The US Air Force Tries To Do Reusables

But is not a real RLV program. It’s just a narrow test for one technology. Hence I think naming it Reusable Booster System Pathfinder is misleading.


They overspecify the problem by requiring a glide landing. Why is it superior to powered landing? At the moment, there’s no clear reason to believe it is! Both need to be developed further to understand their advantages and drawbacks. To my knowledge, there have been only six liquid rocket VTVL prototype manufacturers so far: McDonnell Douglas, JAXA (who was the contractor?), Armadillo Aerospace, Blue Origin, Masten Space Systems and Unreasonable Rocket. Only a few of those have flown to higher than a few hundred meters. The design and operations space is mostly totally unexplored.

Nevermind the large number of other alternatives to boostback. Jon Goff had a recent “lecture series” about these.

I understand that this is just one program, but this should not gain the status of the reusables approach of the air force – stuff like that easily happens.

Master Design Fallacy

They also discard evolution and competition – instead just requiring a single masterfully designed prototype before something operational. Sure, this is much better than starting a multi-billion dollar program without a first lower cost prototype, but nevertheless, it sucks. Somebody brief them on newspace! Rand Simberg, Monte Davis, Jonathan Goff, Clark Lindsey, or one of the numerous people who get it. Or one of the prominent company leaders: John Carmack, Jeff Greason, David Masten.

An Ideal Program

Just specify some boost delta vee points and let companies demonstrate progress towards that. A popup tailflame lander would perhaps give more vertical velocity while some good glider or even a booster that has engines for cruising back could boost far down range to give lots of horizontal velocity. There ain’t a clear winner – there might not even be and multiple approaches would have their uses.

I quote from the document (some emphasis mine, and comments in [] brackets):

  1. Responsive and low-manpower operations
  2. The use of Liquid Oxygen (LOx) and Kerosene (RP) as main propellants
  3. Reusable, highly reliable systems with extensive use of Integrated System Health Management (ISHM)
  4. Aeroconfiguration Traceability to envisioned Reusable Booster System (e.g., high L/D configuration with horizontal landing, main engines encapsulated in nacelles, etc.)

The RBS Pathfinder program is structured with four phases.

1. Phase I – Initial Design. This phase consists of trade studies and analysis to mature the RBS Pathfinder system concept and test plan.

2. Phase II – Detailed Design and Integrated Propulsion System Test. [It’s good that you test the propulsion before building the craft]  This phase consists of completing the RBS Pathfinder system design and test plan. Also included in this phase is a ground test of the Pathfinder’s propulsion system. This test should include as much flight or flight-like propulsion system hardware as practical.

3. Phase III – Ground Launched Flight Test. This phase consists of the fabrication, assembly, check-out, and delivery of the RBS Pathfinder flight system and operations control center. It also includes support for the ground launched flight tests of the RBS Pathfinder. At least two ground launched tests are expected in Phase III. [I expect more tests as there will be envelope expansion.]

4. Phase IV – Rocket-back Flight Test. This phase includes support for the rocket-back flight tests of the RBS Pathfinder. At least three rocket-back flight tests are expected in Phase IV. [Nice that multiple tests are required. Though if the vehicle is lost, I wonder how this goes… perhaps the contractors will build more examples than one]

Table 1 lists the Key Performance Parameters that must be addressed by the RBS Pathfinder program. Examples calculations and further explanation the Phase IV KPPs are included in Appendix A to this announcement.

Table 1: Key Performance Parameters

KPP Threshold Goal
In Phase II – Detailed Design and Integrated Propulsion System Test
Propulsion System Test Mission Profile Traceability Match one Phase III Mission Profile, including duration. Match one Phase III and Most Stressing Phase IV Mission Profile, including durations.
In Phase III – Ground Launched Flight Test
Flight Modes Demonstrated Subsonic and Transonic Powered Ascent; Subsonic and Transonic Glide; Approach and Landing Subsonic, Transonic, and Supersonic Powered Ascent; Subsonic, Transonic, and Supersonic Glide; Approach and Landing on a Prepared Surface
In Phase IV – Rocket-back Flight Test Program Conditions (Thresholds required on every flight, Goals desired across flight test program, not simultaneously on a single flight)
Simulated Staging Point in Nominal Ascent Condition for a Spacelift Mission (power on, starting the rocket-back maneuver)
Mach Number 3.5 7.5
Flight Path Angle 35 degrees 20 degrees
Freestream Dynamic Pressure 100 psf 25 psf
During the Rocket-back Maneuver
Ground Track Azimuth Change During Rocket Powered Flight 120 degrees 180 degrees
Maximum Angle Between Velocity Vector and Roll Body Axis 90 degrees 180 degrees
Max Normal Acceleration 4.5 g 3.0 g
Max Axial Acceleration 7.0 g 5.0 g
Rocket-back Maneuver Endpoint (Power-off, Equilibrium Glide Established)
Ground Track Azimuth Relative to Azimuth at Simulated Staging Point 150 degrees 180 degrees
Altitude 20 kft 50 kft
Glide Path Angle 20 degrees 8 degrees
Lateral Offset from Simulated Staging Point 4 miles 0 miles
Uprange Distance Relative to Simulated Staging Point 0 miles 5 miles
In Phase IV – Rocket-back Flight Test Program Flight Envelope (Variation of the KPP achieved across all the test flights)
Simulated Staging Point in Nominal Ascent Condition (power on, starting the rocket-back maneuver)
Variation of Mach Number 2.0 4.0
Variation in Flight Path Angle 3 degrees 15 degrees
Variation in Freestream Dynamic Pressure 20 psf 75 psf
During the Rocket-back Maneuver
Variation of Ground Track Azimuth Change During Rocket Powered Flight 20 degrees 60 degrees
Variation of Maximum Angle Between Velocity Vector and Roll Body Axis 10 degrees 90 degrees
Variation of Minimum Engine Thrust During Rocket Powered Flight 0% (based on Nominal Ascent Thrust) 66% (based on Nominal Ascent Thrust)
Variation of Max Normal Acceleration 0.5 g 1.5 g
Variation of Max Axial Acceleration 0.5 g 2 g
Variation of Max Instantaneous Vehicle Rotation Rate (excluding Roll) 5 deg/sec 10 deg/sec
Variation of Min Freestream Dynamic Pressure 0 psf 15 psf
Variation of Max Freestream Dynamic Pressure 100 psf 250 psf

Table 2 lists additional characteristics of the RBS Pathfinder that should be achieved to the maximum extent practical.

Table 2: Additional Characteristics

In Phase II –Integrated Propulsion System Test
Maximum use of Flight or Flight-like Hardware in the Integrated Propulsion System Test
Demonstrate Responsive Pre and Post Flight Operations
Demonstrate Autonomous Operations
Demonstrate Low-Manpower Operations
In Phase III – Ground Launched Flight Test
Demonstrate Responsive Operations
Demonstrate Low-Manpower Operations
Minimize Number of Systems Requiring Replacement Between Flights
Minimize Possible Weather Related Impacts to Flight Test Program

Seems it’s surprisingly heavily dependent on the aerodynamics of the turn. Mach 2-3.5 at only 6 km height. I expect the final vehicle to resemble the X-7 / Starfighter, or then a tailless delta / Mirage. Without the air breathing propulsion of course.

X-7:Relatively short very thin little swept trapezoidal steel wings that can take the heat and don’t produce huge drag. Since the center of gravity after the boost is aft, because there’s not much propellant, the wings can potentially be quite aft and serve as tailfins not killing stability at launch. A separate tail is the easiest from some point of view.

A delta wing might not need to be so thin and could house the landing gear and the elevon mechanisms, freeing up space in the fuselage. Though the turning might be too slow without a tail or the L/D might be bad when turning. This doesn’t probably matter though as all that can be done by the gimballing propulsion system. Landing might require high AoA that means a long nose gear or slamming when landing.

Another alternative is a completely deployable narrow chord high lift cambered wing. A scissor wing, two variable sweep wings, or even a diamond wing like on the Small Diameter Bomb.

sketches of a few wing configurations

Conventional (X-7), Delta (Mirage), Deployable Diamond (SDB) or Scissor (Baikal) / Sweep wing (JDAM) configurations for the rocket glider

One more note:

If the RBS Pathfinder system requires a launch assist system for any of the rocket-back flight tests, the contractor shall perform analysis and verification testing in support of the integration of the RBS Pathfinder system with the Launch Assist System. Launch Assist Systems options include, but are not limited to, serial and/or parallel burn rocket assist systems and/or a carrier aircraft.

So one could use a carrier aircraft or two stages! This gets interesting. Could some old missile do this?

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2 Responses to The US Air Force Tries To Do Reusables

  1. Jim Grote says:

    I agree that the RBS Pathfinder program pre-solicitation is over-specified, and not just for landing mode concerns. I am not sure why the AF is excluding VTOVL vehicles, but that will have the effect of reducing the number of proposal submissions. Perhaps that is their intention. The degree of detail makes it appear that the contractor has a specific vehicle in mind.

    The main reason to require wings is to gain cross-range. It is not clear that cross-range is particularly important for stage 1 vehicle returns. I strongly suspect that in real operations the advantage (if any) of significant cross-range ability will evaporate. Without serious cross-range ability, winged vehicles are almost certainly heavier and will have less payload than vertical-landing pure rockets for this application (but that must be conclusively demonstrated in practical vehicles).

    The problem with wings is that you end up designing a high-supersonic airplane in addition to a rocket, which makes the vehicle complex, heavy and expensive. While vertical reentry and landing of a skinny VTOVL rocket stage can be tricky, will likely be parachute braked, and may end up heavier than expected, I see no reason to foreclose that option at this stage of the competition.

    Finally, I would like to see fuel options other than LOX/Ke included as options. My personal preference is for H2O2/Ke which has some operational and structural advantages over Lox based systems.

    For example, proper resin-infused monofilament and unidirectional carbon and kevlar composite tanks are always lighter than metal, and they are easier to build for peroxide than for lox with its attendant thermal expansion issues. Peroxide is also far more storable (for launch holding durations of issue) than lox. In addition O:F for lox/ke optimizes around 3:1, while peroxide is about 8:1, which ordinarily will make for higher density ISP for peroxide systems, and generally smaller and lighter vehicles. The performance of peroxide/ke engines at reasonable chamber pressures is not significantly greater than Lox/Ke systems, especially when factoring in density issues. Vacuum performance of Rocketdyne peroxide/ke engines approaches 330+ sec.

  2. Jim Grote says:

    Quick correction for my last comment:

    Clearly Lox/ke engines have higher performance than peroxide/ke.

    Last paragraph should read “Performance of peroxide/ke engines . . . is not significantly LESS etc.

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