Talk:CR-7 R.A.P.I.E.R. Engine

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Revision as of 05:07, 5 December 2014 by UmbralRaptor (talk | contribs)
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Suborbital capabilities

An Aeris 3A with an FL-400 fuel tank in place of one of the fuel tanks and a ram air intake mounted on the top with a tailpiece is capable of suborbital flight, exceeding those of the Aeris 4A (with my mediocre skill), up to 150km.— Preceding unsigned comment added by Thompcan (talkcontribs) 15:51, 15 May 2014‎ (UTC)

The Aeris 4A is orbit capable so a suborbital flight won't exceed the capabilities of the Aeris 4A. — xZise [talk] 16:23, 15 May 2014 (CDT)

correction of ISP stats

I mounted a Rapier and a Turbojet on the same plane to compare their Isp-height behaviour. As far as I could see on several runs, they behave exactly the same, i.e. 800 at sea level, about 2500 peak at 5000 m and about 1200 at 35000 m.

Furthermore I created a stationary test-stand to check for air consumption. Not surprisingly both engines take exactly the same ammount of air for the same amount of fuel. Having the same Isp-curve in mind, they both take the same ammount of air for the same generated thrust.

The power up / down curve also is identical.

After all, the Rapier is just a bit weaker Turbojet (190 vs. 225 KN) with the same weight, thus having a slightly worse TWR, otherwise the exact same characteristics. Therefore it generates the thrust of about six Rockomax 48-7s in rocket mode while having a slightly better Isp (360 vs 350), meaning that a spaceplane with the same Air / rocket thrust is lighter with the Rapier than with the Turbojet / 78-7s combinaton.

The main disatvantage is the missing power generation.

P.S.: This is my first Wiki edit,so please be lenient towards me... — Preceding unsigned comment added by Lone Starr (talkcontribs) 18:29, 2 December 2014‎ (UTC)

If you look into the part configurations you also see that the stats are the same, which is interesting because then there is basically no need for the TurboJet Engine. And it doesn't look like the TurboJet Engine has been nerved or the R.A.P.I.E.R. Engine has been buffed. And it's alone from the Isp values already better than the 78-7S and the thrust itself is not that important. They also have the same mass, so you can't even argue that the TurboJet Engine saves mass.
Please note that the display of the specific impulse in the infobox isn't very good for jet engines currently.
About wiki edits in general: Add --~~~~ at the end of a comment you make on talk pages to automatically sign it. — xZise [talk] 14:39, 3 December 2014 (CST)

The jet engine is actually quite useful, and yes, it saves mass. For the same trust, you need only 5 jets for every 6 rapiers. The jet+LV-N is an especially potent combo for interplanetary travel. The Rapier ISP is less than half, and the craft would need more efficient engines for far-away travel anyway. Once the jet engines get going fast enough (over 2-2.2km/s) at 32-35km, the LV-Ns can perform essentially a low-trust Hoffman transfer to 70km. It can be done with 0.1 TWR from the LV-Ns. The Rapiers make the ascent easier by providing more trust on rocket mode, but they are much more inefficient. Overall jets+LV-Ns can lift nearly 85% of the initial mass to orbit, which is beyond what Rapiers alone can achieve.

Smartech (talk) 20:49, 3 December 2014 (CST)

Well comparing it with a LV-N is not fair as there is a comparison in TurboJet Engine which says that a TJE and Rockomax 48-7S combination is more efficient than a R.A.P.I.E.R. Engine. Also going alone from the part configuration, the thrust behaviour for speeds up to 2 km/s of the TJE and RAPIER are the same. Just the TJE goes up to 2.4 km/s while the RAPIER goes “only” up to 2.2 km/s.
I had a short conversation with User:UmbralRaptor on Twitter today: [talk] 15:55, 4 December 2014 (CST)
Comparative thrust and fuel on the runway.
Comparison at 2100 m/s.
Comparison at 2159 m/s.
My preferred use of effective Isp is to note that air is "free" (multiplying the nominal Isp by 16), but the thrust curve does not affect fuel consumption. On the pad, you can clearly see turbojets/rapiers burning propellant like they're generating the full 225/190 kN, even though they're actually only putting out 112.5/95 kN. This thrust not affecting fuel use is why turbojets have a higher effective Isp above 2000 m/s, dramatically so above 2100 m/s. (Though setting up pics like this to demonstrate is terribly clumsy) UmbralRaptor (talk) 23:07, 4 December 2014 (CST)