single-stage suborbital vehicle
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Frequently asked questions about the Xerus and our suborbital program
Table of Contents
Part 1 - General Questions
1. What is the Xerus?
2. Why do you want to build it?
a. Is the suborbital market profitable?
2. What makes XCOR different?
3. Why haven't you had renderings of your aircraft previously available to the public?
a. design secrets
b. a legacy of overpromising
c. painting into a corner
4. What about communication satellite delivery?
5. What comes after Xerus?
Part 2 - Technical Questions
6. What is the program status?
7. Why does it have liquid-propellant rather than a solid or hybrid rocket engines?
8. What kind of propellant feed system will Xerus use?
Part 1: General Questions
What is the Xerus?
The Xerus is a suborbital vehicle that is currently in the design stages at XCOR. It is rocket powered, takes off from a runway, and is capable of high altitude, high speed flight.
Why do you want to build it?
To generate revenue. We have identified three markets that it can service. Two of these markets are proven and we plan to compete by offering a lower price than the current vehicles. The first identified market is for microgravity research experiments, which are currently being flown by expendable sounding rockets at a cost of up to $2 million per flight. The second market opportunity is to launch microsatellites into low Earth orbit. We estimate the Xerus will be able to loft 10 kilogram satellites into a low inclination orbit at $500,000 per flight; compared to the current lowest price per flight of $12 million. Our payload is smaller, so that the price per kilogram is not appreciably lower. Our price per flight will help us to gain market share.
The third potential market for the Xerus is unproven but has the potential to generate a very large profit: flying passengers to space. The ride will be to at least 100 km altitude and return within a half hour to the takeoff site.
Is the suborbital market large enough to be profitable?
Yes. XCOR plans to sell and lease our vehicle services to other companies with a presence in our target markets. We already have relationships in place with several such companies and we are developing more. Our development and operating costs are low enough that even very conservative assessments of the market project a substantial profit for the suborbital vehicle.
What makes XCOR different?
We have a proven team that has designed, built, and tested hardware and vehicles. The management team is in place, the engineering team is in place, and we have developed operational systems that actually work.
Why don't you offer more design details publicly?
As a matter of policy, XCOR does not publicize details of future projects. The reasons behind this are:
A. Design Secrets: The most obvious, but not the most important is: "We don't want to give away our plans to our competitors."
B. Painting into a corner: Some companies have said too early what they're going after and how they're going to do it. This creates a lot of pressure not to change the public story about what a company is doing, even if it has improved on its original idea. In the past this prevented people from moving on to better ideas because they were publicly committed to another approach. Our vehicle markets and design will continue to evolve.
C. A legacy of overpromising: The entrepreneurial space industry has suffered damage from companies and individuals overpromising: speaking about hopes as if they were certainties. While this can generate enthusiasm with investors and customers, it is also a big disappointment when it doesn't happen. This creates a skeptical customer base that inhibits the formation of customer relationships and investor capital.
Therefore, we are releasing this information not as a promise of what we will do, but as an illustration of the general type of vehicle that is currently in development.
What about communication satellite delivery?
Launching today's market of large communications satellites is not in XCOR's near-term business plan. They do not fit our next generation vehicle business plan because of their large size and infrequent launch needs. Although the comsat launch market is large in dollar amounts, the small lift rate is not appropriate for a reusable launch vehicle, and we are not ready to tackle such a large vehicle or program.
What comes after Xerus?
When Xerus starts making money for the company, profits will be turned into further vehicle development. It is too early to speculate on exactly what that will be, but it will be bigger and faster, and will go higher for longer.
Part 2: Technical Questions
What is the program status?
The Xerus program is in the design phase. We are currently conducting engineering analysis and design. This involves refining the configuration, identifying potential problems, and determining performance. Vehicle weight calculations are based on bottom-up structural and system layouts. Performance numbers are based on computer trajectory programs developed by XCOR and other partners. Rocket engine performance is extrapolated from XCOR’s past and current engine development experience verified by our currently operating engines.
This is a trade analysis that has to be done by partially designing the vehicle instead of using a parametric analysis. Some critics have briefly looked at the design and not understood how we can store the necessary propellant. Rest assured, however, that the Xerus has several design margins appropriate to this level of maturity to accommodate the inevitable real world surprises and setbacks. The shape you see here will change as we add detail analysis.
Why does it have liquid-propellant rather than solid or hybrid rocket engines?
Rocket engine design is one of XCOR’s strengths. The Xerus vehicle design is not compromised to fit a pre-existing engine. XCOR looked at all possible types of rocket engines and decided on liquid propellants for several reasons.
Solid rocket motors are not acceptable for the Xerus. Their safety features are inadequate, and they have a number of performance disadvantages which are, in decreasing order of importance:
Solid rocket motors do not have an Off switch. Once ignited, they burn to completion and cannot be throttled. This is not safe and is unacceptable for a commercial manned system.
They are traditionally expensive to reload or replace between flights. The cost could be reduced, but the nature of the toxic chemical ingredients and hazardous handling processes will always be a disadvantage. Twice in the past 20 years, factories producing the ingredients (ammonium perchlorate) have exploded and the U.S. supply was interrupted for months. Ammonium perchlorate is currently being examined as an environmental contamination problem at many sites.
They have to be located at the vehicle center of gravity (CG). In any high performance vehicle, propellant mass is a large fraction of the total, and burning the propellant must not be allowed to shift the CG so much that it makes the vehicle unflyable. Since the Xerus is a winged vehicle, and it has to be able to fly as a glider when either full, empty, or during an abort, this is a fatal constraint. (We plan to dump the LOX but not the fuel during abort.)
The exhaust plume is very toxic. This has not traditionally been a problem but the coming commercial and widespread applications will have to address this.
Solid rocket motors create severe shock and vibration environments. This is not a critical problem on a tourist vehicle, but requires the addition of a payload isolation system for many satellite launch applications. This adds a significant amount of weight, cost and complexity.
Hybrid rocket motors have advantages and disadvantages and must be compared to solid and liquid motors for each specific application. Hybrids have several advantages compared to solids, but this FAQ is about the Xerus. (The interested reader is referred to the list of references in our reference library.)
Compared to liquids, they have the advantage of lower development cost. While claims have been made that hybrids are inherently safer, hybrid motors have exploded in the past. Both NASA MSFC and Amroc have experienced case ruptures on large motors that would have destroyed a manned vehicle. Although the energy release is less than a solid motor exploding, it’s still high enough to be fatal in a manned vehicle. Of course, liquid rocket engines have also exploded in the past, but XCOR’s designs start with safety as the highest priority. In over 2,000 engine runs and in all the history of XCOR, we have never had an engine burst or explode. For the Xerus, hybrid propulsion was not chosen (in decreasing order of importance) because:
Center of gravity (CG) maintenance is insufficient for the winged Xerus layout. For a lower performance vehicle, CG shift during burn or abort can be acceptable. The Xerus has a mass ratio of approximately four and cannot accept large CG shift. This much propellant must be arranged about the center of the vehicle, and packaged within the aerodynamic shell. Xerus stores the LOX in a cylindrical fuselage tank and the fuel in the wings and strakes. The relatively lightweight engines are far aft of the CG.
Operations cost. In traditional rocket vehicle operations, propellant cost is not an issue, but XCOR is looking toward commercial operations where the difference between a $3,000 retank and a $50,000 turnaround cost is significant. Part of the higher cost of reflying a hybrid relates to the extra task of casting the fuel inside the motor case or winding the case around the precast grain. XCOR’s planned 40 flight test program for the Xerus cannot afford $2 million just for propellants.
Weight of blast shield. XCOR does not subscribe to the belief that hybrids are inherently safe. All our rocket engines get blast containment shields, and these are far lighter for liquid propellant engines. The combustion chamber for a hybrid motor encloses all the fuel, and a blast shield over that is unacceptably heavy.
System density and packaging are unacceptable. Hybrid motors have lower packaging density than either solids or liquids, and an awkward form factor. We couldn’t find enough volume inside the vehicle to package all the propellant for a hybrid motor.
Low specific impulse (Isp). Lower Isp means a heavier propellant load to do the same job. This would have lowered Xerus performance too much. Xerus has other missions than carrying tourists to 100 km, and these other missions would not have been served.
Poor propellant utilization. To be safe and reliable enough for manned operation, case burnthroughs have to be prevented. This means either adding an additional ablative coating to the combustion chamber, or shutting down the motor before all the fuel is burned. Either of these options adds weight and reduces performance.
Smoke is inherent in the plume. This is not a fatal problem, but we are looking toward future routine operations and air quality will eventually become a critical concern. The EZ-Rocket had no smoke in its plume, Xerus will have very little.
What kind of propellant feed system will Xerus use?
Xerus is being designed with liquid propellant rocket engines that have integral piston pumps for both propellants. We chose piston pumps over turbopumps or pressure feeding for several reasons. Turbopumps have worked well on many past rocket engine designs, but are not appropriate for the Xerus. Most turbopumps are designed for much larger engines and it is not clear that they can be made efficiently in this small size. Turbopump development costs are much higher than the Xerus program can afford, and once designed, they are difficult to scale to other sizes. Changing the size of a turbopump is equivalent in cost to designing a new pump entirely. Also, turbopumps do not start and stop rapidly and the Xerus will be making multiple in-flight restarts during the flight test program.
A pressure-fed design, such as used on the EZ-Rocket, is unacceptable for Xerus. The reason the EZ-Rocket fuel tank is underslung instead of in the wing strakes (where the AvGas was carried when it had a piston engine) is because the tank must be configured as a pressure vessel. This mandates spherical or cylindrical geometry, not the complex shape of the Long-EZ or Xerus wings and strakes. Fitting a fuel tank pressure vessel inside the fuselage in line with the LOX tank would have degraded vehicle performance unacceptably.