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Convair Nexus SSTO

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In 1962 the NASA Future Projects Office called for a post-Saturn launch vehicle for the 1975-2000 time frame. One concept put forward by Convair was the one million pound payload, reusable NEXUS SSTO.

At the time NASA's forward looking plans for the 1970’s and 1980’s called for lunar bases with permanent crews of 100 or more and an ambitious plan for manned exploration of the solar system. This vision helped drive the economics and scale of NEXUS—without a large number of launches its cost would be unsustainable. Large manned interplanetary spacecraft need large liquid hydrogen tanks, nuclear engines were also foreseen … not just solid-core nuclear thermal engines like NERVA, but gas-core nuclear rocket engines and nuclear-pulse engines. These are massive systems and require massive boosters.

The baseline NEXUS could lift 1 million lbs to LEO, or low Earth orbit, this could be extended to 2 million lbs. The booster was a flexible design and could be configured for different payload sizes and weights.

The baseline NEXUS was powered by a system known as a plug cluster engine, which consists of a number of high pressure liquid hydrogen liquid oxygen rocket engines arranged around a centerbody, or plug.  In the diagram, the image top right shows the base of the NEXUS booster and the plug cluster rocket engine.

Plug cluster engines belong to a class of altitude compensating nozzles much like the aerospike. The NEXUS booster design was flexible, the number and type of engines could be varied to fit different payload lofting needs, regardless of number of individual rocket engines the entire engine configuration is referred to as a plug cluster engine.

Beside the plug cluster system there were a number of different engine arrangements for the NEXUS including a single 54-foot diameter, ablative nozzle, 24 million pound thrust Rocketdyne L-24 AH engine, which is, as far as I am aware, the largest single chemical rocket engine ever designed.

For control during ascent four clusters of control rockets were to be employed. Each cluster contained six hydrogen/oxygen rockets. Throttling the engines up and down provided pitch and yaw control due to the long distance from the vehicle centerline.

After deploying its payload NEXUS would reenter Earth’s atmosphere nose-first, relying on the large blunt shape of the booster's forward face for drag to slow the vehicle during descent. Since the booster is base-heavy and would tend to tumble end-for-end, 4 flaps, arranged around the flanks of the booster and hinged at the top, would fold out from the sides, these would stabilize the vehicle aerodynamically. The free-falling vehicle had characteristics akin to a badminton shuttlecock as it plummeted toward the ocean.

Landing rockets (not shown here) mounted in the vehicles nose would fire to apply terminal braking immediately before splash-down. The solid rockets reduced velocity by 200 feet per second via a brief burst of 9,000,000 pounds of thrust, completely canceling free-fall velocity just above the surface of the water. The vehicle would float upside down on the ocean’s surface, probably stabilized by an inflatable air-bag flotation system, until recovery, thus keeping the engines safely out of the corrosive salt-water.

The curved top of the booster is an ablative material which would take the brunt of re entry heat. Below this lay a thick Styrofoam crushable-structure designed to absorb the force of ocean impact and protect the booster’s internal structure and large propellant tanks. Thermal protection and crushable-structure would be replaced between every launch. 

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Convair Nexus Reference Links, courtesy of Scott Lowether’s Unwanted Blog:

Convair Nexus SSTO
Convair Nexus 1million Lb Payload/2 million Lb Payload Comparison
Nexus + Gas Core Nuclear Second Stage
Nexus Gas Core Nuclear SSTO

Related Images

Nexus Booster Comparison
Uprated Nexus + Gas Core Nuclear Second Stage


Convair's two million pound reusable SSTO is the basis for the Martian Nexus designed for my Orion's Arm future history setting, which differs only in the significant detail that it makes a powered tail-landing, image here: First Flight.

Acknowledgements

Scott Lowther of Aerospace Projects Review put in the hours to dig up the information on Nexus, and the data in this post is credited to his hard work. What I've included here is just the barest essential detail, for the full story on Nexus, along with diagrams and facts covering area's of the program not available elsewhere, I highly recommend  Scott Lowther's complete article on Nexus, available here V3N1 of Aerospace Projects Review.

Photo credit: Atlas scale reference image courtesy of NASA via OSU.edu library, NASA History.
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Comments15
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detailedatream1991's avatar

That Good Design Rocket Booster and If They back to 1960s Put-up On Saturn 5 called: 3.5

LordOmegaZ's avatar
kinda looks like a massive aerospike o~o;
William-Black's avatar

Technically it is a plug cluster engine, a number of high pressure hydrogen oxygen rocket engines arranged around a centerbody, or plug. Plug nozzles belong to a class of altitude compensating nozzles much like the aerospike. The NEXUS booster design was flexible, the number and type of engines could be varied to fit different payload lofting needs, regardless of number of individual rocket engines the entire engine configuration is referred to as a plug nozzle engine.

LordOmegaZ's avatar
wait so you could enlarge its size and amount of rockets and it would STILL lift even heavier loads o____O;

also if we HAD made say at least one of these could it put the ISS up in one go? (sea dragon i heard could do that and ORION could put TWO up in one launch) XD
William-Black's avatar

The baseline NEXUS could lift 1 million lbs to LEO, this could be extended to 2 million lbs. The booster was a flexible design and could be configured for different payload sizes and weights, not infinitely, but substantially. There were a number of different engine arrangements including a single 54-foot diameter, ablative nozzle, 24 million pound thrust Rocketdyne L-24 AH engine, which is, as far as I am aware, the largest single chemical rocket engine ever designed.

 

Five general families of Post-Saturn launch vehicles were examined. P/S-A examined what were essentially advanced growth versions of the Saturn V, using uprated engines and redesigned tanks on the first stage with increasingly powerful nuclear upper stages.

 

The P/S-B family looked at what were essentially updated versions of the million-pound-payload two-stage Nova booster concepts that had preceded the Saturn V, again with nuclear upper stages.

 

P/S-C looked at new single-stage to orbit vehicles capable of putting one million pounds or more of payload into orbit, or of lofting even more massive upper stages. This was the NEXUS family.

 

While NEXUS had the chemical and nuclear stages laid out serially, the next family of designs, P/S-D, also known as Helios, used a parallel arrangement. Helios had chemical engines wrapped around nuclear engines, in much the same way that the two booster engines on the Atlas ICBM were on a droppable a structure that wrapped around the sustainer engine. And like the Atlas booster engines the chemical engines, and their associated oxygen tanks, would be dropped from the Helios vehicle after burnout, allowing the vehicle to continue under pure nuclear propulsion.

 

None of the Helios vehicles used a NEXUS or NEXUS-derived chemical core.

 

The fifth family, P/S-E integrated the nuclear and chemical engines into a single stage. This eliminated the operational issues involved with staging and multi-stage recoveries. But the weight penalties of this arrangement were such that the only nuclear engines that would make the concept feasible were gas-core nuclear engines.

 

There is a NEXUS vehicle in the P/S-E class with four gas-core nuclear engines integrated into the NEXUS first stage capable of delivering 1 million pound payload to the lunar surface, or 3 million pounds to LEO. This was the Super NEXUS. 440 feet tall with a second stage 170 feet in diameter, which was a sectionalized hydrogen fuel tank for the gas-core nuclear engines.

 

For a lunar payload delivery mission the vehicle would launch from the Earth’s surface, park itself in a temporary Earth orbit, boost itself to the moon, park itself in a 20-mile altitude lunar orbit, then de-orbit to an altitude of 2000 feet, where it would hover. The payload would then separate and land itself; the Super NEXUS would then boost itself back to Earth and put itself in Earth orbit.

LordOmegaZ's avatar
super nexus o_o...;

jeezus!

wait so it could go to the moon then BACK?!
demonbadger777's avatar
So cool. I can imagine the earth shaking for miles on lift off of something this big. Nicely done. 
RevealedFromtheVoy's avatar
Why do they never realized those projects ? Was it because of the lack of money ?
William-Black's avatar
David Portree wrote extensively about this in his Beyond Apollo Blog, and what I know is formed largely by his scholarly knowledge of the period. In brief, following the Apollo 1 fire, and the skyrocketing cost of the war in Vietnam, extended missions and the hoped-for build-up of near Earth infrastructure was already frowned upon in Congress – the ambitious expansion envisioned by NASA, and its contractors, and ideas such as the Integrated Program Plan (IPP), with its high-traffic models projected for the Reusable Nuclear Shuttle, proposed space stations, lunar base, and Mars missions, were dealt a bitter blow.

Portree wrote that the resignation in September 1970 of NASA Administrator Thomas Paine, an enthusiastic proponent of the Integrated Program Plan, meant an abrupt end for NASA’s enthusiastic long-range plans of the period. The agency's last Apollo-era Mars study report, completed in February 1971, reflected new realities; it examined how NASA might accomplish an "austere" Mars mission in the absence of both nuclear propulsion and the Saturn V rocket. NASA would not resume Mars planning until 1984.
LordOmegaZ's avatar
just read this.

so may i ask how would you (if you could alter history) do to change stuff to make a more extended space program?

no vietnam?
JFK stays alive? (leading to MAYBE a joint USSR/USA moon landing)
etc, etc...
WafflesToo's avatar
Urgh... nuclear propulsion systems would make a Mars flight so much easier!  Though I can kind of understand everyone's qualms about flinging a potential nuclear hazard into orbit it's still really frustrating.
RevealedFromtheVoy's avatar
Thank you very much. But what a pity ! Thinking we could be walking on Mars at that time...
1Wyrmshadow1's avatar
Where did Convair plan on launching this from? Just looking at the sheer size of it, the standard launchpads at Canaveral would have been inadequate. 
William-Black's avatar
Nexus was just one of several HLLV’s considered for a 1970’s post-Saturn program; a common design element was a payload of one million pounds to low Earth orbit. Designs such as the Douglas ROMBUS, the Aerojet Sea Dragon, the Boeing MLLV and the Convair Nexus were all a part of this. Sea Dragon was obviously sea-launched, the rest (and certainly Nexus) would have required modification to the crawler-transporter and one or more of the Saturn era launch pads.
LordOmegaZ's avatar
ah the sea dragon ;D my fave giant rocket.

and being sea launched was a good plan too.
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