All-Liquid:  A Super Heavy Lift Alternative?

by Ed Kyle, Updated 11/29/2009

The recent Augustine Committee (Review of U.S. Human Spaceflight Plans Committee) report evaluated a series of launch options for deep space human exploration.  All beyond-low earth orbit (LEO) options required development of new heavy lift launch capability.  This article will examine potential "all-liquid" approaches for Super Heavy Lift (or, as NASA calls it, Heavy Lift Launch Vehicle (HLLV)).

Augustine's Options

The Augustine Committee deemed NASA's existing Ares I/V based Constellation lunar exploration architecture technically sound, but too expensive both to develop and to fly.  Although it presented a series of options rather than recommendations, the Augustine Committee report tended to support cancellation of the Ares I crew launcher in favor of a smaller "commercial" crew launch system that could carry crew to ISS.  It also tended to support development of a smaller-than-Ares V heavy lift launch system.  The smaller Augustine heavy lifter would support a multiple-launch architecture, carrying both crew and cargo in support of broadly defined "flexible" missions that might or might not one day return U.S. astronauts to the lunar surface. 

The currently planned Ares V would be a powerful super-rocket, able to lift 160 tonnes to LEO (a number that includes the weight of the EDS stage and its unburned propellant load).  It would stand nearly 118 meters tall and would weigh nearly 3,705 tonnes at liftoff.  Ares V would use two 5.5 segment solid rocket boosters to lift a 10 meter diameter core powered by six RS-68B engines.  The 1,587 tonnes of core stage propellant alone would weigh more than two complete Ariane 5 rockets.   A single J-2X engine would power the upper EDS (Earth Departure Stage).  The EDS would serve double duty by first inserting itself and an Altair lunar lander into low earth orbit (LEO), and then firing to propel an Altair/Orion stack toward the Moon.

Augustine's report described an alternative "Ares V Lite" that would be powered by 5-segment boosters and would use only five RS-68 core engines.  It would still be a substantial, potent booster, able to lift 140 tonnes to LEO.  Ares V Lite would be "man-rated" to support a dual-launch architecture.  Money would be saved by developing a single launch vehicle to perform the mission, rather than the currently-planned two launch vehicles (Ares I and Ares V).   NASA's own 2005 ESAS (Exploration Systems Architecture Study) noted that dual-launch architectures were the lowest-cost approach to lunar missions.

Atlas 5 Phase 2

The report also described Shuttle-Derived and EELV-Derived super-heavy alternatives.  Shuttle-derived options were assumed to use existing four-segment boosters, SSME (Space Shuttle Main Engine), and 8.4 meter diameter "ET" type tanks.  Both "Side Mount" and "In Line" options exist that are expected to be able to lift as much as 100 to 110 tonnes to LEO.   One "In Line" option that has received some attention in the popular press is called "Direct". 

NASA is said to be currently evaluating a variety of Shuttle-Derived alternatives.  Shuttle-Derived promises lower development cost than other alternatives, but would cost more to operate.

The Committee described an EELV Super Heavy that could lift 75 tonnes to LEO.  The rocket described in the Augustine Report is "Atlas 5 Phase 2 Heavy".  It would consist of three 5-meter diameter units arranged in a core plus strap-on configuration.  Each unit would be powered by a pair of Russian RD-180 LOX/kerosene engines.  Tanks for this rocket would be formed using Delta 4 tooling.   The upper stage would be powered by four RL10 engines.  A smaller rocket could be created by using just one of the 5-meter units as a first stage.

An attractive feature of Atlas 5 Phase 2 is that it would be an "all-liquid" rocket.  All-liquid rockets offer the potential of lower lift-cycle costs by eliminating an entire contractor chain for solid rocket motors, and by eliminating the costly and hazardous launch processing steps needed for solids.   Offsetting these advantages would be the need to launch twice as many Atlas 5 Phase 2 rockets for a given mission.

All-Liquid, Super-Sized

The Committee did not mention the possibility of all-liquid launch vehicles able to lift more than Atlas 5 Phase 2.  Such vehicles were studied during the 2005 ESAS work.  An interesting alternative was also examined by the Aerospace Corporation in a report issued during early 2009.  Recently, news reports have suggested that NASA may be seriously considering bigger "All-Liquid" options. 

A "Super Delta" Concept Capable of Outperforming Ares V

These Super Heavy alternatives tend to use either 8.4 meter ET type tanks, or the 10 meter diameter tanks planned for Ares V and/or Ares V Lite.   For payloads in the 100 tonne class, they tend to add strap on boosters based on existing EELV Atlas 5 or Delta 4 cores.  Known alternatives use either RS-68 or RD-180 engines, though NASA is thought to favor RD-180.  Russian-built RD-180 engines cost much less, as much as one-third less, than an equivalent U.S.-built version would cost, but building RD-180 in the U.S. might be necessary for this application.  If built in the U.S., RD-180 would cost about the same as the existing U.S-built RS-68 engine. 

In order to lift 100 tonnes or more to LEO, seven or more EELV-class booster engines would be required.  These could be arranged in groups of five to six at the base of the core stage.  Additional engines could be added via. strap-on boosters. 

Since it would use dense kerosene fuel, an RD-180 powered rocket could use 8.4 meter diameter core tanks.  An RS-68 powered Super Heavy would almost certainly require 10 meter diameter core tanks.  While an RD-180 powered rocket would have a smaller core stage than an RS-68 powered rocket, the RS-68 powered rocket would be able to use a substantially smaller, lighter upper stage. 

Both alternatives would be shorter than Ares V.  Both would weigh vastly less during stacking and rollout, since loaded solid motor segments would not be used.  All-liquid alternatives would not require the new super-crawlers, mobile launch platforms, and new crawlerway needed by Ares V.

The drawing accompanying this article illustrates several potential All-Liquid Super Heavy designs and compares them to Ares V.  The following paragraphs describe the concepts. 

1.  RS-68 "Core Only"

A fully fueled Ares V core stage topped by a fully fueled EDS cannot lift itself without solid rocket booster thrust, but a smaller core seems worthy of consideration.  The concept shown uses six RS-68B core stage engines like Ares V, but its upper stage needs more than one J-2X engine to maximize its LEO potential.  The first stage carries 1,046 tonnes of usable propellant.  Its engine cluster produces 1,908 tonnes of liftoff thrust and burns for nearly four minutes, providing nearly half of the vehicle's total ideal delta-v.  The two J-2X engines burn 234 tonnes of propellant during their more than 6.5 minute burn.  The rocket can accelerate 93 tonnes of payload to 9,300 m/s delta-v, which should be sufficient for LEO.

2.  RS-68 Core Plus 2xCBC

A pair of Delta 4 Common Booster Cores (CBC) serve as strap-on boosters in this concept.  In this case, only five RS-68B engines power the core.   At liftoff, the combined seven RS-68B engines produce 2,226 tonnes of thrust to boost the 1,855 tonne rocket.  The core is loaded with about 1,000 tonnes of useable propellant while the two boosters combine to add 408 tonnes more. The boosters burn out and separate about four minutes after liftoff.  The core continues alone for another 77 seconds.  Only 145 tonnes of propellant must be burned by the single J-2X upper stage engine to propel 105-110 tonnes to LEO. 

A larger upper stage could act like a two-burn EDS, providing both LEO and trans-lunar burn capability.  Such a stage could boost more than 40 tonnes toward the Moon. 

A second J-2X upper stage engine would allow this rocket to lift 120 tonnes to LEO, but would not increase its TLI capability.  A dedicated RL10-powered third stage could increase TLI capability to 50-55 tonnes.   

3.  RS-68 Core Plus 4xCBC

Additional CBC strap-on boosters provide more payload.  In this case, four CBCs allows at least 130 tonnes to be orbited.  A version with six CBC strap on boosters, firing a total of 11 RS-68B engines at liftoff to produce more than 3,500 tonnes (7.7 million pounds) of thrust, could provide better-than-Ares V capability.

Although some are taken aback at such engine counts, it is worth remembering that each Saturn V used 11 high-thrust liquid rocket engines in its three stages.  The USSR's N-1 rocket used 42 main engines just to reach LEO!  This is what it takes to send humans to deep space!

4.  RD-180 Core Plus 2xCCB

The final example shows the effect of using a staged-combustion kerosene booster.  This concept, identified in the ESAS report as an "Evolved Atlas", uses 8.4 meter diameter tanks that presumably could be built with Shuttle ET tooling.  The rocket is "skinnier" than the RS-68 alternatives, but still stands about the same height.  The core carries 1,323 tonnes of propellant and is powered by five RD-180 engines.  Two Atlas 5 Common Core Boosters, each partially loaded with 210 tonnes of propellant, augment the core.  Altogether, seven RD-180 engines combine effort at liftoff to produce a total 2,730 tonnes (6 million pounds) of liftoff thrust.   The rocket would weigh perhaps 2,270 tonnes at liftoff. 

The twin boosters would burn for about 168 seconds before being jettisonned.  The core would throttle down after the first minute of flight, allowing it to burn for 300 seconds or more. 

This concept, which could lift 110 tonnes to LEO, requires a larger, heavier liquid hydrogen upper stage than the all-hydrogen concepts previously described.  Two or three J-2X engines would be needed to power the upper stage, which would be loaded with more than 235 or more tonnes of propellant.  The heavier stage would not be able to efficiently serve double-duty as a trans-lunar injection stage due to its larger dry mass.  It would only be able to boost 35 tonnes to TLI.  A smaller RL10-powered stage about the size of a Delta 4 Heavy upper stage would do better, able to boost 45 tonnes or more to TLI.  A heavier TLI stage would be able to accelerate 50 tonnes toward the moon.  

Including its 110 tonne payload, this "Evolved Atlas" would only weigh 314 tonnes at rollout, far less than Space Shuttle (1,340 tonnes) or even Ares I (777 tonnes) at rollout.  A comparable RS-68 powered all-liquid would weigh about 354 tonnes at rollout, including payload.

Here are some prospective numbers for an "Evolved Atlas" with an 8.4 meter diameter core.  Note that the two booster stages don't want to be fully loaded with propellant.  In this example they are only 74% full.

If two J-2X engines power the upper stage, and if the upper stage initial T/W is allowed to be 0.7, the numbers work out as follows.

2xCCB Boosters (Combined, 1xRD180 each)
M_propellant_usable = 420 tonnes
M_total = 462 tonnes
Thrust combined = 780 tonnes
ISP_avg = 329 sec
Tburn = 168 sec

Core (5xRD-180)
M_propellant_usable = 1,323 tonnes
M_total = 1,423 tonnes
Thrust = 1,950 tonnes
ISP_avg = 329 sec
Tburn ~= 305 sec (assuming throttle down to 50% at T+60 sec)

Stage 2
M_propellant_usable = 235 tonnes
M_total = 262 tonnes
Thrust = 266 tonnes
ISP = 448 sec
Tburn = 397 sec

Payload Fairing = 10 tonnes
Payload = 118 tonnes
Ideal Delta-V = 9,208 m/s

Vehicle GLOW = 2,275 tonnes
Liftoff Thrust = 2,730 tonnes
Liftoff T/W = 1.2