Hovering capability for the reusable Falcon 9, page 3: hovering ability can increase the payload of a RLV.

Copyright 2015 Robert Clark

 Blue Origin successfully landed their New Shepard rocket after reaching suborbital space:

 Observing the last portion of the video showing the landing, deviations from the vertical are visible but the ability to hover allowed it sufficient time to correct.

 Comparing this to the SpaceX Falcon 9 failed attempts at landing it is apparent the inability to hover for the F9 did not allow it sufficient time to make the needed corrections.

 SpaceX has said they want their next test landing to be on land at the launch site. My opinion, they might succeed on the next test or two but they will always have failures without hovering ability.

Merlins in a pressure-fed mode.
 Achieving hovering is not even difficult. In the blog post "Hovering capability for the reusable Falcon 9, page 2: Merlin engines in a pressure-fed mode?" I suggested giving the Merlin the ability to run in a pressure-fed mode. The question was whether this was technically feasible. I found in fact that this process of giving a turbopump powered engine a pressure-fed mode, called an idle mode, had been successfully tested during the Apollo days on the J-2 upper stage engine.

 In giving the J-2 an idle mode though, it was changed from the gas generator cycle that is used by the Merlin 1D to a tap-off cycle:

Rocketdyne J-2.
https://en.wikipedia.org/wiki/Rocketdyne_J-2#J-2S

 However, there is an engine that uses the gas generator cycle and has an idle mode, the LE-5 upper stage engine of the Japanese space agency:

Development of the LE-X engine.
https://www.mhi-global.com/company/technology/review/abstracte-48-4-36.html

 In this idle mode though the thrust is significantly less than at full thrust, only 3% in the LE-5 case. If it is a similar low percentage for the Merlin's then all 9 engines would have to be used in this idle mode to allow it to hover on landing.

 The idle mode has an additional advantage since it does not use the turbopumps. It could be used to burn both residual liquid propellant and gases in the tanks. This would mean much less residual fluid would be left in the tank. This then reduces the amount of propellant that needs to be kept on reserve for the landing.

 Elon Musk has also recently said in his Twitter account that the F9 first stage has single-stage-to-orbit (SSTO) capability. For an SSTO the residuals in a first stage can subtract a significant amount from the payload it can deliver to orbit. Then the ability to run in an idle mode with minimal residuals left over can significantly increase the payload for an SSTO. So this would be a further advantage of giving the Merlins an idle mode.

Hovering by use of flexible nozzle extensions.
 In the blog post "Altitude compensation attachments for standard rocket engines, and applications", I discussed another method of achieving hovering capability, attaching nozzle extensions to the bottom of the engines that would allow restriction of the thrust. The flexible high temperature materials already exist in the reentry materials used in NASA's Inflatable Re-entry Vehicle Experiment (IRVE). This has the advantage that the nozzle extension would have to only be applied to the one central engine to reduce its thrust on landing.

 However, the extendable nozzle attachments also have an advantage to the SSTO case. By using an extension that can be retracted at launch and fully extended at high altitude, you can get engines usable at sea level that can reach the high vacuum Isp's usually reserved for upper stage engines. In this way the 311 s vacuum Isp of the Merlin 1D can be raised to the same level of 340 s as the Merlin Vacuum. An increase in the vacuum Isp to this extent can as much as double the payload of a SSTO.

 Note that both of these techniques, idle mode or flexible nozzle extensions, would mean hovering capability can actually increase the payload rather than reduce it.

    Bob Clark

Hovering capability for the reusable Falcon 9, page 2: Merlin engines in a pressure-fed mode?

Copyright 2015 Robert Clark

 It is understandable that SpaceX wants to use the "hover-slam" approach, which allows no hovering capability, for their vertical landings of the Falcon 9 first stage. This means they would have to make no modifications to their rockets. However, it has always been taken as a given that vertical landing reusable launchers would have hovering capability:

Horizontal vs. vertical landing (Henry Spencer; Mitchell Burnside Clapp).

http://yarchive.net/space/launchers/horizontal_vs_vertical_landing.html

 In the blog post "Hovering capability for the reusable Falcon 9", I suggested various attachments to the Merlin engine nozzles that could serve to give the first stage hovering ability. Here I'll suggest some methods that will provide different means of producing lower thrust from the engines.

 The Merlin's are turbopump fed engines that use moderately high chamber pressures to produce high thrust. But what we want for the landing is actually low thrust. Then rather than using the turbopumps can we use the engines in a pressure-fed mode? The idea will be that during landing the propellant is presented to the engines using the pressurization from the tanks alone, bypassing the turbopumps.

 This requires some care however. If you have additional piping that leads from the tanks directly into the engine combustion chambers bypassing the turbopumps, then you definitely can not have the turbopumps operating at the same time. The reason is the turbopumps will provide combustion at high pressure within the engines and the low pressure coming directly from the tanks would allow hot combustion gases to travel back up these lines into the propellant tanks.

 Indeed, for all engines, pump-fed or pressure-fed, the pressure of the propellants from the piping into the engines is always significantly higher than the pressure within the combustion chamber. This is to ensure the combustion gases do not travel back up into the propellant tanks.

 Another possibility is to just use the usual piping that goes into the turbopumps but insure the turbopumps are turned off during this mode. There are various types of operating cycles used in rocket engines however. Is the gas generator engine cycle used by the Merlins amenable to this mode where the turbopumps are not turning and the propellant is allowed to flow straight through from the tanks into the engine?

Gas-generator rocket cycle. Some of the fuel and oxidizer is burned separately to power the pumps and then discarded. Most gas-generator engines use the fuel for nozzle cooling.

   For instance, with the relatively low fuel flow possible without the turbopumps would this supply sufficient cooling to the combustion chamber and nozzle?

 There is also the question of how much thrust you can get at this low pressure. Typically for pump-fed engines, the propellant tanks are only held at pressure ranges of about 2 to 3 bar. Necessarily then the pressure within the combustion chamber would have to be even lower than this. You would then have the problem that the pressure in the combustion chamber would be only slightly higher than the surrounding air pressure at sea level, making it difficult to get net thrust. If it does work, likely you would need to use more than one Merlin for the landing, possibly all of them,

 Another possibility for getting lower thrust would be to emulate the "thrust augmented nozzle" proposal of Aerojet. This works in analogous fashion to an afterburner for fighter jets. It would inject additional propellant into the nozzle to get higher thrust, so you have actual combustion going on both in the combustion chamber and in the nozzle.

 A modification to this idea would be to just inject the fuel, not the oxidizer, into the nozzle. Since this is to be used just for landing you would have sufficient air for combustion. The advantage of this is that you would save on the total propellant required for the landing since the oxidizer would not be used.


  Bob Clark

UPDATE, July 8, 2015:

 If the Merlin can not be made to be pressure-fed, SpaceX does have a pressure-fed engine, the Kestrel. It was used on the upper stage of the Falcon 1. It had an approx. 3,000 kilogram-force vacuum thrust. Being an upper stage engine it would have reduced thrust at sea level.

 Moreover, it was designed for the 200 psi tanks of the Falcon 1 upper stage. The Falcon 9 tanks, for the pump-fed Merlins, would be at lower pressure, perhaps in the 50 psi to 100 psi range. This would mean the thrust would be even further reduced.

 For a ca. 15,000 kg dry mass F9 first stage you might need 8 to 10 of the Kestrel's with their reduced sea level thrust. The mass penalty would not be severe since they only weighed 52 kg. And they would weigh even less than this in this application since you would greatly reduce the nozzle size to operate at sea level. There is also the fact that for a first stage, extra mass added to the stage only subtracts a fraction of this added mass from the orbital payload capacity.