The Purpose of Scramjets

[statickid]

Really they are just there for fun. If you want to fly a realistic mission, use NASSP, shuttle ultra, or latest greatest soyuz

---------- Post added at 07:58 PM ---------- Previous post was at 07:50 PM ----------

I wouldn't call it an error, its just that winged ascent to space (currently sci-fi concept) follows a different flight paradigm than current rocketry mission profiles. If you design a machine with certain requirements to operate, its not an error to use it correctly. If you don't accomplish your goals while using it correctly, then you are using the wrong machine.

 

[Urwumpe]

If you look at the flow rate (and the thrust produced by the Scram) you will see that fuel flow to the DG-S Scram falls from 6kg per second at 20,000m to less than 1kg/sec by 30,000m.

This trend continues into higher altitudes.

Yes... what do you think does "throttle down" mean?

Some Scramjet facts for you:

• The lower the air density, the less fuel you can maximally use.
• The higher the speed, the more effective the engine will get.
• The higher the speed, the air density and the throttle setting, the higher the exhaust temperature.
• The only limit for speed is the exhaust temperature.

 

[C3PO]

The best way to use them is for a relatively brief burn as you pass up and through their operating altitude - so only bring about 75kg scramjet fuel.

If you want to be efficient the best way would be to use a vertical launch and staying ballistic all the way into orbit. A vertical launch would save a lot more fuel then a small blip with the SCRAMs at optimal altitude.

I did a fair bit of testing to find the best way to launch a DG, and the segment where you loose most of your efficiency is the RWY takeoff. If the DG had just remotely realistic weight/ISP it wouldn't get off the ground. The efficiency you gain by getting out of the thicker air ASAP is much higher then using the lift from the wings.

The DG is a demonstrator that a beginner can get into orbit if he has a basic knowledge of orbital mechanics.

 

[Kinetics]

Yes... what do you think does "throttle down" mean?

Some Scramjet facts for you:

• The lower the air density, the less fuel you can maximally use. -
  [OBVIOUSLY - basic chemistry]
  
• The higher the speed, the more effective the engine will get. -
  [WRONG - it is not a linear relationship - theoretical and practical limits range from Mach 20 to c.Mach 6]
  
• The higher the speed, the air density and the throttle setting, the higher the exhaust temperature. -
  [INCORRECT - drag and dynamic pressure]
  
• The only limit for speed is the exhaust temperature.
  
  [INCORRECT - some concern on the bench]
  

There are optimum ranges for both speed and density - it's not a simple linear relation. From your claims regarding "speed" and air density you do not appear to have grasped the basic concept of drag being proportional to the square of velocity. In practical terms achieving +Mach 6 in very dense air is practically impossible.

As far as speculating that the DG-S is throttling down due to temperatures you must try to remember that correlation is not causation.

The scramjet limits on the DG-S are:

1: Atmosphere : The engine loses function above 30km altitude. Output is around 100% up to 22km altitude. It then reduces to less than 1% at 40km altitude. The rate of reduction is not linear - there is large reduction in thrust passing altitude of 25km.

2: Temperature : The relationship between output and temperature is unclear. Peak thrust is achieved at altitudes under 15-22km. Above this thrust declines.

3: Speed : the engine loses function at speeds below around Mach 3.5 and speeds greater than Mach 7.5. Peak thrust of around 600 kN is produced at Mach 5 to 6.

4. Air density : It is not possible to operate in very dense air (civil airspace) due to dynamic pressure which will prevent speeds approaching Mach 3 being achieved - therefore the scramjet can not operate. Even at at 15km altitude Mach 6 cannot be exceeded due to potentially catastrophic levels of drag.

6. Peak cruise performance is achieved at 35km altitude and speed of Mach 7 under full throttle. Actual engine output of 5% in this configuration will perfectly balance drag. Lift will balance weight.

7. Flying technique - optimum distance for non orbital flights can be achieved by climbing rapidly to 40km, accelerating to MAch 7, switching off main engines and using scramjets alone to maintain cruise speed by surfing the atmosphere:

ONCE MACH 7 and 40 KM IS ACHIEVED TRIM USING SCRAMJET PITCH GIMBALS!!!!

LEAVE SCRAMJETS OPEN AND SWITCH OFF MAIN ENGINES!

THEN DO NOT ATTEMPT TO MAINTAIN ALTITUDE OR SPEED

The DG will then oscillate up and down between 30km and 40km every 40 to 60 seconds.

Due to the relationship between scramjet power and altitude - the scramjets will increase thrust when it dips and reduce thrust when it climbs!!

It will maintain speed and altitude indefinitely by literally surfing the denser part of the atmostphere - the scramjets act as autopilot due to the relationships between speed, lift, altitude and thrust.

For example

1) if drag reduces speed then lift will also drop. This will reduce altitude. This will in turn cause the scramjets to increase (gimballed) thrust. This increased thrust will correct the speed and the pitch - and therefore lift and altitude.

Vice versa, 2) if scramjet thrust increases speed too much then the altitude will increase above 40km, causing scramjets to turn off. This will cause speed to reduce.

The benefit of this approach is that you bounce in and out of the low drag environment above 30km without having to use radial accelaration to maintain it. Fuel usage is in the region of 0.6kg/sec.

For short distances (transatlantic), this is more efficient than climbing into orbit due to the huge delta v required for orbital velocity to compensate for the loss of lift at altitudes exceeding 40km.

 

[PhantomCruiser]

Both you guys are smarter than me regarding the scramjet practicalities, but I'm curious as to how these factors play in when using th XR fleet rather than the "stock" DG w/scrams?

 

[Urwumpe]

As far as speculating that the DG-S is throttling down due to temperatures you must try to remember that correlation is not causation.

1. I know what a Scramjet is, why we have only minutes of Scramjet flight experience and why there are maximal Mach numbers for practical applications. No need to lecture me, especially if you are looking at the Ramjet as if you have never seen a jet engine.
2. This is not speculation. You can read it in the source code of the DG-S Ramjet simulation code. Also such control is commonly done in real ramjets.

Follow this link for the theory behind ramjets and scramjets:


http://www.grc.nasa.gov/WWW/k-12/airplane/ramth.html

---------- Post added at 06:33 PM ---------- Previous post was at 06:31 PM ----------

Both you guys are smarter than me regarding the scramjet practicalities, but I'm curious as to how these factors play in when using th XR fleet rather than the "stock" DG w/scrams?

Basically, as much as I know, the XR is based on the same code, but uses different parameters for the Scramjets for better performance at higher speeds. No real difference in the behavior.

 

[Andy44]

If you want to be efficient the best way would be to use a vertical launch and staying ballistic all the way into orbit. A vertical launch would save a lot more fuel then a small blip with the SCRAMs at optimal altitude.

I did a fair bit of testing to find the best way to launch a DG, and the segment where you loose most of your efficiency is the RWY takeoff. If the DG had just remotely realistic weight/ISP it wouldn't get off the ground. The efficiency you gain by getting out of the thicker air ASAP is much higher then using the lift from the wings.

The DG is a demonstrator that a beginner can get into orbit if he has a basic knowledge of orbital mechanics.

+1

Which is pretty much why all real life rockets do it this way, including the space shuttle.

 

[statickid]

Both you guys are smarter than me regarding the scramjet practicalities, but I'm curious as to how these factors play in when using th XR fleet rather than the "stock" DG w/scrams?

:2cents:

DG-S scrams less useful for orbital insertion and more similar in performance to x-43, a real scram jet vehicle.

XR-fleet reflects the future vision of scram use in space flight, that is, a powerful long duration scram engine with a high performance spectrum that kicks you out to the very limits of "suborbital flight" so all you need is tiny rocket boost into orbital insertion

 

[Cras]

If you do it right, 1 bay tank of SCRAM fuel will get your XR-2 from Mach 3 to Mach 20.
You may even hit Mach 20 with a bit still left in the tank, but your diffuser temp will be too hot that you will still be forced to shut them down and switch to mains at that point.

Then use mains for the rest. Also the SCRAMS at max efficiency produce something like 300 kN more power.

 

[Kinetics]

1. 
   Follow this link for the theory behind ramjets and scramjets:
   
   
   http://www.grc.nasa.gov/WWW/k-12/airplane/ramth.html
   
   ---------- Post added at 06:33 PM ---------- Previous post was at 06:31 PM ----------
   
   

1. 
   
   I downloaded that NASA EngineSim before but never really used it much until today. It's interesting looking at the Ramjet data - shame they don't have scramjets on it.
   
   For ramjets, obviously, net thrust is gross thrust less ram drag.
   
   The applet shows that ramdrag increases with the square of speed, whereas gross thrust increases at a much lower rate compared to speed. It follows that at high speeds (or high power outputs) ramdrag will gradually reduce net thrust to zero.
   
   It shows that overheating is not a risk factor at high altitudes - the reduced air intake limits the burn rate and therefore potential heat generation. Additionally, the speed required to maintain the air supply at altitude causes excessive ramdrag. This reduces net thrust to zero before overheating can occur. Note that temperature reduces with altitude - further reducing overheating risks.
   
   
   So, why would the code cause the DG-S scramjets to overheat at increasing altitudes?

 

[Urwumpe]

The applet shows that ramdrag increases with the square of speed,

Actually both thrust and inlet drag increase at the same rate, if you look at the terms. Both are velocity * mass flow, and mass flow is density * velocity.

Also the engine exhaust temperature has to be limited additionally to prevent thermal chocking for scramjets, since heating the air by combustion increases the local speed of sound in the exhaust and can turn it accidentally subsonic, resulting in an engine stall and explosion.

Temperature of the combustion is simply said "Initial temperature after compression" plus "Combustion temperature". Temperature after compression or inlet temperature depends on the compression, plain old gas dynamics.

 

[Kinetics]

Actually both thrust and inlet drag increase at the same rate, if you look at the terms. Both are velocity * mass flow, and mass flow is density * velocity.

Example 1:
@ 200kph Gross thrust = 30,949N Ram drag = 1,860N
@ 400kph Gross thrust = 32,819N Ram drag = 3,901N

- Speed doubled, thrust remained constant and drag doubled.

Example 2:
@500kph Gross thrust = 34,264 Ram drag = 5,051
@1,000kph Gross thrust = 47,745 Ram drag = 13,328

- Speed doubled, gross thrust increased 39%, ram drag increased 164%

Example 3 at altitude of 18,105m:
@400kph Gross thrust = 3,004 Ram drag = 333
@2,475kph Gross thrust = 27,199 Ram drag =15,334

-Speed increased 500% , thrust increased 800% ram drag increased 4500%.

Source: Nasa EngineSim1.7b :hello:

 

[Urwumpe]

You use subsonic ramjets in your examples. Can you make a comparison with Mach 2 and Mach 4 at different altitudes and also say which material and which fuel you use?

Also, speed doubled and ram drag doubled is not what you can expect - it should be drag x 4. I think you have done some errors in your simulation set up. The inlet area of your engine had shrunk in your first two examples and grew in the third.

Example 2:
Inlet Mass flow@500 km/h : 36 kg/s
Inlet Mass flow@1000 km/h : 48 kg/s

Example 3:
Inlet Mass flow@400 km/h : 3 kg/s
Inlet Mass flow@2475 km/h : 22,3 kg/s

The mass flows increase not linear with the velocity, which suggests that there is a change in engine inlet area.

 

[Kinetics]

You use subsonic ramjets in your examples. Can you make a comparison with Mach 2 and Mach 4 at different altitudes and also say which material and which fuel you use?

Also, speed doubled and ram drag doubled is not what you can expect - it should be drag x 4. I think you have done some errors in your simulation set up. The inlet area of your engine had shrunk in your first two examples and grew in the third.

Example 2:
Inlet Mass flow@500 km/h : 36 kg/s
Inlet Mass flow@1000 km/h : 48 kg/s

Example 3:
Inlet Mass flow@400 km/h : 3 kg/s
Inlet Mass flow@2475 km/h : 22,3 kg/s

The mass flows increase not linear with the velocity, which suggests that there is a change in engine inlet area.

Re:

-Well spotted but there was no change in engine inlet area. Example 3 was at 18km altitude; examples 1 and 2 were at ground level.

-The relationship of aero-drag to speed is complex. It may not be the square (it's not my area of interest) but it clearly increases at a faster rate than gross thrust.

-Everything was left on default so should be repeatable. Mach 2 and Mach 4 would be interesting but given this data is for ramjets it's unclear whether it can be extrapolated to scramjets (which are optimised for hypersonic speeds).

 

[Urwumpe]

The simple problem is: Your data is invalid, because it can be shown (by the inlet drag force) that a design constant (engine inlet area) had changed.

So, there has to be another simulation with corrected parameters. Also, the fidelity of the simulation will be pretty wrong anyway, so no need to expect much accuracy. There is currently no simulation model that can handle hypersonic flow in the engine properly.

 

[Face]

Moderator note: thread closed pending staff review

 

[dbeachy1]

O-F Staff Note: thread reopened after cleanup. Let's please keep it civil, all, per O-F Rules.

 

[Urwumpe]

Well, some clarifications and definitions to restart the discussion:

General aerodynamics:

If you look at a plain tube in an airflow from outside as black box, without knowing what happens inside, you can see that air enters the tube and air leaves the tube again. A jet engine of any type is similar to that tube, but it accelerates the air inside it to make it leave the engine faster than it enters it.

The air that leaves the engine produces thrust force. Not really surprising.
The air that enters the engine produces the reverse, a drag force.

The force of the leaving air/exhaust is called gross thrust.
The force of the entering air is called inlet drag

The difference between gross thrust and inlet drag is net thrust.

The mass flow through the engine is constant: Aside of pressure fluctuations, only as much mass can leave the engine, as mass enters it.

For both Ramjets and Scramjets:

Inlet drag and gross thrust are both proportional to velocity squared.
For both exists a maximal velocity at which fuel mass flow has to be throttled to zero, because adding more fuel would not result in a gain of energy (ramjet cycle thermodynamics)
For both is common, that they have a very low net thrust compared to their weight.

Both engines would without control at full throttle accelerate until they unstart or overheat. For most real engines, the overheating temperature is reached before the inlet unstarts (For example the SR-71 engine in ramjet mode)


For Ramjets:

Ramjets are mostly limited by technology. While you can continue increasing their maximum speed by using stronger and more heat resistant materials, this gets more and more ineffective at higher speeds. The more you reduce the speed of air in the inlet to get it subsonic, the higher you compress it. At Mach 10, this would result in a pressure of 3000 times sea level air pressure, higher than in any rocket engine (and any piston engine as well, only some guns could compete there). That limitation is the main motivation for using Scramjets, which don't need to reach such high compression ratios, but then also have a much higher minimum speed to operate. Ramjets on the other hand can operate even at subsonic speeds (if designed for it, they are then not capable of supersonic flights) or at only low subsonic speeds (around Mach 1.5) for supersonic ramjets.


For Scramjets:

At Mach numbers below Mach 5, you can compress air with only small increases in temperature. Past Mach 5, any small increase in air pressure comes with a strong increase in temperature (See spacecraft reentry as example there). Thus Scramjets already reach high temperatures at the end of the inlet (unless you would add precooling), which limits their thermodynamic efficiency (Lower specific impulse than ramjets, but still better than rocket engines).

Scramjets have additionally a limitation because of the high speed inside the combustor, the blow out limit. If you exceed a speed, the exothermic chemical reactions will happen in or behind the nozzle and the net thrust approach zero.

Scramjets can additionally get problems at low Mach numbers, if you use too much fuel, thermal choking. When air gets hot, the speed of sound in the air increases. If this happens in a scramjet, with the air in the combustor being only slightly supersonic, it can result in the air in the combustor becoming subsonic (despite traveling at the same velocity) and the airflow getting blocked, gross thrust dropping to zero suddenly (you get an "unstart")

The next limit for Scramjets is the vehicle it is installed in: Flying low at high Mach numbers results in very high dynamic pressure and thus aerodynamic forces and heating. This limit exists for all aircraft, but it is not as problematic, as it is for a Scramjet, which has already a high dynamic pressure at its minimum speed.

The operating envelope of a Scramjet is thus a very narrow band that slowly increases in altitude with increasing velocity. And it is really easy to leave that narrow band. Or to produce conditions inside the engine which unstart it.

(There are also other real problems in Scramjets at higher speeds: The air turns into plasma at higher temperatures, changing chemistry and aerodynamics)

 

[Kinetics]

Rather than obscure or complicate the issue perhaps let's stick to the only point in disagreement : Does ram drag increase at a greater rate than thrust at increasing aircraft speeds?

My finding:

"Ramdrag increases with aircraft speed at a greater rate than thrust

- I provided supporting ramjet data from Nasa on gross thrust and ram drag at a range of speeds to support this assertion

Urwumpe's reply:

Actually both thrust and inlet drag increase at the same rate, if you look at the terms. Both are velocity * mass flow, and mass flow is density * velocity.

And, consequentially:
- Nasa's engine simulator data must be flawed
- Engine inlet size must have changed
- my data is "invalid"

My reply

F = [m dot * V]e - [m dot * V]0 + (pe - p0) * Ae

Urwumpe's comment assumes:

1) mass flow (m dot) and velocity (V) in the first(e) and second(0) terms is identical; and
2) V in both terms is equivalent to "speed"

Both assumptions (1) and (2) are incorrect. The exit mass and the free stream mass are not identical. The exit velocity and the free stream velocity are not identical.

The incorrect assumptions (1) and (2) have led Urwumpe to infer that the products of the first and second terms (gross thrust and ram drag) will increase at identical rates at increasing aircraft speeds. Urwumpe's conclusion is wrong.

 

[Urwumpe]

You assume that the products of the first and second terms are identical and move only in line with changes in V. You assume a identical m and identical v.
That is incorrect and is the source of this misunderstanding.

The exit velocity (Ve) and free stream velocity (Vo) are different as are the exit mass flow and the free stream mass flow.

And...

Exit Velocity is bigger or equal (zero thrust) to free stream velocity (unless you have a unstart - in that case you will have slightly less, momentarily even almost zero)
Exit mass flow is bigger or equal (zero thrust) to inlet mass flow.


The first term (gross thrust) is always bigger or equal than the second term (ram drag) since both factors of the term are bigger or equal then the factors in the second term.

Additionally, you will have a positive third term (pressure compensation), since reducing the pressure at the outlet below the freestream pressure would mean you reverted the flow in your engine and have no ramjet at all at this point.

And stop claiming that I said stuff like "Equating a single V to speed". I never did that anywhere.