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FILL ‘ER UP!

February 12, 2017

This Launcher Evolution Advanced Prototype (LEAP) is a feasibility study, not a finished design.  It will change as the design evolves.  As such we are harvesting ideas and data from other projects and publications.  Seeking the best of this we may use some estimation to project possible value.  Again we offer the question; is this concept worth evaluation?  The answers will require funding to motivate qualified analysis.  By offering these ideas on an open forum we allow you to observe as we discover ideas and barriers along the way.

We are able to aim at best models and seek compromises that move past the observed problems.  There are a lot of ideas out there that may open doors if we find the right combinations.  Trade studies offer comparisons of these combinations to validate best opportunities.  Our aerodynamic ideal will have to accommodate compromises with other issues like mass, thermal heating, and structures.

Rocket Fuel can be bulky and heavy when so much energy is needed to achieve orbit.  We need to make some estimates that can fit the aerodynamic model.  Since I can’t pay rocket scientists, I gather any published data from other projects.  They may not be perfect, but they are probably better that my best guess work.  We gathered published information about the Bristol Spaceplanes Space Cab for comparisons.  We assumed the cryogenic fuels and the volumes they projected.  Similarly we compared to other aircraft for mass estimates.  We end up generating a unique solution as we progressed.  Now we need to explain the high mass of our fuel load indicated here.

mass-1

Cryogenic rocket fuels are hard to fit in an aerodynamic airfoil shape.  They are usually cylindrical or spherical as the most mass efficient container for high pressures.  For the Rockwell Star-Raker they proposed to make shaped tanks with shared flat sides.  They had problems with that on the X-33, and I am not sure we can do that now.  As such the best shape is a tapered cylinder like the Soyuz, which can fit an airfoil shape better.  I did try simple flat tank proposal to fit our wing though.

star-raker-system-design-features-2

STAR RAKER CONCEPTS

The Rockwell Star-Raker proposed an “air mattress” shape with flat sides to strengthen its wings.  We proposed a little less bold application by placing tanks between linear structural members.  We have to reserve space for structures in our proposal too.  This structural shape is still just an estimate that targets known needs for landing gear and propulsion forces.

Tanks can add structural value when filled under pressure.  A beer can with the top sealed can take a big load if you want to stand on it.  After you pop the top is easily crushed.  But the strength is a little less in lateral forces, as it can be dented even when filled.  This benefits the Atlas rocket, and Star-Raker proposed to use this to reinforce wings.  We only proposed to squeeze tanks in line with linear structures for some advantage.  It isn’t an easy thing to do.

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We can’t really pack tanks that tight because we have to clear landing gear and the taper of the airfoil.  And attempts to build complex tanks were not too successful on the X-33 program.  We can squeeze more, but there will still be unused volume.

htp3

When mating these tanks to a winged vehicle, even a simple modification of flat sides still leaves a lot of space un-used.  Jet fuel can fit in wing tanks, but cryogenic tanks are bulky.

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Unfortunately cryogenic tanks aren’t flat sided yet, and still tend to fit this form, which is harder to package within an optimized blended wing body.

cryotank1

There will be an increase in mass once fuel tanks materials and insulation are factored in.  There may be advantages to propulsion if we can overcome this though.  We are considering new air breathing engine technology.  The Air Force wants to test a small version of the Sabre engine as shown here.  This initially interested us for our airframe.

scm4

SMALLER SABRE PROTOTYPE

I think we can see more mass than a comparable turbine engine but there may be enough thrust and oxidizer load reduction to justify that.  This may justify using cryogenic fuels, but we don’t have a good way to package without the less efficient fat fuselage.  We may need to consider a different solution though.

1sg6

We wanted to show a better proposal than what the Air force is now considering.  In-line staging can reduce drag, increase lift, and offers other mass reduction methods.  this may be critical to launching from a runway.

af2s1

Our first design was far short of the volume needed for cryogenic fuels.  We can try to cram more in, but there is a big gap to close.  We should look at an all new HTP version next.  Here we can see that our wing body is far short of the fuel load of the Space Cab.

cryo-comp

COMPARING THE LEAP VEHICLE TO THE SPACECAB

We can build shape conforming tanks with non-cryogenic fuels, but they still offer challenges.  Hydrogen Peroxide is an alternative but it has much greater density and mass.  Now we need to compare thrust and mass properties to consider feasibility.  Can we make the mission with these alternative fuels?  (Or are they an alternative reality?)

SpaceCab and LEAP have similar empty mass, but we need more oxidizer mass.  We observed data from the British Black Arrow HTP rockets and compared them to cryogenic vehicles to generate a factor comparing performance.  Our spreadsheet magic predicted a need for 25% more fuel mass than a cryogenic system, not counting tank mass differences.  To be conservative, we targeted 50% more fuel.  Better to have extra tank volume if needed to balance in flight.  Lighter tanks will recover some of that, and they can fill all the available volume in an airfoil shape.

This NASA technical report confirmed our interest in HTP.  “Summary and Conclusions  A trade study considering two alternate oxidizers, liquid oxygen or 90% hydrogen peroxide, for a rocket based combined cycle demonstrator vehicle was completed. Given the limited energy requirement (AV) of the demonstrator vehicle (Mach 0.7 to 7), the higher density and mass ratio of 90% hydrogen peroxide yielded similar vehicle performance when compared to LOX. Additionally, hydrogen peroxide provided system simplification, increased flight safety and packaging advantages. After consideration of the technical and programmatic details, 90% hydrogen peroxide was selected over liquid oxygen for use in the ISTAR program.”

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htp1

PEROXIDE AND JET FUEL TANKS

With HTP we can effectively fill the gaps between the structures and follow the airfoil form fully.  This delivers the required fuel for the mission and allows vacant tank volume for in-flight balance in supersonic flight.  We will also maintain low pressure (30 psi) in these tanks, so structural strength is aided, and mass is reduced.  The Star-Raker concept lives again!

 htp4

What does a heavy fuel load mean to our mission?  We have to remember that the fuel load will drop quickly during takeoff.  Compared to commercial craft we will not nurse the fuel in a long cruise.  Indeed this is a space mission where a lot will come from thrust.  But wing loading is not too unreasonable in historical comparisons.

3-charts

So fuel will shape the mission and the vehicle.  But it will also affect costs in shipping and storage.  Peroxide also comes off as a fairly clean fuel.

ROCKET FUEL:  TANKS AND STORAGE

Rocket cost can’t be divorced from the storage and transportation of rocket fuel.  This is what you can expect with cryogenic fuels; not a cheap plan to deliver cold fuels.

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Hydrogen peroxide and jet fuel are room temperature low cost storage issues.  Thin tanks and fuel bladders work in about any shape.  Even plastic water tanks work, as seen here at Frontier Astronautics’ facility in Wyoming.

htp-store

If we can build around the fuel load we can go on designing tanks, airframe, and mechanisms.  All this will feed data as the CAD tools will record mass and balance data.  While this not a final design, it may provide a fair prediction of good design.  That comes when we can make paychecks.  There is a lot more innovation coming that can justify that investment…stay tuned!

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