XCOR Lynx: Aerodynamics–Supersonic wind tunnel tests

LITT TUNNEL

In this image, XCOR engineer Brandon Litt makes final preparations to a Lynx supersonic wind tunnel model before performing a test at the storied NASA Marshall tri-sonic wind tunnel in Huntsville, Alabama.

NASA TUNNEL

Above, a NASA Marshall spaceflight engineer tests a similarly sized supersonic wind tunnel model of the Space Shuttle in the same facility during the early years of the Shuttle program. The results of the Shuttle tunnel tests were critical to the go-forward decision to build and fly the Space Shuttle.

We believe that test pilots and customers alike require confidence that the spacecraft they fly on has been evaluated and tested extensively prior to flight. And in this day and age, many rely on computer simulation to be sure that their aircraft can perform at the level required.

XCOR used Computational Fluid Dynamics (CFD) and other software analysis tools to design the shape of the Lynx aeroshell. However, things really accelerated when we were able to perform a series of seven subsonic and supersonic wind tunnel test campaigns at United States Air Force (USAF) and NASA labs.

Typically, the faster the tunnel, the smaller the available space. Supersonic wind tunnel models are traditionally smaller than their subsonic counterparts due to the available space in supersonic tunnel test sections. The models must not only fit, but have room to change pitch and yaw inside the tunnel without approaching the side walls of the tunnel. If the model is too close to the walls, edge effects from the walls can be picked up in the data, and ruin the test.

Because supersonic wind tunnels are so small, Lynx models themselves must be highly precise to adequately simulate the full scale spacecraft. The models have many pieces that are interchangeable to test different configurations of the vehicle. For example, a model may have 5-10 different nose shapes, 5-10 different permutations of wing aileron settings, and so on. Because of this, the models can become quite expensive, sometimes approaching several hundred thousand dollars.

Because the actual Lynx is so much smaller in real life than the Space Shuttle attached to the solid rocket boosters and center fuel tank, the Lynx model shown above is actually much larger (almost 3-4 times larger) in scale than the Space Shuttle model.

Tomorrow we answer some of the week’s questions and let you know where you can find XCOR on the road in the near future. In particular, we’ll have some updates for future appearances of XCOR team members and the Lynx full scale model.

XCOR Lynx: Fuselage 101

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The Lynx flight weight fuselage is mounted on the fire wall test stand, awaiting re-installation of the engine truss structure.

On Wednesdays we highlight progress on various structural components such as the fuselage, strakes, wings, nose and landing gear.

Always build a house with a strong foundation. Always anchor a suspension bridge into bedrock. A similar principle holds true for spacecraft: they must always be built on a solid structural core that can stand up to flight in harsh environments.

The Lynx structure consists of several critical components: the fuselage (containing the liquid oxygen tank), the wings and control surfaces, wing strakes (containing our kerosene fuel, and main landing gear), cockpit, and nose (containing the nose landing gear).

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Our spacecraft is a complex assembly of highly-engineered structures designed to maximize strength and minimize weight. Each have their own functional requirements, and must serve the function of the vehicle as a whole. More so than any other application, form follows function very tightly on a high performance launch vehicle.

At the center of the Lynx is the fuselage. The fuselage holds the liquid oxygen tank, and all other major structural components attach to the fusleage. These include the truss structure that holds the propulsion system (Monday’s post), the cockpit (yesterday’s post), the wing strakes that hold the kerosene and the main landing gear, the nose structure, and the wings with their control surfaces.

The fuselage can be seen in the photo above. In it, the carbon fiber structure has been painted to protect it from the sun’s UV radiation, as UV can degrade the carbon fiber structure. The fuselage is sitting on its cradle on the Lynx firewall test stand, waiting for the propulsion system to be re-attached.

XCOR Lynx: Cockpit and Avionics 101

Searfoss Gilman fitting
XCOR Chief Test Pilot and former NASA Astronaut Rick Searfoss sits in a full-scale Lynx cabin mockup to test a prototype helmet and suit with Orbital Outfitters Chief Designer Chris Gilman.

Each Tuesday we detail features of the cockpit, its avionics and life support systems, environmental controls, pressure suits and payload configurations. We will also take a look at fabrication, mockups and other key design points.

When boarding Lynx, you climb right into the cockpit, slide down and strap in next to the pilot. It is an unobstructed view, and within minutes after liftoff you experience the Earth, stars and space at 328,000 feet inside the glass canopy of the Lynx cockpit.

This cockpit is a self-contained pressurized vessel that sits inside the outer shell of the Lynx vehicle. It is attached to the fuselage (containing the liquid oxygen tank), the wing strakes (containing the
kerosene fuel), and the nose (containing the front landing gear).

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Note that the windscreen on the Lynx cockpit actually constitutes *two* different windscreens. The distinctive flat panel look of the front windscreen is high temperature / high strength glass that protects the cockpit from heat and forceful strikes.

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The inner windscreen is a complex curved shape that holds the pressure of the cockpit.

The Lynx cockpit is maintained at approximately 8000 feet “pressure altitude”, similar to what we experience flying New York to Los Angeles in a Boeing 737. To maintain this environment, the cockpit includes a life support system which is also integral to the pressure suits that both pilot and spaceflight participants wear as safety back-up devices in case there is ever a loss of pressure. In normal operating conditions, breathable air is circulated through the cabin and suit from an onboard supply.

Over the next few Tuesdays we will show you the cockpit in various stages of construction.

Tomorrow marks our first day on Lynx fuselage and structures.

More to come.

XCOR Lynx: Engines 101 — What 5K18 really means, and a bit on pumps and regenerative cooling

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This shot of the Lynx 5K18 engine truss and Lynx main fuselage was captured during a hotfire outside XCOR’s bunker at the Mojave Air and Space Port in early 2013. This particular engine test was one of the world’s very first fully piston pump-driven rocket engine firings.

Welcome back!

As discussed last week, Mondays are dedicated to the Lynx propulsion system. This includes Lynx main engines, engine nozzles, reaction control systems, and related technologies such as XCOR rocket piston pumps, valves and piping.

Let’s start with the Lynx main engines, how we name them and a bit about how 5K18 pumps and regenerative cooling work.

How we name our engines

As just mentioned, Lynx main engines include four XR-5K18s. The engine naming and numbering scheme we use follows a specific naming convention, like most things in aerospace or automotive.

The “XR” stands for “XCOR Rocket”.

The “5” stands for the thrust class of this rocket engine.

The “K” in this case is for the fuel, here it’s kerosene.

The “18” is the 18th overall engine design since XCOR was founded. For example, our X-Racer was powered by the XR-4K14 engine, a fourth-generation engine powered by kerosene and liquid oxygen, and the 14th overall engine design we had initiated at the time.

Fully pump-fed engines

Sometimes when testing our engines we use highly-pressurized inert gas, such as helium, to force the oxidizer and fuel into the engine for combustion. Or our engines may be powered by pumps that move oxidizer and fuel from low pressure tanks to high pressure before feeding them into the engine for combustion. And sometimes we do half and half: half pressure-fed and half pump-fed.

In operations, Lynx 5K18 engines will be fully pump-fed.

Regenerative cooling

The 5K18 engine uses “regenerative cooling” to prevent it from melting during the extreme temperatures of rocket combustion. Tiny capillaries inside the wall of each engine (and engine nozzle) have cooling liquid that flows through them to maintain appropriate temperatures. In the 5K18 we use kerosene fuel to cool the engine just prior to it being injected into the engine for combustion.

This is not a new idea. In fact, it has been used on most liquid rocket engines since the early days by pioneers such as Wernher von Braun (Germany, 1930s) and Reaction Motors (US, 1940s).

Over the next few Mondays you will experience a set of behind-the-scenes photos and see some of what it takes to build, prepare and test these engines. We will also highlight the unique features of this propulsion system that make it one of the most advanced and reliable rocket propulsion systems in existence today.

Tomorrow, it’s all about the cockpit!

Stay tuned.

XCOR Lynx: Aeroshell 101

Thursday Aeroshell
The Lynx aeroshell shape demonstrates why author Michael Belfiore calls it a “Space Corvette”

As discussed, Thursday 9am Pacific is the time each week that we discuss aerodynamics, modeling, simulation, test process and results. [Note: we delayed today's post by 20 minutes to make room for some other excellent news from our neighbors].

Developing the outer aeroshell for Lynx has been a long and detailed process. From early concepts to wind tunnel tests and “schlieren” shockwave photographs, we will give you a visual tour of what it is like to design an airfoil for a spacecraft, and hear from the people who are doing just that.

XCOR uses computer modeling, simulation, and analysis tools in the aeroshell development process. We have also used subsonic, transonic and supersonic wind tunnels to test and analyze the aeroshell design from very low speeds up to over Mach 4. This will continue into actual flight tests, where we will use an incremental “expand the envelope” approach from slow speed taxi tests to supersonic flights.

This is the approach most aircraft have used for over 100 years, including the rocket aircraft predecessors of Lynx like the Bell X-1 and the North American Aviation X-15.

Tomorrow, find out how you can communicate with us and share what you learn with others.

XCOR Lynx: How to follow the build

Lynx Cockpit

Your view from inside the Lynx cockpit

Beginning each day next week at 9am PST, Monday through Friday, we will post a piece of the Lynx story online.

We will start with background on Lynx systems and subsystems and how they’ve been developed, tested and integrated. Each day will be reserved for a different set of systems and subsystems.

On Monday, we follow Lynx propulsion system development, starting with the origins of the Lynx main engine, the 5K18. We cover its main parts, a bit about our design-build-test approach, move to subjects such as cold flow tests and hotfires, and take a deeper look inside Lynx thrusters (the 3N22).

Every Tuesday we give you an inside look at the Lynx cockpit and avionics.

Wednesday we cover the Lynx structure, strakes, fuselage, landing gear, nose and wings.

On Thursday we take a look at aerodynamics, modeling, simulation and test processes / results and regulatory matters.

And Friday is reserved for updates on the coming week at XCOR, live chats and ensuring you have your questions answered from the past several days.

Tomorrow we kick off the series with an introduction to Lynx aerodynamics, and in particular with a pretty cool near-final design image.

Stay tuned.

Follow the build

Pictured: XCOR welder, machinist and fabricator Eber West performs a test weld for a fixture to mount the Lynx truss to a firewall test stand.

For many weeks and months the XCOR team has been hard at work on a journey like no other, building Lynx. We would like to share that experience with you, and share what excites and motivates our team in developing Lynx. We will bring you the story daily, a piece at a time, all the way through the flight test program to first commercial flight. Just stay here to follow along. It will be a marathon, not a sprint. But it will be a marathon at Mach speed!

What you will see, hear and feel over the coming year is nothing less than the birth of a spacecraft told by the people who are that craft.

So welcome to the XCOR hangar. It’s going to be quite a ride—and you are coming with us.

Tomorrow – A brief look at Lynx systems, subsystems, and how we tell the story so that you can follow the build.

Thoughts from NSRC 2013: Export controls, suborbital space vehicles and next steps

Just a quick note while I have a moment to stop and reflect in the thick of NSRC 2013. Specifically, I want to address some new rules being proposed by the US State Department on export controls for manned suborbital space vehicles designed for commercial spaceflight.

At the end of May, the Department of State published a Notice of Proposed Rulemaking (or NPRM) Rule 78 FR 31 444 (pdf)– that did a great thing. The DoS proposed a move of commercial satellites from the US Department of Defense (DoD) Munitions List to the Department of Commerce’s commerce control list (CCL).  This is a great step for the industry. Since the time commercial satellites were placed on the munitions list in 1999, the commercial satellite industry was almost wiped out.

As the Aerospace Industries Association clearly illustrates in its fact sheet on US Satellite Manufacturing Job Losses (pdf), the US commercial satellite industry lost over 250,000 jobs. Market share dropped from a dominant position greater than 60%, down to bare relevance at under 30% by 2008 (and less today). The impact on the US launcher community was greater, as it is now practically non-existent. The US does not launch commercial satellites, they are launched elsewhere. Again, more than a quarter of a million jobs were lost mainly due to these restrictions.

Unfortunately, there were some “not so good” inclusions in the Department of State NPRM … it has explicitly proposed to put manned commercial space flight vehicles on the Department of Defense Munitions List. This is the same backward path provided to the US satellite manufacturing and launch community two decades ago that almost decimated that industry.

However, even before we can achieve a meaningful sized global market, and dominate it with US companies, there is the real possibility that we will be hampered before the market is fully opened. The benefits that have a real potential of not being realized are high tech job creation in rural, underserved, and hard hit regions of our country (by necessity, these vehicles are flown in remote areas); creation of a global, multi-billion dollar suborbital space-science research and personal spaceflight industry led by US companies; and the very large influence that these operations will have on our children through enhanced STEM opportunities. We will be turning over the lead to non-US companies.

Let’s push together to move the proposed rules in a better direction. Write to the State Department before July 8th and tell them to move suborbital manned vehicles to the Commerce Control List.

Via the U.S. Department of State:

“Interested parties may submit comments within 45 days of the date of publication by one of the following methods.

• Email DDTCResponseTeam@state.gov with the subject line ‘‘ITAR Amendment—USML Category XV and Defense Services.’’
• Internet: At www.regulations.gov, search for this notice by using this rule’s RIN (1400–AD33).”

Today’s news: One more step

We are excited to announce the most recent major milestone in our Lynx engine program, truly a part of aviation and space history: a 67-second fully pump-fed firing of our XR-5K18 rocket engine (and the first firing of a full piston pump-powered rocket engine in history).

For details, do read the release. Here we will just say that we believe this is leading toward a new era of fully reusable and reliable spacecraft.


XCOR CEO Jeff Greason inspects the Lynx main engine after a hotfire test while Chief Test Engineer Doug Jones looks on.

As the Lynx engine program continues to evolve we anticipate that the net result will be a dramatic reduction in per-flight costs and turnaround time, and that it will lead to a serious increase in affordable and routine spaceflight.

And with pumps as powerful as turbines but as reliable as automotive engines, today’s news on the 5K18 program is one more step toward the goal of true reusability and reliability.

We hope you join us.


Shock diamonds are visible during a test of the Lynx main engine. The diamonds are an interaction between supersonic gasses escaping from the engine and the pressure of earth’s atmosphere.


Jeremy Voigt prepares to test fire the Lynx main engine at XCOR’s test site on the Mojave Air and Space Port in Mojave, CA