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G Hudson, 1991, "A Single-Stage-to-Orbit Thought Experiment", 4th ISCOPS, AAS Vol 77, pp 349-351.
Also downloadable from single stage to orbit thought experiment.shtml

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A Single-Stage-to-Orbit Thought Experiment

Since the first proposals for single-stage-to-orbit ( SSTO) launch vehicles in the late 1940s, the chief criticism of the concept has remained the need for high mass fractions (ratio of propellant weight to loaded weight less payload) required for the concepts to be practical. Whether reusable or expendable, in many critics' minds the impossibility of SSTO remains tied to this issue. The purpose of this analysis is to dispel the concern over the issue of mass fraction. We propose to do this by means of a thought experiment.

Can any combination of existing or historical hardware, for which we know the precise weights and performance, be combined in a manner to yield a positive payload in low earth orbit in a single stage configuration?

The two systems which we will review are based upon two flown stages. The first concept employs the Saturn S-IVB stage, while the second uses the Shuttle's external tank ( ET). In both cases we will baseline the Space Shuttle Main Engine ( SSME) as the powerplant, even though higher performance could be achieved with a "clean-sheet" engine design.


The S-IVB was designed by the Douglas Aircraft Company in the early 1960s. At the time it was the largest LOX-hydrogen stage available, but it was soon overshadowed by the five-times larger S-II. The S-IVB was used for ten years, as both the second stage of the Saturn IB and the third stage of the Saturn V. In both applications the stage was subjected to far greater loads than it will see in our SSTO application. We should also note that the technology in this stage is now nearly thirty years old.

Table A shows the relative characteristics of both the S-IVB and the ET SSTOs. Reference (1) was used to compile S-IVB data and reference (2) was used for the ET.

Table A. Data for SSTO versions of the S-IVB and the Shuttle ET
S-IVB Shuttle ET

GLOW (lbs) 330,885 1,826,096
Number of SSME(s) 1 6
Isp (average) 425 425
Payload (lbs)* 10,360 59,064
Injected (lbs) 36,885 203,564
Propellant Weight (lbs) at 6:1 294,000 1,622,532
Propellant Volume (ft3) 13,254 73,081
Average Prop. Density (lbs/ft3) 22.2*** 22.2
T/W (at liftoff)** 1.24 1.34
Lambda Prime 0.92 0.92
Delta V (fps) 30,000 30,000
Mass Ratio 8.971 8.971

* any fairing weight and payload support provisions must be deleted from these numbers

** assumes 109% SSME power level

*** assumes 5.5:1 mix ratio is changed to 6:1 without increasing tank volume, ie a floating bulkhead.

S-IVB Shuttle ET

Basic vehicle weight (lbs) 22,300 68,000
SSME(s) +3,000 42,000
Thrust structure _____ 30,000
Residuals (0.25% as achieved by S-IVB) 725 4,000
Avionics 500 500
Injected (less Payload) 26,525 144,500

The S-IVB SSTO would be capable of placing about 10,000 pounds payload through a velocity increment of 30,000 feet per second. If the velocity requirement could be lowered to 29,300 fps (typical of a launch from the Cape), the increase in payload could be about 2,200 additional pounds. The weight of a payload fairing and support hardware must be subtracted from this number to obtain true capability.

Work by Bono (3) suggested that an S-IVB could be recovered at a penalty of 6,500 pounds. This would suggest that a primitive, but reusable SSTO could be built which would have a payload in the few thousand pound range, using twenty-year-old structural and propulsion technology.


A single-stage could also be made out of the Shuttle external tank by the addition of six SSMEs and a thrust structure to transfer the loads of the engines into the barrel of the tank. A generous 30,000 pounds was allotted to the thrust structure weight budget in Table A. No deletion of unnecessary hardware (such as the SRB load carry-thru structure, the orbiter attach bracketry, or the tank reinforcing beams) was postulated. (A weight savings of at least 10,000 pounds could be made here if desired.) Even so, a payload in the 60,000 pound class could be orbited in the expendable single-stage mode. Again, with a lower target for total velocity change, an additional 12,000 pounds of payload could be obtained.

Nearly 75% of the desired "heavy lift" NLS-class payload could be achieved without the expenditure of a dime on new technology. Wise use of newer high T/W engines, altitude compensating nozzles, high o/f mixture ratios or dual fuel, and modern structures would bring the payload to over 100,000 pounds at little risk, and in a phased developmental program. Recovery of the engines from orbit might reduce operating costs to an affordable level.


We have shown that off-the-shelf (or out-of-the-museum) flight-proven aerospace components can be combined to conclusively demonstrate the feasibility of SSTO. The addition of a dash of innovation, combined with remarkable advances in the state-of-the-art of materials, propulsion and avionics technology which have occurred since the S-IVB, Shuttle ET and SSME were designed, would strongly suggest that a fully reusable, durable, and inexpensive SSTO could be fashioned without breakthroughs or further technology programs.

We do not propose that either of the conceptual vehicles discussed above actually be built. Rather, we suggest that a sensible and low risk program be initiated to explore the limits of present technology before spending vast sums on unproven or speculative programs. Such a program as we propose would have as its short-term goal the flight testing of a small SSTO, such as the Pacific American Phoenix or the Lockheed X-rocket.

  1. W L Osterhout, " Saturn Data Summary Handbook", Douglas Missile and Space Systems Division
  2. Martin Marietta, April 1983, " Space Shuttle External Tank System Definition Handbook, Volume 1, Configuration & Operation"
  3. Philip Bono et al, " The Saturn S-IVB as a Test-Bed for Booster Recovery", Douglas Eng. Paper 3808 - Presented at the 6th European Symposium on Space Technology, May 23-25, 1966, Brighton, England
G Hudson, 1991, "A Single-Stage-to-Orbit Thought Experiment", 4th ISCOPS, AAS Vol 77, pp 349-351.
Also downloadable from single stage to orbit thought experiment.shtml

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