Spaceplanes: minimal Thrust

[hms_warrior]

Hey.

I have been reading a lot of stuff about the Skylon lately, and this got me thinking about two questions i can not realy find an answer to.

Maybe someone here can help?

First: So let's asume we have a Spaceplane-SSO.

To make things easier i pull some specs:

Think about something like the F14 Tomcat. Variable geometry, 14 tons empty, 20 tons fuelled. We have some magical fusion-drive giving us 10.000 isp (so no big t/w change on the way up...just to make things easier^^).

Now how powerful does this magical engine needs to be? as a plane we don't need a t/w above 1 to lift off.

Those wings will help us trick gravity. But somewhere on our way to space the air will be so thin that our wings turn into useless pieces of metal and all we have to finish our trip to orbit is our engine. But how much acceleration do we need to keep up to reach orbital velocity before tumbling down again?

The second(kinda bonus like) question i have trouble finding a good answer for is:

How far up is that point our wings get useless?

I guess for both questions will depend partly on the speed we can maintain inside the atmosphere...so let's say our Space-Tomcat can sustain a climbrate of 15 degrees all the way up to the "wings get useless" point. And our hull is good enough to go mach 5 at 20Km whitout melting...and an additional 3 per 10km extra. so 30km: mach 8 max. 40 km mach 11 max and so on.

Anyone have a rule-of-thump for this cenario?

Edited November 25, 2016 by hms_warrior

[Steel]

16 minutes ago, hms_warrior said:

Anyone have a rule-of-thump for this cenario?

There probably isn't a rule of thumb for this, mainly because you've just asked for a rule of thumb for a scenario that is absurdly specific!  Rule-of-thumbs tend to come out of experiences that happen often so people begin to intuit how to do things well. Flying a Tomcat up to orbital velocity with a magical fusion drive at a constant angle is not one of those experiences!

Now to attempt an answer to your questions:

16 minutes ago, hms_warrior said:

Now how powerfull does this magical engine needs to be? as a plane, we don't need a t/w above 1 to lift up. Those wings will help us trick gravity. But somewhere on our way to space, Air will be so thin that our wings turn into useless peaces of metal and all we have to finish our trip to space is our engine. But how much alleleration do we need to keep up to reach space?

It will likely need to give a TWR above 1 so that it can, if needed, overcome the effects of gravity. In theory if you're on a suitably high suborbital hop you can use incredibly weak engines to get into orbit, but in your scenario you'd want something comfortably above a TWR of 1.

 

16 minutes ago, hms_warrior said:

The second(kinda bonus like) question i have trouble finding a good answer for is: How far up is that point our wings get useless?

I guess for both questions will depend partly on the speed we can go inside the atmosphere...so let's say our Space-Tomcat can sustain a climbrate of 15 degrees all the way up to the "wings get useless" point. And our hull is good enough to go mach 5 at 20Km whitout melting...and an additional 3 per 10km extra. so 30km: mach 8 max. 40 km mach 11 max and so on.

Anyone have a rule-of-thump for this cenario?

How far up you can go is entirely dependent on your velocity. The Karman line is actually defined as the point where you would have to be traveling faster than orbital velocity to generate enough lift from a wing to support itself.

As for your specifics, I can't really give you an answer. There are so many variables in play here (speed, AoA, heating, engine thrust, aircraft TWR e.t.c) that there isn't really a single, satisfactory answer. Someone could tabulate the air pressures and densities at various altitudes and work out whether you could generate lift with your criteria above, but I don't have the time at the moment!

 

One final point: an aircraft like a tomcat actually relies remarkably little on its wings to sustain flight at high altitudes and high speeds anyway, and is mostly just a missile with sightly larger wings (i.e the engine does most of the work keeping is flying, the wings are just there in case you want to change direction).

Edited November 25, 2016 by Steel

[Gaarst]

19 minutes ago, hms_warrior said:

Hey.

I have been reading a lot of stuff about the Skylon lately, and this got me thinking about two questions i can not realy find an answer to.

Maybe someone here can help?

First: So let's asume we have a Spaceplane-SSO.

To make things easier i pull some specs:

Think about something like the F14 Tomcat. Variable geometry, 14 tons empty, 20 tons fuelled. We have some magical fusion-drive giving us 10.000 isp (so no big t/w change on the way up...just to make things easier^^).

Now how powerful does this magical engine needs to be? as a plane we don't need a t/w above 1 to lift off.

Those wings will help us trick gravity. But somewhere on our way to space the air will be so thin that our wings turn into useless pieces of metal and all we have to finish our trip to orbit is our engine. But how much acceleration do we need to keep up to reach orbital velocity before tumbling down again?

The second(kinda bonus like) question i have trouble finding a good answer for is:

How far up is that point our wings get useless?

I guess for both questions will depend partly on the speed we can maintain inside the atmosphere...so let's say our Space-Tomcat can sustain a climbrate of 15 degrees all the way up to the "wings get useless" point. And our hull is good enough to go mach 5 at 20Km whitout melting...and an additional 3 per 10km extra. so 30km: mach 8 max. 40 km mach 11 max and so on.

Anyone have a rule-of-thump for this cenario?

As it turns out we defined space when trying to answer your questions.

The Kármán line (standing at about 100km) is usually quoted as the boundary between atmosphere and space, and is historically defined as the altitude at which you would need to go faster than orbital speed to generate enough lift to stay up. In other words, under 100km, you need wings, over 100km wings are useless since the centrifugal force is stronger than the lift they could generate.

For your engine's power, you just need enough to be able to reach orbital speed, that is to overcome drag at all points in order to be able to accelerate. No need for TWR >1 (in fact most upper rocket stages have TWR below 1).

Edit: ninja'd

Edited November 25, 2016 by Gaarst

[UnusualAttitude]

You could experiment in KSP (RSS)...

1) Build something with wings and a rocket engine.

2) Add/drain fuel until TWR < 1.

3) Turn on infinite fuel and see if you can get to orbit. See how low a TWR you can get away with.

4) Report your results. :-D 

[cantab]

https://en.wikipedia.org/wiki/Lift-to-drag_ratio#Supersonic.2Fhypersonic_lift_to_drag_ratios

So as you go faster, your attainable lift-to-drag ratio approaches about 4. It's about 5 at orbital speed. That means you'd need a TWR of at least 0.2 just to maintain straight and level flight. Which seems surprisingly low actually. I think in practice you'd want higher, but I reckon 0.5 would do it.

[AeroGav]

On 25/11/2016 at 9:35 PM, cantab said:

https://en.wikipedia.org/wiki/Lift-to-drag_ratio#Supersonic.2Fhypersonic_lift_to_drag_ratios

So as you go faster, your attainable lift-to-drag ratio approaches about 4. It's about 5 at orbital speed. That means you'd need a TWR of at least 0.2 just to maintain straight and level flight. Which seems surprisingly low actually. I think in practice you'd want higher, but I reckon 0.5 would do it.

Also, I'd like to point out that wings just don't abruptly stop producing lift.  

Your craft has a certain weight - actually that's something that's constantly decreasing as fuel burns off, but whatever -  and you need to counteract that.

Just after takeoff, lift counteracts 100% of that gravity.

When you achieve stable orbit, the centrifugal effect of hurtling around the planet at 18000 mph cancels 100% of the gravitational force, and you are above the atmosphere so lift is 0%.

But in between there is a gradual transition.

So, when you're nearing orbital velocity, you might only need lift equivalent to half of the vehicle's current weight, because centrifugal force is supporting the other half.    This again cuts down on the thrust requirement.

 

[p1t1o]

At very high velocity, aerodynamic lift can be had at extremely high altitudes - far above the speed and altitude envelopes at which current air-breathing propulsion can provide meaningful thrust/acceleration. It is also approaching the regime where you are travelling so fast that gaining aerodynamic lift is getting offset by aerodynamic drag - time to exit the atmosphere!

According to the Skylon "user's manual" this point is around 100,000ft.

**edit**

Fun Fact - at the high velocities being talked about, sufficent lift can be obtained without wings, relying entirely on body-lift.

Edited December 2, 2016 by p1t1o

[AeroGav]

49 minutes ago, p1t1o said:

At very high velocity, aerodynamic lift can be had at extremely high altitudes - far above the speed and altitude envelopes at which current air-breathing propulsion can provide meaningful thrust/acceleration. It is also approaching the regime where you are travelling so fast that gaining aerodynamic lift is getting offset by aerodynamic drag - time to exit the atmosphere!

According to the Skylon "user's manual" this point is around 100,000ft.

**edit**

Fun Fact - at the high velocities being talked about, sufficent lift can be obtained without wings, relying entirely on body-lift.

The wikipedia article on hypersonic lift drag ratio states you really need a "wave rider" design to get good l/d numbers, the skylon's wing looks like something optimised for mach 2 or 3 tops.  Actually in a single stage vehicle you're never going to achieve the ideal l/d ratio because that means optimising heavily for a single mach number which of course is going to change hugely over the flight.

Also the skylon will have insane twr close cycle mode with much fuel burned off, so it's probably easier to just work the vertical.       The insane twr of conventional rocket engines is why they don't bother with wings.  Yes, wings would allow smaller rocket engine to do the job, but rocket engines are so light it's not a worthwhile saving.    Air breathing engines have much worse TWR which is where wings make sense.