Sunday, October 30, 2016

Altitude compensation attachments for standard rocket engines, and applications, Page 4: the double aerospike.

Copyright 2016 Robert Clark
(patent pending)

 On the blog page "Altitude compensation attachments for standard rocket engines, and applications", I suggested various methods to accomplish altitude compensation with already existing engines. One method was a sort-of "inverted aerospike". It consisted of a movable spike pointed inward, rather than pointing outward as with the standard aerospike:

 There are two disadvantages to this method. First the spike has to be movable so that adds mechanical complexity. Secondly, the size of the outer, fixed nozzle in order to achieve high Isp at high altitude has to be large. But this nozzle will be used all the way from the ground, so this will induce high drag at low altitude.

 The reason why this nozzle has to be large is because you are not really using the altitude compensating capacity of a shaped spike on exit from the nozzle. The only purpose of the movable spike is to vary the size of the exit plane of the nozzle, to provide a variable area ratio.

 But could we use a fixed nozzle and the usual outward-pointing aerospike? This would have the advantages that we could use the altitude compensating capacity of the usual aerospike, so we could use a shorter nozzle, and also have a fixed spike, reducing mechanical complexity. 

 The problem with this with a usual engine is you would need to change to a toroidal combustion chamber, an expensive change to an engine. So instead of this, we will also use an inward pointing spike so that the exit of the nozzle has a toroidal shape:

  This now has two advantages. We will be using this as an attachment to a usual ground-firing engine and nozzle. Since these already expand the exit gases to a certain extent, you would need a much shorter, slimmer and lighter outward-pointing spike to accomplish the rest of the expansion at high altitudes. The usual aerospike has to accomplish the full expansion from ca. 100 bar combustion chamber pressures to near vacuum pressures at high altitude, requiring a large and heavy spike.

  Another advantage is that nozzles for sea-level-firing engines actually overexpand the exit gases at sea level. This is because you want a longer nozzle to achieve at least moderate performance also at high altitude. But now, with the addition of the inward-pointing spike you can reduce the pressure at exit of the nozzle to that of sea level by reduction of the exit plane area. This will also improve the performance at sea level.

   Bob Clark


  1. but the spike would have to be fixed in the nozzle somehow

    the radial vanes holding the spike would reduce the exit plane area and thus make a larger nozzle necessary (with the resulting weight and drag penalites) and would create additions resistance and turbulence in the gas stream

    so i wonder if thiss would crrate a bottom line advantage

  2. Hi. This is an interesting idea, but I have a few questions:

    -What is the Isp gain of such a configuration?

    -How is the plug/cone cooled? If using active cooling, how is the coolant delivered? Can in be recycled into the combustion chamber?

    -How much drag does the plug/cone impose on the gas flow? Will it be significant?

    -Will it be possible to pass a stiff rod through the middle of the nozzle to connect the plug/nozzle to an actuator on the spaceship?

    1. Active cooling was to be used on the aerospike for the X-33/VentureStar so presumably would work here as well for the exterior spike.
      Higher temperatures would be experienced by the interior spike. We might need higher temperature materials such as ceramics:

      In regards to the Isp gain ideally it would be the same as the vacuum Isp of the engines with the nozzle extensions. For instance for the Merlins it goes from 312 s to 342 s. For the aerospike it doesn't recover the full Isp of the vacuum optimized nozzle but close.

      For the drag questions due to the cone or support structures, that would require computational-fluid dynamics simulations. I hope to partner with researchers with experience in the field to address such questions.

    2. The concept sounds interesting at 400 or so Isp all the way into orbit, an altitude compensating nozzle will allow for a two-fold reduction in the mass ratio required to get into orbit, compared to a rocket engine that averages 300 Isp.

      Liquid cooling: This cuts into effective Isp, as I do not think a regenerative high pressure loop can be run directly through the exhaust stream. You mention ceramics, but aren't they avoided in rocket engines due to the propensity to fracture due to vibrations?

      Another question: have you considered half-nozzles? Like the X-43a's 'aerospike' that only opens to the atmosphere on one side. Current nozzles can be plugged. It would provide a channel for cooling to be delivered to the spike without exposing the coolant lines to high-temperature exhaust..