Reverse Rocket

This is a post about an idea by Doug Plata. His idea is to put multiple propellant tanks on top with just enough structure to keep them intact and drop them in pairs as they drain. Under the tanks is the payload. Under the payload is a plug nozzle/heatshield that multiple engines expand against for altitude compensation. The configuration is chosen to be fault tolerant of both engines and tanks. This allows the use of somewhat questionable engines and tanks. The concept is specifically for the purpose of launching very large payloads on vehicles with relatively low development costs.

From the top down, this concept starts with a composite aerodynamic fairing that is also a fuel tank for kerosene. Next is a cluster of 19 tanks each of which holds either fuel or oxidizer with no common bulkheads. Next comes the payload volume which is the full diameter of the 19 tanks and whatever length required for the particular mission, with cylindrical sections added or removed as required. Under the payload volume is a full diameter plug nozzle that is also a heatshield for reentry. Against the sides of the plug nozzle are multiple engines of relatively low expansion ratio with the plug nozzle making up the difference at all altitudes.


The cartoon is a rough representation of Dougs’ concept. The payload can be a habitat cylinder 15 meters in diameter and 30-45 meters long. By launching it dry, all of the permanent fittings can be installed and tested on the ground as well as some of the transient components that are used early on.

The habitat is conceived as useful for an all up station in one go without the fitting problems of an expandable structure that is volume constrained at launch. It is also possible to design it in such a way that lunar mass could be added for radiation shielding in Lunar orbit or one of the L points. The rigid structure is also suitable for the Lunar surface with the capability of handling a thick regolith covering

The aerodynamic fairing that is also a fuel tank is handles aerodynamic loads only with both the fuel and pressurant gas providing support through the atmospheric portion of the flight. It is expected that mass of the shroud/tank will be on the order of 2% of the mass of fuel it holds. The shroud tank is sized to empty as the vehicle reaches low dynamic pressure at altitude when it is jettisoned.

The 19 tanks under the shroud/tank are protected from  head on pressure while in the atmosphere. As the shroud tank is jettisoned, the empty oxygen tanks in the 1 ring are sent off as well leaving only full tanks to carry. As each pair of either oxygen or kerosene tanks are drained on opposite side of the vehicle, the empties are kicked off from the 1 ring, then the 2 ring. The pressurant gasses in each tank are used as a cold gas thruster to ensure clean separation. There are as many as 6 tank staging events as the vehicle climbs out. Each tank being 3 meters diameter by 50 or more meters long, available propellant volumes are at roughly 350 cubic meters per tank. Tank mass can be on the order of 1% of propellant mass. Along with the shroud/tank, propellant mass can be on the order of 7,000 tons.

At the plug nozzle/heat shield, there are as many engines as required by a given mission. Since the concept is for launching massive one off missions, engine mounting must be modular similar to the tank concepts. Shrapnel shields and other safeguards must be designed in as the concept is for very low flight number vehicles with inherent infant mortality. Extra engines are a requirement for a couple of reasons. One is that it allows a more efficient flight profile than the normal thrust limited take offs of most launch vehicles. The other, more important reason is that available engines will be used with variable reliability and  availability. Careful attention to this detail should make it possible to use the remaining AJ-26 inventory of orbital-ATK as well as used Merlins and anything else the contractor can get his hands on. Unreliable engines can be compensated by having fail safe ways of shutting them down and jettisoning them. This approach allows buying engines from motivated sellers.

An additional advantage of Dougs’ concept is that it allows the multiple engines to use a variety of propellants. A mixture of kerosene, methane and hydrogen engines is quite possible in this configuration.

As the vehicle sheds propellant and tank mass, acceleration will rise. As it reaches the maximum desired, pairs of engines will be dropped in a manner similar to the way the tanks are treated. Each engine can be fitted with a decelerator and parachute as long as the velocity is low enough that there is a reasonable expectation of recovery. At higher velocities, expended engines will be lost as they are dropped. The ones that make it to orbit can be packaged into the heat shield for return to the ground.

The plug nozzle/heat shield has a multiple use as a heat shield in addition to its’ nozzle duties. In case of abort, engines and tanks are jettisoned and the heat shield  is used to protect the payload for a return to sea level. The idea is that the nozzle and vehicle shell may be lost, but the interior equipment could be saved for use on a replacement mission. For returning the payload from orbit, or atmospheric entry to another planet, the heat shield works in the normal manner. On a nominal mission where the payload is not returning to Earth, the heat shield is used to return the remaining engines and any other valuable gear that needs to return to the ground.

Figuring the payload to orbit is interesting. It turns out that with so many small staging events, the dropped tanks and engines can be treated as propellant for calculation purposes. With ~7,000 tons of Kero/LOX propellant, it is possible to place a 350 ton space station in orbit with well over 6,000 cubic meters of living and work space.  This is moreover, work space that doesn’t have to be cramped into tiny cubicals and corridors.

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I do construction for a living and aerospace as an occasional hobby. I am an inventor and a bit of an entrepreneur. I've been self employed since the 1980s and working in concrete since the 1970s. When I grow up, I want to work with rockets and spacecraft. I did a stupid rocket trick a few decades back and decided not to try another hot fire without adult supervision. Haven't located much of that as we are all big kids when working with our passions.

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8 Responses to Reverse Rocket

  1. DougSpace says:

    Wow. That’a quite the write-up. To be clear, I’d say that this final design us about 15% mine and 85% John’s. True, my concept had the payload just above the engine. That’s about all of my concept that I can recognize.

    I love the idea of the plug nozzle which I think is an aerospike. My concept didn’t specify an engine or propellant type. I conceived of only one engine but I think that multiple engines around the plug nozzle is ingenious as it allows for multiple propellants.

    The biggest difference was that I envisioned two sets of drop tanks. The upper pair would drop first and the lower pair would drop second hence the reverse nature. I know that people playing KSP love to have numerous lateral boosters dropping off at different points. I imagine that there is some crossover point where dividing propellant into more and more tanks eventually makes it less efficient. 19 tanks seem a lot to me. Certainly it’s a lot more than my four.

    The fuel shroud is an interesting idea that I wouldn’t have thought of. The top of my drop tanks would be aerodynamically shaped. John’s approach would keep the length of the rocket the same whereas mine would shorten as the upper tanks dropped. I wonder about the mechanics of John’s tanks being used to support the fuel shroud as well as the aerodynamics as pieces of the center part of the rocket goes missing.

    All-in-all I appreciate John running with the seed if my idea and am intrigued with the result. Thanks.

  2. Andrew Swallow says:

    The payload bay is a different shape to the 19 fuel tanks so calling it a fuel tank just adds confusion.

    To maintain structure 3-4 polls may be needed to support the fuel shroud.

  3. johnhare John hare says:

    I’m missing something in your comment. I don’t call the payload bay a fuel tank. I do call the aerodynamic shroud a tank.

    The payload area may indeed need more structure to carry the propellant tanks. The tanks however, can carry the shroud tank easily.

  4. Juan T Suros says:

    Could the outer rings be fuel bags hung below the fuel shroud? The vehicle spine needs to be strong enough to work with or without the outer rings of tankage, so why not slim down the outer thanks to a non structural fuel bladder?

  5. Andrew Swallow says:

    Sorry. I think of fairings as covering for payloads.

    A full set of tanks can certainly carry the shroud tank, check what happens when you are down to one tank and under high acceleration.

  6. Bob Steinke says:


    I think the concept is that the shroud tank is the first one that is jettisoned. If any tanks underneath it get empty before dynamic pressure is low enough to jettison the shroud they are just kept in place until the shroud tank is jettisoned. More likely they are sized so that the first LOX tanks empty just after the shroud tank.

  7. Matter Beam says:

    Just how flexible is this design?

    Could the reverse rocket be used for smaller designs?

    An idea that popped up was fuel tank structural support. By making the nose and core of the rocket a fuel tanks, and putting the payload at the bottom instead of on top and ‘sitting’ on the propellant, we can use very lightweight side tanks that ‘hang’ instead of ‘sit’.

  8. johnhare johnhare says:

    It’s a concept rather than design, so scaling would depend on a trade study with the particular use in mind. Doug and I did discuss using 7 tanks on smaller units. Part of the reasoning for 19 tanks was road-ability of the components. Any number of 3 meter tanks could be highway transported, while 4 or more meter wide units would be more restricted.

    As far as tanks hanging instead of sitting, that would be adding a structure for them to hang from without reducing the tank mass very much, if any. The tank structure should be dominated by the pressure and the liquid head. The pressure already has the tank in tension, so hanging it would add to the material stress while sitting on its’ bottom would have the tank mass in compression partially balancing the tensile load.

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