Jump to content

How much delta v will jet engines give me?

noahtech
 Share

Recommended Posts

noahtech

noahtech

Posted
Posted (edited)

I'm trying to get into orbit and I can't seem to figure out a way to caculate how far the basic jet engine, and the turbo jet engine will take me before I need to switch to liquid fuel+oxidizer engines. I've noticed that it seems that the basic jet engine only takes me up to the top of the light blue bar in the atmosphere indicator, and the turbo jet engine only seems to take me to half way into the next bar in the atmosphere indicator.

Edited by noahtech
question was answered
Link to comment
Share on other sites
Hodo

Hodo

Posted
Posted

It depends on how high and fast you manage to get before you fire the rocket engine. If you manage to get to mach 5 or about 1500-1600m/s and 30km alt you wont need much d/v to get to orbit.

Link to comment
Share on other sites
GoSlash27

GoSlash27

Posted
Posted

It depends on a lot of external factors. A properly designed spaceplane can cover almost the entire load with jets, only needing 100 m/sec for circularization.

The altitude your jets will get you to is a function of your wing loading, drag, and intake area... but the altitude doesn't matter as much as speed. If you can get over 2,300 m/sec, you'll need almost no rocket DV to finish the job.

Best,

-Slashy

Link to comment
Share on other sites
UmbralRaptor

UmbralRaptor

Posted
Posted

The problem with talking about it in terms of ÃŽâ€V is that jet ascents are very different from rocket ascents. It's quite possible to burn 7-10 km/s getting a jet into an unstable orbit (eg: 80 x 30 km), but this doesn't matter much due to their absurdly high effective Isp.

The trick is to use turbojets, have semi-decent TWR (~1 on the runway), enough intakes that each engine effectively gets 0.03+ intake area, and a lowish angle of attack.

Link to comment
Share on other sites
Starman4308

Starman4308

Posted
Posted (edited)

First off, no basic jets. Turbojets will get you off the runway, and will give you far more power during the crucial high-altitude acceleration to orbital velocity, at which point basic jets are dead mass. RAPIERs are about as good: if you have a dedicated circularization rocket, turbojets probably have the edge, while RAPIERs are good if you don't want to carry a separate engine for orbital maneuvers.

Second, turbojet dV is so weird that you shouldn't pay much attention to it. More important is to have an appropriate ratio of jet engines per tonne of spaceplane mass, an appropriate ratio of intakes per jet, and an appropriate number of wing sections per tonne of aircraft: after that, it's trial-and-error to see how much LF you need to dedicate for the atmospheric section of your ascent.

EDIT: Also, basic spaceplane profile: dash up to 20 km (out of le soup), and then go horizontal. Try to balance climb rate with intake air needed and drag: the higher you are, the less drag you get, but the less intake air you get. You can somewhat compensate for low intake air with high velocity (the faster you go, the faster air is rammed into the intakes), but at some point, you can't go higher without a lack of intake air choking off jet thrust to below drag.

Edited by Starman4308
Link to comment
Share on other sites
capi3101

capi3101

Posted
Posted (edited)
More important is to have an appropriate ratio of jet engines per tonne of spaceplane mass, an appropriate ratio of intakes per jet, and an appropriate number of wing sections per tonne of aircraft: after that, it's trial-and-error to see how much LF you need to dedicate for the atmospheric section of your ascent.

EDIT: Also, basic spaceplane profile: dash up to 20 km (out of le soup), and then go horizontal. Try to balance climb rate with intake air needed and drag: the higher you are, the less drag you get, but the less intake air you get. You can somewhat compensate for low intake air with high velocity (the faster you go, the faster air is rammed into the intakes), but at some point, you can't go higher without a lack of intake air choking off jet thrust to below drag.

And in case you're wondering, the aforementioned ratios are generally considered as follows:

- 1 Basic Jet per 10 tonnes takeoff weight, 1 RAPIER per 13 tonnes takeoff weight, 1 Turbojet per 15 tonnes takeoff weight.

- No less than 0.03 intake area per engine regardless of the type used. Moar is generally better. Which one is "best" is a matter of debate; as a rule, you can't go wrong with RAM Intakes or the old-style Radial intakes; two radials are roughly equivalent to a RAM. Put them towards the back of your craft if you want good dynamic stability (they tend to have high drag ratings).

- About 1.15 units of lift per tonne of takeoff weight (~2.3 tonnes per Delta Wing). A craft-wide lift:mass ratio of 1:1 is easier to remember and provides more lift than you should need in most cases.

Basic Jets do suck as main ascent engines; where they excel is low-speed, low-altitude flight where they have insane specific impulse values. They're particularly useful in VTOL applications and at those low altitudes the Circular Intake will provide sufficient air for their operation.

You can make a reasonable guess as to how much LF and oxidizer you'll need based on the engine selection you make - 40 units times the maximum takeoff weight (in tonnes) your engine selection is capable of, and 25 units times max take off weight in oxidizer. That's assuming you're going with RAPIER engines - in truth, if you've got a decent TWR on the runway (say, somewhere in the neighborhood of 1.2 or higher) and a high intake:engine ratio your jet engines should be capable of reaching orbital velocity on their own and you shouldn't need that much delta-V to circularize. Enough fuel for 400 m/s or so of rocket thrust for circularization is generally a safe assumption given the recommended ascent profiles, though I myself have been able to get a spaceplane up to orbit with just over 50 m/s of delta-V before (that was with a pair of turbojets that had serious intake spam going on - 0.09 intake area.

The old fuel rule used to be one Mk1 fuselage per engine. Nowadays there's not much reason to not use the Mk2 tank - it provides four times as much fuel with a slightly better mass:fuel ratio as compared to 4 Mk1 fuselages and it provides a not inconsequential amount of lift. The amount of oxidizer provided in a Mk1-2 adapter is generally more than sufficient for most orbital ops.

In general you want to keep your ascent profile shallow; a climb rate of no greater than 100 m/s is what you want and the lower you can get it during that period where your jets have maximum thrust, the better. As a rule, if your surface velocity is rising rapidly, you're ascending too quickly; if it's barely budging, you're not ascending fast enough. That bit just takes experience to judge effectively.

Edited by capi3101
Link to comment
Share on other sites
John FX

John FX

Posted
Posted (edited)

The quick answer is `most of the Dv needed for orbit as long as you are not using FAR` and if you want more power higher up and faster add more intakes.

Turbojets and ram air intakes.

Edited by John FX
Link to comment
Share on other sites
GoSlash27

GoSlash27

Posted
Posted

Drag has a lot to do with it. Once above 32KM altitude, it's all a matter if drag.

If you're having issues with it slowing and falling, it could be insufficient wings (1 CL per tonne), insufficient intake area (.01 M^2 per engine), insufficient engines (1 turbojet per 15 tonnes) or you're too nose heavy (CoL should be centered in CoM or just behind).

If all that is good, it's your takeoff profile. Limit your climb rate to NMT 100 m/sec and do not allow your angle of attack to exceed 22 degrees. You want to be doing about 900 m/sec when you cross 20 KM altitude.

Best,

-Slashy

Link to comment
Share on other sites
GoSlash27

GoSlash27

Posted
Posted

^ Expanding a bit on the launch profile:

Although Kerbin's atmospheric pressure logarithmically declines with altitude, in practice spaceplanes behave radically differently in different sections of altitude.

Below 10 KM, it's a thick soup.

From 10-25KM, it's noticeably thinner and thinning at a rate that helps you climb ballistically.

From 25-32KM, it's very thin and makes lift difficult to achieve

Above 32 KM it's nearly vacuum and makes thrust hard to produce.

Below 10 KM, you won't be able to accelerate much. You just want to climb as quickly as you can without losing your stability. Pitch as steep as you can without it getting wallowy and maintain a minimum airspeed of 100 m/sec.

10-25, it's easy to ballistically climb and get yourself too slow, so you want to limit your climb rate and build speed. Maintain a climb rate of 100 m/sec.

25-32, your wings are struggling to maintain your progress upwards. It's important to limit your angle of attack to manage drag and let speed build while taking whatever climb rate you can get. Maintain an angle of attack of not more than 22 degrees.

Above 32, your wings no longer serve any purpose and you're trying to keep the engines lit. The good news is you're going so fast that the centrifugal force is helping you climb. It's just a thrust vectoring thing at that point. Limit your climb rate to keep your apoapsis ahead and keep the engines working by throttling down. Keep a positive rate of climb and get all the speed you can.

Once you get to the point where your speed is no longer increasing, your jets have done all they can for you. Time to switch to rockets and close the intakes.

Best,

-Slashy

Link to comment
Share on other sites
noahtech

noahtech

Posted
  • Author
Posted

It seems to be lacking enough thrust. It has a LF+O engine on it for when it gets into orbit, but if I use it on takeoff then it can get into the air, but when I turn it off it falls again.

Is there some way I can upload the plane to here for someone to check on?

Link to comment
Share on other sites
numerobis

numerobis

Posted
Posted
noahtech,

Not sure what happened, but the craft file is gibberish. I'm guessing it got corrupted. Please try uploading again.

Best,

-Slashy

It's a PDF document advertising internet marketing. ...?

- - - Updated - - -

Fun forum feature I just learned; whiskey tango foxtrot becomes an ellipsis.

Link to comment
Share on other sites
numerobis

numerobis

Posted
Posted
I have no idea how you figured that out!

Open it in a text editor. It says PDF near the top. Rename the file to have a .pdf extension, open it in Preview or acrobat or whatever.

apparently the link goes to a web page where you can download the file from.

Clicking the button on that web page is where GoSlash27 and I found the PDF document. Your file is borked.

Link to comment
Share on other sites
This thread is quite old. Please consider starting a new thread rather than reviving this one.

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
 Share
Go to topic listing
×
×
  • Create New...