehicles for a standard LEO mission. The curves vertically asymptote at the maximum structural coefficient limit where mission criteria can no longer be met:
In comparison to a non-optimized TSTO vehicle using restricted staging, a SSTO rocket launching an identical payload mass and using the same propellants will always require a substantially smaller structural coefficient to achieve the same delta-v. Given that current materials technology places a lower limit of approximately 0.1 on the smallest structural coefficients attainable, reusable SSTO vehicles are typically an impractical choice even when using the highest performance propellants available.