Slide 52 of 125
By 1989, the NASA/Langley Advanced Manned Launch System (AMLS) follow-on study had investigated
airbreathing HTHL TSTO systems in addition to the vertical-launch, horizontal-landing TSTO and
SSTO "Shuttle II" rocketplane designs covered earlier.
Only TSTO systems appeared feasible if 1992 technologies (structures, propulsion, subsystems
etc.) must be used, and the AMLS team investigated five concepts (left to right): a fully
reusable VTHL TSTO (Mach 3 staging), a similar system with expendable orbiter fuel tanks, a
partially reusable system consisting of a Mach 3 VTHL flyback booster+expendable core
stage+manned glider, a 2-stage expendable launcher+manned glider, and an air-turborocket HTHL
TSTO w. Mach 6 staging. The requirement was still a 9,072kg manned payload capability to SS
Freedom & 5443kg to low polar orbit by 2005. STME, STBE, SSME propulsion system options were all
considered (methane,hydrogen fuel). Most unmanned payloads would be launched on unmanned
heavy-lift expendables such as the Advanced Launch System (ALS). The important conclusions were
The highest gross liftoff weight vehicle is the fully reusable VTHL TSTO; increasing use of
expendable components tends to reduce the weight. All-hydrogen fueled TSTO vehicles also weigh
less than their methane-boosted counterparts at launch, and the dry weight only increases
slightly despite larger tanks, lower thrust-to-weight engines etc.. Hence, the AMLS team
concluded that Space Transportation Booster engines (STBEs) should be eliminated to avoid
unnecessary engine development & maintenance expenses; all-hydrogen propulsion will be more cost
effective. The fully reusable VTHL TSTO option will require 25000 man-hours of maintenance
between missions vs. 90000 for the existing Shuttle, mainly because the External Tank and Solid
Rocket Boosters are eliminated.
The drop-tank VTHL TSTO requires no hydrogen fuel tank maintenance but ground processing hours must
be added for handling and checking operations. Advantages include reduced overall size and
weight plus additional room for internally mounted payload canisters. Propellant crossfeed
systems will be more complex, however.
This configuration would have removed the onboard propulsion system+all propellants from the
orbiter and placed them in an expendable core stage. The core main engines and avionics are
recoverable while the tanks are expendable. This system would weigh less than the fully reusable
TSTO, but the recurring cost per flight would be higher and the vehicle is operationally complex
since it consists of four elements (reusable flyback booster, core engine/avionics recovery
module, core stage propellant tanks, manned orbiter).
This concept retains the same manned orbiter of the previous booster/core vehicle, but boosted
to orbit using an expendable two-stage launch vehicle (ELV). This reduces the development cost
and the size/mass of the recoverable vehicle that must be refurbished between missions, but
recurring costs are driven by the replacement value of the ELV.
The airbreathing HTHL TSTO vehicle has the least gross liftoff mass of all but the dry mass is
the heaviest which suggests it will be the most expensive and difficult option to develop.
For example, the booster aircraft's air/turborocket propulsion system would require a thrust-to-weight
ratio of 20 -- a goal considered ambitious for a system based on existing technology.
The AMLS team concluded TSTOs will be much smaller (e.g. a 747 aircraft could easily transport
the vehicle to the launch site) and more economical than SSTOs if only moderately advanced
technologies corresponding to a 25% dry mass reduction from the existing Shuttle dry mass.
However, SSTO will become more feasible when the technology level reaches the equivalent of
35-40% dry mass reduction. With advanced technologies (60% dry mass reduction), the SSTO will
have the same gross liftoff mass as TSTOs and be much more cost effective since only a single
vehicle must be developed and maintained. Preliminary life cycle cost tradeoff analyses also
indicate partially or fully reusable concepts cost more to develop than expendables but their
total life cycle cost (development+operations) will be less. The caveat is fully reusable systems
have to perform reliably without excessive verification and costly refurbishment between flights
since the cost of losing a vehicle is very high.
"Advanced Manned Launch Systems" -- Talay & Morris, ESA 1989/SP-293/p.117 (Proceedings of the 2nd European Aerospace Conference on Progress in Space Transportation, 22-24 May 1989)